BPVI-13-02

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BPVI-013MILK PROCESSING &

PACKAGING

Indira GandhiNational Open UniversitySchool of Agriculture

Block

2PROCESSING OF MILK

UNIT 4

Clarification, Separation, Bactofugation and Standardization 5

UNIT 5

Pasteurization 24

UNIT 6

Homogenization 38

UNIT 7

Sterilization and Ultra-High-Temperature Processing 54

UNIT 8

Preparation of Designated and Special Milk 72

Processing of Milk

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Programme Design Committee

Prof. H.P. Dikshit Prof. Panjab Singh

Vice Chancellor Vice Chancellor

IGNOU, New Delhi Banaras Hindu University, Banaras (U.P.)

Prof. S.C. Garg Shri A.N.P. Sinha

Pro-Vice Chancellor Additional Secretary,

IGNOU, New Delhi Ministry of Food Processing Industries, Delhi

Ministry of Food Processing Industries, Milk Plant, Gwalior :

New Delhi : Shri M.E. Khan, Manager - Plant Operation Mr. K.K. Maheshwary Mr. R.K. Bansal, Consultant Delhi Milk Scheme, Delhi : Mr. V.K. Dahiya, Tech. Officer Shri Ashok Bansal, DGM

(Milk Products)

CITA, New Delhi :

NDRI, Karnal, Haryana : Shri Vijay Sardana Dr. S. Singh, JD (Academics) Dr. S.P. Agrawala, Head (Dairy Engg.) Mahaan Protein, Mathura (U.P.) : Dr. Rajvir Singh, Head (Dairy Eco.) Dr. Ashwani Kumar Rathor, GM Technical Dr. K.L. Bhatia, Ex-Principal Scientist Dr. S.K. Tomar, Principal Scientist IGNOU, New Delhi Dr. B.D. Tiwari, Ex. Principal Scientist (SOA Faculty Members) : Dr. Dharam Pal, Principal Scientist Dr. M.K. Salooja, Dy. Director Dr. A.A. Patel, Principal Scientist Dr. M.C. Nair, Dy. Director

Dr. Indrani Lahiri, Asstt. Director

Mother Dairy, Delhi : Dr. P.L. Yadav, Sr. Consultant

Dr. P.N. Reddy, Quality Control Manager Dr. D.S. Khurdiya, Sr. Consultant Sh. Jaya Raj, Sr. Consultant Sh. Rajesh Singh, Consultant

Programme Coordinators: Prof. Panjab Singh, Dr. M.K. Salooja and Dr. P.L. Yadav

Block Preparation Team

Writers: Dr. A.A. Patel (Unit-4), Editors: Dr. P.L. Yadav

Dr. M.K. Salooja (Unit-5) Dr. M.K. Salooja

Dr. RRB Singh (Unit-6 & 7)

Dr. Alok Jha (Unit-8)

Course Coordinators: Dr. M.K. Salooja, Dr. P.L. Yadav and Dr. A.A. Patel

Material Production

SOA, IGNOU :AR (Publication) - Ms. Pushpa GuptaSecretarial Assistance - Mr. Vinay Sehgal

July, 2006

© Indira Gandhi National Open University, 2006

ISBN 81-266-

All rights reserved. No part of this work may be reproduced in any form, by mimeograph or anyother means, without permission in writing from the Copyright holder.

Further information on the Indira Gandhi National Open University courses may be obtained fromthe University's office at Maidan Garhi, New Delhi-110 068 or the official website of IGNOU atwww.ignou.ac.in.

Printed and published on behalf of Indira Gandhi National Open University, New Delhi byProf. S.C. Garg, Pro-Vice Chancellor, IGNOU.

Printed atPaper Used: Agro-based Environment Friendly

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BLOCK INTRODUCTION

We have learnt that milk is collected and transported to the dairy plant for furtherprocessing. Now we will know the different operations, which are performed in thedairy plant for processing the milk like clarification, separation, standardization,pasteurization, homogenization and packaging of milk. The method for preparingsterilized milk also has been explained. We will know the methods for preparingstandardized, toned, flavoured, recombined and reconstituted milk.

Unit 4: Milk clarification, separation, bactofugation and standardization are thecommon operations in a dairy plant. In this unit, we will learn about the purposeand operational features of filtration and clarification of milk. Methods of separation,separation of milk, factor affecting yield and fat contents in cream are also includedin the text. Bactofugation and clarification are important processes in modern dairyunits for removal of heavy dirt particles and bacteria from raw milk. Standardizationof milk for fat and SNF is important to ensure the desired and designated compositionin the milk and milk products. We will learn about the purpose and procedure ofstandardization of milk. Simple methods for preparation of different types of milkwith varying fat % and SNF % have been explained. This will help us to calculatethe required quantity of fat and SNF for preparation the milk of desired/designatedstandards.

Unit 5: Pasteurization is one of the most important heat treatment processes in anymilk plant. In this unit, we will study the fundamentals of pasteurization process,reasons for pasteurizing milk, theory of pasteurization, type and important parts ofpasteurizer, procedure for operating a pasteurizer and cleaning of plant.

Unit 6: Homogenization is widely accepted process in dairy industry to reduce thesize of fat globules. We will learn in this unit the reasons for homogenization of milk,theory governing the homogenization process, design of homogenizer, factors relatedto homogenization efficiency, effect on physico-chemical properties of milk due tohomogenization and operational details of homogenizer.

Unit 7: Milk is highly perishable commodity. To preserve its quality, differentprocessing treatments are employed. One of such processes is sterilization. Theunit “Sterilized and UHT Processing” gives the details such as definition, theoreticalbasis, type of plants, processing details, changes during processing of milk andaseptic packaging.

Unit 8: Special types of milks are prepared by altering natural constituents of milk.In this unit, we will study about the preparation of different types of special milklike toned milk, double toned milk, standardized milk, skim milk, recombined milk,reconstituted milk and flavoured milk.

Processing of Milk

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UNIT 4 CLARIFICATION, SEPARATION,BACTOFUGATION ANDSTANDARDIZATION

Structure

4.0 Objectives

4.1 Introduction

4.2 Filtration and Clarification of Milk

Filtration

Clarification

4.3 Separation of Milk

Methods of Separation

Factors affecting the Skimming Efficiency

Factors affecting Yield and Fat content of Cream

4.4 Other Centrifugal Processes for Milk

Bactofugation

Clarifixation

4.5 Standardization of Milk

Standardization of Milk for Fat

Standardization of Milk for Fat and SNF

4.6 Let Us Sum Up

4.7 Key Words

4.8 Some Useful Books

4.9 Answers to Check Your Progress

4.10 Some More Questions to Check Your Progress

4.0 OBJECTIVES

After reading this unit, we should be able to:

state the purpose of some of the basic milk processing operations.

differentiate between filtration and clarification of milk.

define what ‘separation of milk’ means and what factors affect the same.

enumerate other centrifugal processes viz. bactofugation and clarifixation.

specify ‘standardization’ means and how to carry our the same.

4.1 INTRODUCTION

As we have studied earlier, that it is essential to keep the milk cool soon aftermilking till it reaches the processing plant. At the milk plant it may be (i) processedfor distribution in fluid form as market milk, or (ii) converted into various products.In either case, it is required to be subjected to certain basic treatments beforefurther processing.

The treatments that milk is required to undergo at a dairy plant include filtration orclarification, separation and standardization. These are aimed at purification andcompositional modification of the milk. In the present unit we shall discuss theobjectives of such treatments, and ways and means of carrying out the same.Clarification and separation of milk are, in practice, achieved by centrifugation of

Processing of Milk

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milk (in specially designed centrifugal machines.certain other centrifugation-basedprocesses such as ‘bactofugation’ and ‘clarifixation’ relevant to milk processingwill also be briefly discussed in this unit.

4.2 FILTRATION AND CLARIFICATION OF MILK

i. Purpose

Raw milk as produced on the farm and transported to the collection centre or adairy plant generally contains varying amounts of visible, invisible impurities. Thisforeign matter includes straw and hair pieces, dust particles, leukocytes (somaticcells or white blood cells), insects, etc. If not effectively removed, such extraneousinsoluble matter can result in deposits in milk handling equipment such as cooler,etc., and, more importantly, cause unsightly appearance.

Relatively large pieces of such material e.g. straw, hair and insects, are usuallyremoved by ‘straining’ (passing the milk through a fine metal–gauge strainer ormetallic sieve on the farm, at the collection centre or at the processing plant.Tubular sieves located in the milk inlet pipe to the processing unit (e.g. pasteurizer)are also used.

However, finer foreign matter to be eliminated requires clarification using a specialfilter or a centrifuged clarifier. These steps of aesthetic improvement of product areparticularly useful for overcoming the problem of sediments in fluid milk and liquidmilk products in general, and homogenized milk in particular.

ii. Filtration

Filtration (or, clarification using a filter-bag) refers to making the milk pass througha filter-cloth or filter-pad. The filtering medium has a pore size (25-100 mm) thatpermits most of the foreign matter to be retained on it. The milk filter consists ofa nylon filter-bag or a filter-pad supported on a perforated stainless steel (SS)support held in an SS enclosure with a tight-fitting lid, milk distributor, and inlet- andoutlet- connections. Milk usually passes from top to bottom. In case of twin filters,three way valves in the inlet and outlet lines enable switching from one filter to theother when the first is to be cleaned. Sometimes, filters may be provided in theform of cylindrical bags or ‘stockings’ fitted over perforated SS tubes as in themodern continuous pasteurizing plants (high-temperature short-time, or HTSTpasteurizers

Filtration can be carried out either on cold milk (about 10oC) or warm milk (40-45oC). Since warm milk filtration is more rapid due to lower viscosity of warm milk,it is universally used. For cold filtration, the filter is located in the line connectingthe milk receiving tank or holding tank and the pasteurizer. Since warm filtrationrequires preheating, the filter of this type is placed between the regenerator and thefinal heating section of the HTST pasteurizer.

The filter-bag must periodically be cleaned. Accordingly, the operation run mayvary from 2 to 10 hours depending on the level of foreign matter and the filter poresize. Generally, twin filters located in parallel are employed to permit cleaning ofone filter while the other is in use. This enables continuous process run.

We should be able to realize that filtration removes only the gross impurities, anddoes not remove bacteria from milk. Accordingly, it does not improve the keepingquality of the milk. In fact, bacteria may grow in the filters if they are used forunusually long times before cleaning.

iii. Clarification

Definition and objective : As an alternative to filtration, clarification can also be

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employed to remove insoluble impurities especially the finer ones. It involves theuse of a centrifugal machine called ‘clarifier’. Thus, clarification is a process ofsubjecting milk to a centrifugal force in order to eliminate the finer but heavierparticles from milk, somatic cells, dust particles, etc. Although part of bacteria arealso removed along with the extraneous matter, clarification cannot be consideredan effective means of bacteria removal. Hence, one should be aware that it cannotbe a substitute for a suitable heat treatment in order to ensure safety againstpathogenic (disease-causing) microorganisms.

Principle of clarification : As we have studied, when milk is introduced betweentwo adjacent rotating conical discs (in a stack of several discs) of a centrifugebowl, it is subjected to a centrifugal force. This force causes the heavier dirtparticles to be thrown out into the sludge space surrounding the discs where it iscollected during the run, while the comparatively lighter milk continuously flowsinward and upward to the outlet. There is no separation of fat globules (cream) andskim milk in a clarifier.

Operation of a clarifier : Raw milk is made to pass usually under a pumppressure, down a central pipe of a rotating bowl and led to the outer edge of theclarifier discs through a distributor in the bottom and then onto the spinning discs,where milk and dirt are separated. The milk is led to the discharge port at the topof the bowl whereas the dirt is accumulated in the sediment space. The accumulatedsludge is removed from the bowl by dismantling the clarifier at regular intervals.The interval may range from 1 to 8 hours depending on size of the clarifier and theamount of impurities in the milk. However, most large-size modern clarifiers areself-desludging or ‘partial desludging’ type in which periodical sludge removal takesplace during the clarification process, without interruption of the clarifier operation.Such desludging results in about 0.05-0.10% of milk being lost and the sludge beingliquid rather than solid as in the non-self-desludging machines.

As for the milk filter, clarifier may be located in the raw milk line between the rawmilk tank and pasteurizer. Alternatively, milk may be clarified warm/hot by placingthe clarifier at a suitable point in the regeneration section of the HTST unit orbetween the regeneration and heating sections.

The clarifier sludge or clarifier ‘slime’ consists primarily of dust and dirt particles,blood cells, microorganisms and milk protein. Its composition will depend on whetherit is liquid (82-86% water, 6-8 % protein), or solid (65-69% water, 24-28% protein).

Check Your Progress I

1. What type of impurities are removed from milk by filtration?

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2. What will happen if milk is not clarified?

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3. State about the milk filter.

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Clarification, Separation,Bactofugation and

Standardization

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4. What is the purpose of twin filters?

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5. What is clarification?

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6. How does a clarifier remove milk impurities?

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7. What is the main operational advantage and limitation of a self desludgingclarifier?

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8. Where should a clarifier are located?

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9. What is ‘clarifier sludge’?

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4.3 SEPARATION OF MILK

We have studied that milk contains fat and non-fat constituents, also called solids-not-fat (SNF). Fat is present as globules whereas the SNF form an ionic solution(e.g. certain salts), true solution (e.g. lactose and whey proteins), or a colloidalsolution (e.g. casein micelles) in the water part of milk. Thus, milk represents anemulsion in which the relatively large fat globules are dispersed in the continuousaqueous phase (serum). Since fat globules are lighter as compared to other solids,they tend to readily separate out from the serum (or skim milk), as can be seenin the formation of a ‘cream’ layer on the top of milk held undisturbed in acontainer for a few hours. Cream is that portion of milk, which is rich in milk fat,but poorer in SNF. This suggests that much of the fat can be easily separated inthe form of cream from milk, leaving behind the skim milk containing very little fat.

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Cream separation enables the processor to manufacture a variety of fat-rich dairyproducts such as cream of various types, butter, ghee, etc. Cream separation alsomakes it possible to adjust the composition of milk with respect to its fat and SNFcontents. Such compositional modification (vide Sec. 4.5) may be desired for productsmanufacture as also for meeting the legal requirements of different types of fluidmilk

i. Methods of Separation

Two methods of separation of cream from milk are commonly used: (i) gravityseparation and (ii) centrifugal separation. Both these methods rely on the basicprinciple of separation of two immiscible liquids having different densities, under theinfluence of gravitational or centrifugal force.

Gravity Separation: As mentioned above, when milk is allowed to stand undisturbedfor some time, a layer of cream (or ‘malai’) forms on the top due to rising of thefat globules which are initially dispersed throughout the bulk of milk. The upwardmovement of the lighter fat globules (density, 0.93 g/cc at 20oC) in the heavierserum (density, 1.035 g/cc) takes place owing to gravity. Creaming may becomeevident in as short time as half an hour.

The rate of cream separation is directly proportional to the difference between thedensities of fat and serum and to the square of fat globule diameter, and inverselyproportional to the viscosity of serum. Thus, for a given sample of milk, the creamingrate will be maximum when the density difference is maximum and viscosity isminimum. Both these factors are, in turn, affected by temperature of milk. As thetemperature rises, the ratio of the density difference and the serum viscosity increasesfavouring the separation process. This increase is particularly prominent between10o and 30oC and much less above 50oC.

Cream separation by gravity is, however, a very slow and inefficient process. It isof little practical value for commercial purposes. Hence, mechanized cream separationemploying a centrifugal machine is most commonly used in the dairy industry. Evenfor a very small scale separation involving, say 10-20 litres of milk, a centrifugalseparator is used, be it hand-driven or motor-driven.

Centrifugal Separation: In principle, this method of cream separation is similar togravity separation but gravity as the driving force is replaced by the centrifugalforce for which a rotational machine is used. Since the latter force is much largerthan the gravitational force, separation is greatly accelerated. The centrifugal separatoris similar to the clarifier discussed in the earlier section, but milk entering throughthe bottom of the separator bowl holding a stack of conical discs rises up throughholes located somewhere in the middle of the inner and outer edges of the discs.The milk between discs is subjected to a centrifugal force in the rotating bowl andthereby tends to fly out from the centre. The skim milk fraction, being heavier,moves away and forms a layer on the outer edge of the discs, whereas the fatglobules gather on the inside edge. The incoming un-separated milk forces theseparating layers further and upward out at the top of the bowl. Thus, there aretwo outlets in a cream separator, one for skim milk and the other for cream, thecream outlet being nearer to the centre.

The rate of cream separation in ease of a centrifugal separator is influenced by thesame factors affecting gravity separation, but the speed of the separator bowl andthe disc diameter are also very important here. The higher the speed of the bowlor larger the diameter of discs, the greater will be the separation rate.

ii. Factors affecting Skimming Efficiency

Since fat removal from milk is the principal function of a cream separator, theefficiency of the process, also called skimming efficiency, is determined by the


Standardization

Processing of Milk

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effectiveness with which the fat content of the out-coming skim milk is reduced.The residual fat content of skim milk is usually in the range of 0.01 – 0.05% in themodern machines. A fat content higher than 0.06% represents poorer separationefficiency. The skim milk fat content is inversely related to fat recovery in thecream. Hence, the skimming efficiency is often defined as the percentage of totalfat in whole milk recovered in the cream separated from it. For a given fat contentof whole milk, the higher the fat content of skim milk, the lower the skimmingefficiency. The factors that affect the skimming efficiency are related to either themilk being separated or the separator.

Intense agitation of milk prior to separation, air incorporation (or foaming) and highacidity of milk adversely affect the separation efficiency. Further, if the proportionof smaller fat globules (especially below 2 mm in diameter) is greater, the skimmingefficiency will go down. It should, therefore, be obvious that homogenized milk withits very small globules (please see Unit 3) cannot be separated. Gravity or centrifugalseparation of fat globules from skim milk is faster at a higher temperature. Thusskimming efficiency increases with increasing temperature of milk up to about80oC, beyond which increasing viscosity of milk tends to make the separationprocess less efficient. Depending upon location of the cream separator in the milkprocessing line (particularly with respect to HTST pasteurization), the separationtemperature may range usually from 35-75oC, optimum being 50-55oC (‘warmmilk’ separation). However, cold milk separators’ may operate at 5-10oC giving anadvantage of less foaming, but partial churning of fat, bowl clogging and reducedflow rate (separator capacity) are the associated disadvantages.

Adjustment of the ‘cream screw’ for high-fat cream (above 55%), or excessiveflow rates of feed milk reduce the skimming efficiency. However, feeding ratesbelow the normal separator capacity does not enhance the skimming performance,but it may lead to undesirable air incorporation. A higher bowl speed gives higherskimming efficiency but, since increased speed requires greater energy input, normalrange of 4000-7000 rpm (sometimes as low as about 2000 rpm) giving efficientseparation is normally not exceeded in the separator design. Poor disc condition(e.g. deshaped, dirty or scratched one), vibrations of the separators, and defectivegaskets in the cream section could cause unacceptable skimming efficiency.Excessive slime getting collected in the sludge space of the bowl would also havean adverse impact on the separator performance.

iii. Factors affecting Yield and Fat Content of Cream

The yield of cream and skim milk can be given by the following formulae:

i) Yield of cream (% of feed milk) =

m s

c s

f - fx 100 ..... (Eq.1)

f - f

ii) Yield of skim milk (% of feed milk) =

c m

c s

f - fx 100 ....... (Eq. 2)

f - f

where,

fm

= fat in milk, %

fs

= fat in skim milk, %

fc

= fat in cream, %.

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All those factors which affect the skimming efficiency can be expected to influencethe cream yield too. Conditions leading to a higher skimming efficiency would givea better yield. However, the fat content of cream is obviously the major factorinfluencing the yield of cream. Accordingly, the adjustment of the cream screw orskim-milk screw is critical with regard to cream yield.

The position of the cream screw i.e. a valve provided in the cream outlet controlsthe flow rate of the cream being discharged. Turning the screw ‘inward’ reducesthe cream discharge rate thereby increasing the fat content of cream. Adjusting thevalve by ‘outward’ movement has the opposite effect. Similarly, manipulation of theskim milk screw so as to decrease the flow rate of the exiting skim milk willdecrease the fat concentration of cream, and vice-versa. Thus, changing the positionof the cream screw or skim-milk screw alters the ratio of cream to skim milk; anincreased ratio decreases the fat content of cream and a decreased one raises it.

Further, a lower separation temperature and a higher fat content of milk lead to anincreased fat content of cream, whereas an increased feed rate causes a decreasedrichness of cream, and vice-versa.

Check Your Progress 2

1. What are the objectives of cream separation?

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2. What are the methods of separation of milk?

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3. What is creaming?

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4. What are the factors affecting the creaming rate?

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5. What is the effect of temperature on the rate of creaming?

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6. What is the major drawback of gravity separation as compared to centrifugalseparation?

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7. How is fat separated from skim milk in a centrifugal separator?

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8. How do the machine parameters affect the rate of fat separation in a centrifugalseparator?

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9. What do you understand by ‘skimming efficiency’?

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10. List the factors affecting the skimming efficiency.

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11. How is the fat content of cream exiting the separation adjusted?

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12. How does the fat content of feed milk influence the richness of cream?

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13. What is the effect of temperature of separation on the fat content of cream?

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4.4 OTHER CENTRIFUGAL PROCESSES FOR MILK

The principle of differential movement of heavy and light components of milksubjected to a centrifugal force has been utilized in a few applications other thanclarification and cream separation. These include bactofugation and clarifixation.

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i. Bactofugation

We may recall clarification of milk wherein it was stated that the centrifugalremoval of heavy dirt particles etc. results in elimination of a part of bacteria aswell. Such bacteria removal is made more effective using a special high-speed disc-bowl centrifuge called ‘bactofuge’. This process known as ‘bactofugation’ isparticularly applicable to removal of bacterial spores from milk, which are not onlydifficult to inactivate by heat treatment but also heavier (or denser) than vegetativecells.

The bactofuge is kind of high-speed (up to 20,000 rpm) clarifier provided withdischarge nozzles in the bowl wall. The centrifugal force generated in it is upto10,000 g (g = gravitational acceleration). The bacteria in milk being bactofuged arecollected in the form of ‘bactofugate’ in the sludge space. The bactofugate isapprox. 3% of the feed volume and contains primarily bacterial spores and milkproteins. Anaerobic spores are removed to an extent of 98-99%. A double-bactofugetreatment at 73oC yields more than 99.9% reduction in bacterial spore count ofmilk. However, since bactofugation does not effectively eliminate all microorganisms,pathogens in particular, the milk must ordinarily be pasteurized so as to ensuresafety of consumption.

The main application of this expensive process is in the field of cheese makingwhere removal of anaerobic (clostridial) spores from milk is useful in avoiding theproblem known as ‘late blowing’ in hard and semi-hard cheeses. Bactofugation hasalso been employed to extend the shelf life of milk under refrigeration.

In order to destroy the bacteria contained in the bactofugate and improve theeconomy of the process by utilizing the milk protein in it, a process called‘Bactotherm’ has been evolved. Clarified and standardized milk is bactofuged at60-75oC followed by pasteurization employing the HTST process. The bactofugateis deaerated in a vacuum chamber and sterilized at 130-140oC for 3-4 sec usingsteam injection, and finally mixed with chilled bactofuged milk for further processing.

ii. Clarifixation

A clarifixator is a machine working on the same principle as that of the centrifugalseparator but has an additional provision to effect reduction of the size of fatglobules in the cream fraction before it is remixed with the outgoing skim milk. Theresulting milk, sometimes called ‘stabilized’ milk has a reduced tendency to creamingupon undisturbed storage because of small-size fat globules.

The break-down of fat globules is brought about by the peripheral spikes orprotrusions on the ‘paring disc’ provided in the cream passage at the top of thecentrifuge. A paring disc is a fixed circular structure acting as a stationary centripetalpump. The cream separated from milk strikes the protruding obstacles beforeentering the paring disc. The fat globules thus experiencing an intense turbulenceor shearing action are broken down to a smaller size (less than 2 mm). The creamis then mixed with the incoming milk to be recycled through the bowl. The fatglobules of sufficiently reduced size will not get re-separated when the creampasses through the bowl discs again and will therefore, exit the separator throughthe skim milk outlet which thus discharges ‘homogenized’ whole milk. However,because of its lower effectiveness as compared to pressure homogenization,clarifixation has not been used to any significant extent in the dairy industry.


1. What is the function of a bactofuge?

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2. How is a bactofuge different from a clarifier?

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3. Whether bactofugation can be a substitute for pasteurization.

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4. Give the major applications of bactofugation.

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5. What is the ‘Bactotherm’ process?

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6. What is the difference between a cream separator and a clarifixator?

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7. What is a ‘paring disc’?

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8. How is clarifixated milk different from homogenized milk?

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4.5 STANDARDIZATION OF MILK

i. Purpose and Definition

We know that liquid milk sold in the market is of different types with regard to itscomposition. Since the milk available to the processor may not necessarily be ofthe same composition as desired in the milk to be marketed, it is a very common

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practice to adjust the composition as per the requirement. Also, compositionalmodification (or adjustment) is necessary if the milk is to be converted into a certainproduct. A product must conform to the legal standards prescribed for it, or thequality standards set by the manufacturer. Product manufacture without appropriatecompositional manipulation of milk may lead to poor quality product, or a productthat does not meet legal requirements, or it may be an economical loss to theprocessor.

Standardization thus refers to the process by which the milk composition is adjustedto the desired level. The most commonly considered compositional parameters arefat and SNF for market milk, although sometimes fat alone may be taken intoaccount for standardization. For certain specific, product,-manufacturing applicationseven protein content may be adjusted. Accordingly, the process of standardizationinvolves lowering or raising the level of a particular constituent(s) to the desiredvalue specially fat.

ii. Standardization of Milk for Fat

Often milk may be standardized to a certain value of a single component i.e. fator SNF alone. This can be achieved by adding to the milk, a calculated quantity ofa fat-rich product such as cream if the fat level is to be raised, or a low-fat or fat-free component e.g. skim milk, if the milk has excess fat. The fat content of milkcan be reduced also by separating a calculated amount of cream of known fatpercentage.

The calculation of the quantity of cream or skim milk to be added to milk, or creamor skim milk to be separated from it can be made by a simple method known asthe Pearson’s square method. It consists in drawing an arbitrary square (or, arectangle), placing at the left concerns of the square, the values of fat content ofthe two products to be mixed and at the centre of the square, the desired fatpercentage. Then, subtractions are made diagonally across the square, the smallervalue being deducted from the larger one, and the differences are entered at thecorrespondingly opposite corners on the right- hand side. These two new values atthe right corners are summed to obtain a third value. All the three values placedat the right represent the proportions or relative amounts of the given products tobe mixed (top right figure for the amount of the product to the top left, bottom rightfigure for the product to the bottom left, and the sum for the final product). Thefollowing is an example of such a calculation:

500 kg of milk testing 6.5% fat to be standardized to 3.1% fat using skim milkcontaining 0.05% fat.

Thus mixing of 3.05 kg of 6.5% fat milk with 3.40 kg of the skim milk will yield6.45 kg of milk containing the desired fat level i.e. 3.1%. Therefore, the quantityof skim milk required to be added to 500 kg whole milk will be

3.4 x 500= 557.38 kg

3.05

6.5

0.05

3.05

3.1

3.406.45


Standardization

Processing of Milk

16

Accordingly, 557.38 kg of 0.05% skim milk mixed with 500 kg of 6.5% fat wholemilk will yield 1057.38 kg of milk having 3.1% fat.

The single-component (fat-based) standardization is commonly used for creammeant for butter-making. It generally involves adjusting the fat percentage of ahigh-fat cream to the desired level by mixing it with the calculated quantity of skimmilk (or, whole milk). Blending of the two components i.e. cream and skim milk orwhole milk can be carried out by transferring the calculated quantities of the two(one after the other), to a tank (or, silo) with a provision for adequate mixing.

Continuous, on-line blending is much more desirable in a large-scale operation. Thiscan be achieved on the cream separator itself by allowing sufficient cream to remixwith the skim milk so that the mixture is a milk with the desired fat content; thebalance cream flows through the cream line into the cream tank. This requires thatthe separator is fitted with a standardizing device.

iii. Standardization of Milk for Fat and SNF

When milk is required to be standardized for both fat and SNF, the basis ofcalculation of the quantity of skim milk or cream to be added is the ratio of fatto SNF, and the total solids (TS) content. If the desired fat-to-SNF ratio is higherthan the actual ratio in the available milk, then skim milk will be required to beadded. On the other hand, when the desired ratio is lower, cream needs to beblended into the milk. It is, therefore, necessary that both the fat and SNF contentsof the milk to be standardized, and those of cream or skim milk to be used areknown. If the fat content of cream or skim milk (separated from a milk of knownfat and SNF contents) is known, the SNF content can be estimated as under

cm

m

100 - fi) SNF in cream, % = SNF x ....... (Eq. 3)

100 - f

sm

m

100 - fii) SNF in skim milk = SNF x ....... (Eq. 4)

100 - f

where,

SNFm

= SNF percentage in milk

fc

= fat percentage in cream

fm

= fat percentage in milk

fs

= fat percentage in skim milk.

The amount of skim milk or cream required to be added to a given quantity of milk(so as to attain the desired levels of fat and SNF in it) can be worked out by usingthe following formulae:

m mc m

c c

m ms m

s s

(R x SNF ) - Fi) Q = Q x ......... (Eq. 5)

f - (R x SNF )

(f /R) - SNFii) Q = Q x ....... (Eq. 6)

SNF - (f /R)

where,

Qm

= Quantity of milk to be standardized

Qc

= Quantity of cream required

Qs

= Quantity of skim milk required

R = Fat/ SNF ratio desired.

17

fm

= Fat percentage in milk

fc

= Fat percentage in cream

fs

= Fat percentage in skim milk

SNFm

= SNF percentage in milk

SNFc

= SNF percentage in cream

SNFs

= SNF percentage in skim milk.

Alternatively, an algebraic method can be used taking ‘x’ quantity of cream or skimmilk of known fat and SNF contents required to be added to the given quantity ofmilk with certain fat and SNF levels, and then solving. (for x) an equation of thedesired fat-SNF ratio:

mc s m

mc s m

Qxf (or f ) x + f ( )

100 100R = ....... (Eq.7)Qx

SNF (or SNF ) x + SNF ( )100 100

where all values except that of x are known.


1. What is ‘standardization of milk’?

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2. The Pearson’s Square method is used for standardizing milk for fat and SNF.True or False?

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3. Single component standardization is applicable to cream for butter-making.True or False?

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4. How can an in-line cream separator be used for standardization of milk?

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5. When standardizing milk for both fat and SNF, how is it determined as towhether cream should be added or skim milk?

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Standardization

Processing of Milk

18

4.6 LET US SUM UP

There are certain basic operations in relation to processing of raw milk into marketmilk (or, fluid milk) or dairy products. These are primarily aimed at removing thephysical impurities (insoluble foreign matter) from the milk and ensuring the desiredcomposition of the finished product.

The processes that are supposed to ‘clean’ the milk include straining through ametallic sieve to remove coarse particles, filtration using a filter-bag which eliminatessmall particles and body cells, and centrifugal clarification meant for removal ofvery fine dust, dirt and cells. While in-line straining and filtration are most commonmilk treatments followed in a dairy plant, clarification (using a centrifugal machine)is employed only to a limited extent, usually when filtration is not practiced.

A centrifugal separator helps fractionate milk into a fat-rich component i.e. cream,and an essentially fat-free component i.e. skim milk. Several milk- and machine-related factors affect the efficiency of the separation process, also known as‘skimming efficiency’. Similarly, the yield of cream and its fat content are governedby several variables, which must be controlled by the operator in order to obtainthe desired quality and quantity of cream.

Besides cream separation, clarifixation and bactofugation are other centrifugal dairyprocesses. Intended to make milk resistant to fat separation, clarifixation causesbreak-down of the fat globules, whereas bactofugation eliminates microorganisms(especially bacterial spores) from milk.

In order to meet quality and legal requirements milk meant for market purpose orproduct manufacture may be adjusted to a prefixed level of certain compositionalparameters such as fat or non-fat solids (SNF) alone, or both. Such standardizationis usually practiced on batch milks in tanks and silos by blending of the milk withcream or skim milk. Alternatively, standardization can, more conveniently, be carriedout on a separator in a continuous manner.

4.7 KEY WORDS

Bactofugation : It is a process of removing microorganisms,particularly bacterial spores, from milk bymeans of a centrifugal machine called‘bactofuge’, which is essentially a high-speedclarifier.

Bactotherm : It is a process in which the ‘bactofugate’ (thebacterial mass entrained in milk protein gettingcollected in the sludge space of a bactofuge)issubjected to ahigh heat treatment (130-140oCfor 3- 4 sec) and then remixed with the milk.

Clarification : It is a process of removing insoluble extraneousmatter such as dirt particles, somatic cells (or,blood cells), etc. from milk by using a clarifier’,which is a centrifugal machine similar to acream separator but having only one outlet i.e.the one for the clarified milk.

Clarifier sludge : It is the semi-solid impurities separating frommilk and getting collected in the sludge spaceof a clarifier bowl. It primarily comprises dustparticles, leucocytes, microorganisms and milkprotein.

19

Clarifixation : It is a process of reducing the size of fatglobules in milk (and making it stable againstfat separation) by using a specialized centrifugalmachine called clarifixator in which fat globulesare separated and broken down before beingremixed with the milk.

Cream : It is the fat-rich fraction of milk obtained uponseparation of fat globules from milk under agravitational or centrifugal force (the latter usinga disc-bowl centrifuge).

Creaming : It is the phenomenon of cream separation ingeneral, but refers particularly to gravityseparation of fat globules in the form of creamfrom whole milk; a fat-rich cream layer formsat the top of undisturbed milk.

Cream separator : It is a centrifugal machine in which milk issubjected to a whirling action resulting inseparation of lighter fat globules (cream) fromheavier non-fat fraction (skim milk).

Filtration : It refers to removal of extraneous impuritiessuch as hair pieces, dust, dirt, insects, celldebris, etc. from raw milk by passing it througha filler-bag usually placed in-line during milkprocessing.

Self desludging : It refers to automatic, intermittent removal ofclarifier slime (or, separator slime) from thesludge space (without dismantling the bowl)while the process of clarification or creamseparation is going on.

Separator slime : It is the semi-solid material (impurities)separating from milk and getting collected inthe sludge space of the bowl of a creamseparator. It is similar to the clarifier slime.

Serum : The portion of milk excluding fat globules. Itcomprises water and the non-fat constituentsviz., lactose, protein and minerals.

Skim milk : The non-fat component of milk obtained uponseparation of cream from it. It contains verylittle fat, usually less than 0.10%.

Skimming : It refers to the effectiveness with which milkfat can be separated as cream from milk. It isdefined as the percentage of the total fat in thewhole milk recovered in the cream that hasbeen separated from it.

Solids-not-fat : The solids (or constituents) in milk other thanfat are called SNF. These include milk protein,lactose and minerals (or, ash).

Standardization : It is the process whereby the composition ofmilk (in terms of fat, or both fat and SNF) isadjusted to a predetermined level.

Straining : Refers to removal of coarse impurities such ashair pieces, insects, etc. from milk by passingit through a cloth piece or metallic sieve (calledstrainer).


Standardization

Processing of Milk

20

Yield (or, out-turn) : It is the amount of a product (e.g. cream),expressed in terms of percentage of milk used(or separated).

4.8 SOME USEFUL BOOKS

De, S. (1980). Outlines of Dairy Technology. Oxford University Press, Delhi.

Kessler, H.G. (1981). Food Engineering and Dairy Technology, Verlag A. Kessler,Freising (Germany).

MacWalter, R.J. (1962). Clarifying, cooking and storage of milk. In: Milk Hygiene.WHO, Geneva.

Spreer, E. 1998. Milk and Dairy Product Technology. Marcel Dekker, N.Y.

Towler, C. (1994). Developments in cream separation and processing. In: ModernDairy Technology. Vol. 1 Advances in Milk Processing, Sec. Ed. R.K.Robinson (ed.) Chapmen & Hall, London.

Walstra, P., Geurts, T.J., Noomen, A., Jellema, A. and vanBoekel, M.A.J.S. (1999).Dairy Technology: Principles of Milk Properties and Processes. MarcelDekker, Inc., New York.

Warner, J.N. (1976). Principles of Dairy Processing. Wiley Eastern Ltd., NewDelhi.

4.9 ANSWERS TO CHECK YOUR PRGRESS

Your answer should include the following points:


1) i. Insoluble foreign matter such as straw, hair pieces, insects, dust, dirtparticles, somatic cells, etc.

2) i. Unfiltered or unclarified milk will tend to cause deposit formation inequipment used in handling of the milk and give rise to unsightly sedimentat the bottom of the container.

3) i. A milk filter is made up of a nylon cloth with fine pores (25-110 mm) andit is supported over a perforated stainless steel basket or tube.

4) i. Twin filters permit uninterrupted run of a continuous milk process (e.g.in an HTST pasteurizer): Since at a time one filter is in use, when onefilter is required to be cleaned, the other is brought into use merely byturning appropriate valves.

5) i. Clarification is a process of removing milk impurities comprising fine dustand dirt particles, body cells, etc. by means of a centrifugal machine(although ‘filtration’ is also sometimes termed as one of the methods ofclarifying milk).

6) i. When milk is subjected to a whirling motion, the centrifugal force actingon throws the heavier dirt particles away into the sludge space while themilk, being higher, moves inward and upward between the conical discsof the clarifier bowl.

7) i. The clarifier slime in a self-desludging clarifier keeps dischargingautomatically at a definite interval of time so that the machine can workfor a long period without the need to stop it for cleaning. The disadvantagehowever, is that desludging takes place by means of milk jets. Hence,0.05 – 0.10% of milk is lost with the sludge.

21

8) i. If cold-milk clarification is desired, the clarifier should be located in theraw milk line to the HTST pasteurizer. For warm-milk clarification, theclarifier may be located within the regeneration section, or between theregeneration and heating sections.

9) i. The clarifier sludge (or clarifier slime) is the liquid or semi-solid (or solid)material separating from milk and gathering in the sludge space of theclarifier bowl. It is the milk impurities consisting of dust particles, bodycells, leucocytes and bacteria in addition to milk protein.


1) i. Separation of milk into cream and skim milk makes it possible to adjustthe milk composition with respect to fat and SNF, and also facilitatesmanufacture of certain products such as cream, ghee, butter, skim milkpowder, etc.

2) i. Cream separation (or, separation of milk) can be achieved by either (i)the gravity (quiescent storage) method or (ii) centrifugal separation.

3) i. Creaming is the phenomenon wherein the lighter fat globules in undisturbedmilk tend to rise and form a fat-rich layer (of cream) at the top underthe influence of gravity.

4) i. The factors affecting the rate of fat separation in milk include the densitydifference between fat and serum, size of fat globules, and viscosity ofserum. All these factors particularly the first and the last ones are in turn,influenced by the temperature of milk.

5) i. As the temperature increases, there is an increase in the ratio of thedensity difference (between fat and serum) to the viscosity of serum upto about 80oC, the increase being more prominent up to about 50oC. Thusthe creaming rate is higher at higher temperatures, up to approx. 80oC,beyond which it goes down.

6) i. Gravity separation being much slower than centrifugal separation, it is oflittle practical utility in the industry.

7) i. The milk distributed between the discs of the rotating separator bowl issubjected to whirling action. Under the resulting centrifugal impact theheavier serum portion is thrown outward whereas the lighter fat globulesmove inward and upward through the passage near the central axis to thecream outlet at the top; the skim milk is continuously forced upward overthe outer edges of the discs and then to the skim milk outlet.

8) i. The higher the speed of the separator bowl and the larger in diameter ofthe discs, the greater is the rate of separation of fat in a centrifugalmachine.

9) i. Skimming efficiency refers to the effectiveness with which fat can beseparated from milk. It is defined as the percentage of total milk fatrecovered in cream. Skimming efficiency is often indicated by the fatcontent of skim milk, which should not be higher than 0.06% for anefficient separation process.

10) i. The factors affecting skimming efficiency include agitation of milk beforeseparation, presence of air in milk, size of fat globules, acidity of milk,temperature of milk, feed rate, position of cream-screw, separator-bowlspeed, condition of discs, vibrations of the machine and the condition ofgaskets.

11) i. The primary method of controlling the fat content of cream is by adjustmentof the cream screw.

12) i. The higher the fat content of the feed milk, the greater with theconcentration of fat in the resulting cream if the cream screw setting andother factors are not changed.


Standardization

Processing of Milk

22

13) i. As the temperature of separation decreases, the viscosity of creamincreases which causes the cream flow rate to fall. Thus, a colder milkgives richer cream at the same feed rate.


1) i. A bactofuge achieves removal of bacteria, bacterial spores in particular,from milk by subjecting it to a centrifugal force, the heavier bacteriabeing thrown away from the axis of rotation and thereby getting collectedin the sludge space of the disc bowl.

2) i. The bactofuge has a higher speed (about 20,000 rpm) providing a largercentrifugal force (10,000 g) to effect removal of bacteria to a greaterextent than in a clarifier.

3) i. Bactofugation does not necessarily eliminate all pathogenic microorganismsfrom milk although the bacterial load is greatly reduced in bactofugedmilk. Hence, heat treatment of such milk is necessary to make it safe forhuman consumption.

4) i. Since bactofugation can render the milk nearly free from anaerobic spores,bactofuged milk is particularly suitable in cheese-making where the sporescause defects like ‘late blowing’. Also bactofuged milk can be used forproducts with extended shelf life.

5) i. The Bactotherm process refers to bactofuging clarified milk, subjectingthe resulting bactofugate to high heat treatment (130-140oC for 3-4 sec)and then remixing the latter with the pasteurized bactofuged milk forfurther processing.

6) i. In a cream separator, milk under the influence of a centrifugal force isseparated into a fat-rich component (cream) and a low-fat component(skim milk), but the fat globule size remains unchanged. On the otherhand, a clarifixator reduces the size of fat globules in the separatedcream to below 2 mm before remixing the cream with the feed milk fromwhich small fat globules are not reseparated but pass into the skim milk.Thus it delivers ‘homogenized’ whole milk.

7) i. A paring disc is a stationary structure in the cream outlet of a creamseparator (or a clarifixator). The product enters the disc in a circular pathat the periphery whereby the rotational energy is converted into linearkinetic energy providing a pumping effect to the cream.

8) i. Both clarifixated milk and homogenized milk have fat globules of reducedsize giving stability against separation, but the mean globule size is largerin the former. Also, the clarifixated milk is also clarified one so that itshows little sediment.


1) i. Adjustment of the composition of milk with regard to fat alone, or bothfat and SNF, so that the milk has predetermined levels of thesecomponents. It usually involves mixing of a calculated quantity of creamor skim milk with milk in or der to attain the desired values of thecompositional parameters.

2) i. False. The Pearson’s Square method is suitable for a single-componentstandardization, not for both fat and SNF.

3) i. True. In butter-making cream is standardized for the fat content only.

4) i. Special pipeline connections with necessary valves and flow meter providedin the outlets for cream and skim milk on a separator enable mixing ofcream into the skim milk at a predetermined rate and thereby allowstandardization of milk.

23

5) i. Comparing the fat-SNF ratio (Ra) of the available milk with the desired

ratio (Rd) one can determine if cream is to be added (R

d> R

a) or skim

milk (Ra

> Rd).

1.10 SOME MORE QUESTIONS TO CHECK YOURPROGRESS

1. How does a clarifier deliver unseparated purified milk?

2. What is separator slime?

3. What is the role of temperature in clarification and separation of milk?

4. What is the function of the cream screw?

5. How are paring discs useful in a semi-closed separator?

6. What are the rotational speeds of a cream separator and a bactofuge?

7. Why is clarifixator not used extensively in the dairy industry?

8. What are the requirements of in-line standardization of milk?


Standardization

Processing of Milk

24

UNIT 5 PASTEURIZATION

Structure

5.0 Objectives

5.1 Introduction

5.2 Definition and purpose of pasteurization

Time-temperature combination

Purpose

5.3 Theory of pasteurization

Limiting factors for heat treatment

Types of heat treatment

5.4 Batch Pasteurizer

5.5 HTST Pasteurizer Plant and its components

Flow diagram of pasteurization process

Components of a HTST pasteurization plant

Plate heat exchanger

Instrumentation

5.6 Operation of pasteurization plant

Starting the plant

Shut down of the plant

Cleaning and Sterilization of the plant

Pasteurization of milk

Trouble shooting

Preventive maintenance

5.7 Test for Pasteurization Efficiency

5.8 Let Us Sum Up

5.9 Key Words

5.10 Some useful Books


5.0 OBJECTIVES

After studying this unit, we should be able to:

define and give the reasons for pasteurizing the milk

explain the theory of pasteurization

list important parts of a pasteurizer

describe the procedure of operating a pasteurizer.

5.1 INTRODUCTION

Pasteurization is a key process in modern dairy plant operations and forms anintegral part of manufacturing of various indigenous and western dairy products.Milk is a perfect medium for growth of micro-organisms, and growth of pathogenicorganisms can cause diseases such as tuberculosis and typhus. Pasteurization killsthe organisms responsible for spread of diseases through milk and makes it safe forconsumption.

The word ‘pasteurization’ has been named after an eminent French scientist, Louis

25

Pasteur. In general terms it is heating milk or its products to such temperature,which destroys nearly all the microorganisms, present in it without affecting thecomposition or properties of the product. Thus it is important to monitor thepasteurization process as improperly/under pasteurized milk can cause the infection.This unit will give us working knowledge of pasteurization process. Let us understandthe process.

5.2 DEFINITION AND PURPOSE OF PASTEURIZATION

Fresh milk produced from healthy milch animals generally contains minimum loadof microorganisms. In the course of handling at the farm, milk is liable to becontaminated by various microorganisms mainly bacteria. Rapid chilling to below4°C temperature slows down the growth of microorganisms in the milk. Milk mustbe treated by an established process so that all pathogenic microorganisms arekilled before it is consumed as fluid milk. This is achieved by heat treatment.Pasteurization is one of the most important heat treatment processes. The term asapplied to market milk refers to the process of heating every particle of milk to atemperature of at least 63°C (145.4°F) for 30 minutes or 71.7°C (161°F) for 15seconds (or to the temp-time combination which is equally efficient) in properlydesigned equipment. Milk is immediately cooled to 4°C and stored in cold storagemaintained at 4°+1°C.

As per definition of International Dairy Federation (IDF) “Pasteurization is a processapplied to a product with an objective of minimizing possible health hazard arisingfrom pathogenic microorganisms associated with milk by heat treatment, which isconsistent with minimal chemical, physical and organoleptic changes in the product”The heat treatments suggested by the IDF for the pasteurization of milk are 15seconds at 71.7°C=161°F or 30 minutes at 62.8°C=145°F can be regarded as“universal” reference treatments. Three aspects emerging from the definition are:(i) level and degree of heat treatment, (ii) minimum chemical, physical and organolepticchanges, and (iii) minimum health hazards. These are elaborated here.

i. Time-Temperature Combination

The time-temperature combinations normally used for pasteurization of fluid milkare as follows:

63°C (145.4°F) and held at that temperature for at least 30 minutes

72°C (161.6°F) and held at that temperature for at least 15 seconds.

The milk is then immediately cooled to a temperature not greater than 4°C. Theselected heat treatment shall be applied only once. This means pasteurization includesheating to a specific time-temperature combination followed by immediate coolingto 4°C.

ii. Purpose

Milk is pasteurized for two purposes:

To make safe for human consumption by destroying pathogenic microorganismspresent in milk.

To improve its keeping quality.

The most heat resistant pathogenic organism at pasteurization temperature is theMycobacterium tuberculosis and hence this has been made as an index organismto achieve complete safety of milk. Any heat treatment, which will destroy thisorganism, can be relied upon to destroy all other pathogenic organisms as well asother organisms involved in milk spoilage. Some bacteria, call thermodurics (heatresisting) may survive during pasteurization but immediately cooling slows downtheir growth and thus prevents them causing spoilage such as flavour taint or

Pasteurization

Processing of Milk

26

souring. Although, the main purpose of heat treatment is to destroy all microorganismscapable of causing disease in humans but pasteurization has two additional benefits,i.e. the destruction of a large number of spoilage microorganisms present in rawmilk and deactivation of some natural enzymes like lipases, which can adverselyaffect the quality of manufactured products, i.e. lipolysis or breakdown of fat intoglycerol and free fatty acid. However, we must be clear that pasteurization is nota substitute for cleanliness during milk production. The pasteurization process shouldonly be applied to raw milk obtained from healthy cow, which is clean, sweet andhas a low bacterial count.


1. Give two reasons for pasteurizing the milk.

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2. Describe the time-temperature combination normally used for milk pasteurization.

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5.3 THEORY OF PASTEURIZATION

We have understood that heating milk to selected time-temperature combinationeffectuates pasteurization to ensure destruction of all pathogenic microorganisms.Theoretically, aspect of pasteurization is “the heat treatment applied to the milk todestroy pathogenic organisms”. The process parameters of heat treatment or time-temperature combinations are elaborated below: (a) Limiting factors for heattreatment and (b) Types of heat treatment.

i. Limiting Factors for Heat Treatment

The upper and lower limits of temperature to pasteurization process are based onthermal death point of tubercle bacilli and beginning of reduction of the cream line.The thermal death time for tubercle bacilli provides the lower limit to heat treatment.The adverse effects on the commercial quality milk provide an upper limit for thepossible time- temperature combinations used in pasteurization. As the cream lineis the first quality to be affected, it is generally used as the standard indicator ofchanges in the chemical, biological and physical properties of milk caused by overheating.

In the early 1920’s, North and Park performed extensive tests by heating milksamples containing tubercle bacilli at different time-temperature combinations thatdestroyed all the tubercle bacilli present in them. The time-temperature combinationsthat destroy all tubercle bacilli are taken as thermal death points. Table 5.1 showsa number of thermal death points for tubercle bacilli.

Table 5.1: Thermal Death Points for Tubercle Bacilli

Temperature Time

°C °F

100.0 212 10 seconds

27

93.3 200 20 seconds

82.2 180 20 seconds

76.7 170 20 seconds

71.1 160 20 seconds

68.2 155 30 seconds

65.6 150 2 minutes

62.8 145 6 minutes

61.1 142 10 minutes

60.0 140 10 minutes

57.8 136 30 minutes

55.6 132 60 minutes

These thermal death points can also be plotted on a graph to give a thermal deathline.

Safety margin: This is the additional amount of heat treatment (time and temperatureabove the thermal death point of the tubercle bacillus) so that, under no circumstances,will any tubercle bacilli be left alive after correct routine operation of a pasteurizer.Certainly, a more intense heat treatment would obtain more efficient antibacterialresults than pasteurization. On the other hand, the milk is not inert to heating; over-heating adversely affects the appearance, taste, nutritional and technological valueof milk. Combination of higher temperature and longer holding time-temperature arealso recommended for HTST pasteurization of dairy products having higher contentsof solids.

ii. Types of Heat Treatment

The heat treatment given in form of (i) holding and (ii) continuous correspondinglyrelate with two methods of pasteurization i.e.

Batch, holding or Low Temperature Long Time (LTLT) method and

Continuous, High Temperature Short Time (HTST) method.

In the batch method, the milk is heated to 63°C in a tank or vat equipped with ahot water or steam jacket and agitators to keep the milk agitated; held for 30minutes and then partly cooled in the batch pasteurizer. The further cooling is doneby surface/plate cooler. This method is mostly used for processing of around 5000liters of milk.

High Temperature-Short Time (HTST) pasteurization is the process, which iscommonly used now a day all over the world. Plate Heat Exchanger (PHE) is usedto heat, hold and cool the milk. Milk is heated to a temperature of at least 72°Cand held at that temperature for not less than 15 seconds and then immediatelycooled to a temperature not greater than 4°C.


1. What are the different methods of pasteurization?

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Pasteurization

Processing of Milk

28

2. Enumerate the temperature–time combination for the two methods ofpasteurization.

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5.4 BATCH PASTEURIZER

The parts of a typical batch pasteurizer are following:

Insulated outer casing

Insulated hinged cover

Stainless steel inner vessel

Agitator and its motor

Outlet cock and heating water distribution pipe.

This system is well suited for small-scale operation, where less than 3000 to 5000litres of milk are available. The vat may be rectangular, but a vertical, cylindricaldesign is preferred for practical reasons. The vat normally consists of an innervessel, surrounded by an insulated outer casting, thus forming a jacket, throughwhich hot water or steam is passed (Figure 5.1). After the milk has reached therequired temperature (63.0°C), it is usually held at that temperature for a certainfixed period (30 minutes). Thereafter, it is cooled as quickly as possible either bycirculating refrigerant/chilled water or through plate/surface chiller. Cooling the milkafter pasteurization by circulating a refrigerant – in most cases cold water –

a. Insulated outer casingb. Insulated hinged coverc. Stainless steel inner vesseld. Agitatore. Agitator motorf . Outlet cockg. Heating water distribution pipe

c g

d

a

f

b

e

Figure 5.1: Batch Pasteurizer

29

through the jacket or the vat may take much time. Therefore, a separate small-capacity surface, tubular or plate cooler may be used to rapidly cool the milk to therequired temperature. This system also has the advantage that the vat will beavailable sooner for the pasteurization of another batch of milk.

Batch pasteurizers have a small heating surface area relative to their contents.Heat transfer is greatly improved by agitating the milk. Agitators of different designare used for this purpose. They may even consist of double-walled paddles or otherdevices with internal steam or water circulation. Care must be taken to avoid foamformation during filling of vat. It is very difficult to heat the milk and foam togetheruniformly and consequently microorganisms present in the foam may survivepasteurization. If the inlet valve is at the bottom of the vat, foam formation caneasily be prevented. A lid or cover on top of the vat promotes a uniform temperatureof the contents and prevents skin formation on the milk.

5.5 HTST PASTEURIZER PLANT AND ITSCOMPONENTS

The HTST system is the most common method used by the dairy plants forpasteurization of milk. The main advantage of HTST pasteurization is its capacityto heat treat milk quickly and adequately with built-in safeguards that preventimproper pasteurization due to under heating of milk. The HTST system employsplate heat exchangers for heating, regeneration and cooling. The system consistsof feed pump, plate heat exchanger, holding section, flow diversion valve,instrumentation, essential services and piping system. The entire process is automaticand is ideal for handling of 5000 litres per hour (lph) or higher quantity of milk. Thisis a continuous flow process and also saves energy due to regeneration section(Figure 5.2). In order to understand a pasteurizer let us go systematically for:

Pasteurization

Figure 5.2: Flow diagram of high temperature short time pasteurizer (H.T.S.T.)

Processing of Milk

30 Figure 5.3: Flow Diagram of Pasteurization

Flow diagram of process;

Different compartments/sections;

Plate heat exchanger, which is the main part; and

Instrumentation

i. Flow diagram of pasteurization process

The schematic flow diagram of HTST pasteurization is given in Figure 5.3.

Raw milk enters the constant heat tank (balance tank), passes to the milk pumpand then through a flow controller to the plate heat exchanger. The plate heatexchanger consists of regeneration section, heating, holding and cooling sections.

31

The raw milk enters the pre-heating (regeneration section), where hot pasteurizedmilk (72°C) flows counter current to the raw cold milk, within adjacent plates,transferring heat for pre-heating of raw milk and pre-cooling of pasteurizing milkresulting in energy saving. The partially heated raw milk passes through a filter orclarifier and homogenizer. It then enters the heating section where it is heated toat least 72°C. The hot milk then passes through the holding section to ensure thatthe fastest moving particles of milk are held at 72°C for at least 15 seconds.

The flow diversion valve diverts the milk to constant head tank if it is not properlyheated to pasteurization temperature. Properly pasteurized milk passes forwardthrough the flow diversion valve into the regeneration section where it is cooled byincoming cold raw milk passing in the opposite direction on the other side of theplates. Milk enters the cooling section and is cooled at 4°C before storage.

An indicating thermometer situated at the outlet of the holding section measures thetemperature of the hot milk and this is recorded on a revolving thermograph. If thetemperature of the milk falls below 72°C, the hot milk-recording pen drops past theset pointer on the thermograph and this activates the flow diversion value, the safe-guard pen and an alarm bell. The flow diversion valve diverts the unheated milk intothe constant head tank for re-circulation until the milk reaches the correcting temperature.

ii. Components of a HTST Pasteurization Plant

The complete pasteurizer plant consist of:

Constant head tank

Milk feed pump

Flow controller

Filters

Clarifier

Homogenizer

Plate heat exchanger consisting of bank of plates compartmentalized intoregeneration, heating, holding and cooling sections,

Flow diversion valve

Instruments associated with indicating controlling and/or recorded functions,

Systems for providing steam, air, water, heating and cooling arrangements, and

Piping system to link various components

iii. Plate Heat Exchanger (PHE)

The Plate Heat Exchanger consists of a bank of plates inter-connected (sections)held in a rigid frame (figure 5.4). The main function of the PHE is the exchangeor transfer of heat from a hot liquid (hot water or hot pasteurized milk) to a coolerone (cold water, chilled water brine or raw milk) across a metal plate. Let us seehow the heat is transferred through plates.

Plates: The plates are thin stainless steel sheets usually rectangular in shape. Theplates are corrugated and cause a turbulent flow, which increases rate of heatexchange. The rate of heat exchange also depends on the surface area of the plate,the thickness and type of metal used in the plates, the rate and direction of flowof the liquids and the difference in temperature between the two liquids involvedin the heat exchange process.

An approximate 3-8 mm space is maintained between the plates by a non-absorbentrubber seal, which is bonded around the edges of the plate. The liquids, which aresandwiched among the plates, enter and leave the interspaces through holes in thecorners of the plates. Open and blind holes route the liquids from one set of plates

Pasteurization

Processing of Milk

32

Figure 5.4: Plate Heat Exchanger

to another. The capacity of the pasteurizer is secured by a corresponding numberof plates.

Regeneration sections: The bank of plates is usually divided into four sectionsseparated by connector grids with inlet and outlet bosses. In the regenerationsection, the incoming cold milk is heated by the hot pasteurized milk and thepasteurized milk is cooled by transferring heat to the cooling medium. This heattransfers process work most effectively when the two liquids involved flow inopposite direction, i.e. counter current flow on either side of the plates. Regenerationsection raises the raw milk temperature from 4°C to 67°C and cools the pasteurizedmilk from 72°C to 10°C. Thus, PHE saves about 92% of heating and coolingenergy. The regeneration efficiency is calculated by using the following formula:

% Regeneration = temperature increase due to regeneration/ total temperature increase

For example: The cold milk enters the pasteurizer at 4°C and attains a temperatureof 60°C after regeneration. The final pasteurization temperature is 72°C. Calculatethe regeneration efficiency.

Increase in Temperature due to regeneration: 600C-40C=560C

Total Temperature Increase: 720C-40C= 680C

% Regeneration efficiency: 560C/680C = 82.36%

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Steam-heated hot water or vacuum steam is used in heating section to raise thepartly heated raw milk to pasteurization temperature. The holding section is eitherplate type or tube type. The plate type will have a number of plates. The partlycooled pasteurized milk is further cooled in cooling section to 4°C.

iv. Instrumentation

The instruments associated with the pasteurization plant are used for performingthree functions (Table 5.2).

Table 5.2: Instruments associated with pasteurizer and their functions

S.No. Type of Instrument Function(s)

1. Indicating Temperature (Milk, hot water, Chilled water),Steam & air pressure

2. Controlling Operating the flow diversion valve, operatingthe steam regulating valve in the heatingsystem

3. Recording functions Recording the hot and cold milk temperaturesand recording the frequency and duration ofdiversions


1. Describe the constructional and operational details of a batch pasteurizer.

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2. Describe the steps involved in the operation of a milk pasteurizer (HTST).

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5.6 OPERATION OF PASTEURIZATION PLANT

We have studied the importance of pasteurization process and the importantcomponents of a pasteurization plant. Now let us see the operations involved inrunning of a pasteurization plant and how to cope with operational problems.

i. Starting the Plant

The following steps should be followed to start the plant:

a) Start the air compressor;

b) Switch on the control panel mains;

c) Fill the hot water tank, start the hot water pump;

d) Open the air vents

e) Start flow of the milk to the float controlled balance tank

f) Start the milk pump

g) Close the air vents when the milk coming out from them indicates that all airhas been displaced.

Pasteurization

Processing of Milk

34

h) Set the temperature controller to maintain the milk at 72°C.

i) Turn on cold water and chilled water and hot water set.

ii. Shut down of the Plant

For shutting down the plant, at the end of the milk run:

a) Make available in the storage tank a sufficient quantity of water (approx. equalto the capacity of the plant).

b) As the last milk is leaving the float balance tank, tip in the water from the tank.

c) When the last of the water is leaving the float balance, turn the three-way cockat the finished milk outlet so that the flow is diverted to the floor.

d) Place a hose in the float balance tank and flush the plant thoroughly with wateruntil the discharge from the finished milk outlet becomes clear.

e) Turn off the cold water, brine or chilled water in the cooling sections.

f) Shut off the steam supply to the hot water set.

g) Admit cold water to the hot water tank and run until the plant is cold.

h) Stop the milk and hot water pumps.

i) If brine is used, flush out with running water.

j) Turn off the air supply and the main electric switch at the panel.

Thereafter, the plant must be thoroughly cleaned.

iii. Cleaning and Sterilization of the Plant

Cleaning the plant: Cleaning is done after completion of pasteurization process.The milk supply is stopped to constant head tank by turning off the valve ofRaw Milk Storage Tank. Clarifier and homogenizer are stopped. The water isadded to the constant head tank. Hot water temperature is set at 70°C. Primarydetergent solution is circulated for 20-30 minutes. Flush the system with lukewarmwater. Secondary detergent solution is circulated for 20-30 minutes. Flush theplant with water.

Sterilization: The plant can be sterilized by hot water or sodium hypochloritesolution. The raw milk tank and pasteurized milk tank are bypassed and hotwater (87-90°C) is circulated for 10 minutes. The sterilization is done beforerunning the plant with milk for pasteurization.

iv. Pasteurization of milk

The operation of plant with the milk is called running of the plant. The plant isstarted. It is sterilized. The plant is run on water. The standardization is done tocheck the flow, operation of flow diversion valve, heating temperature and coolingtemperature. The homogenizer pressure is also set in according to requirements.The flow of milk from raw milk tank to pasteurized milk tank is monitored.

v. Trouble shooting

Pasteurizing problems may occur during start up procedures or during the run.When a problem occurs it is important to be able to identify the problems from thesymptoms, identify the cause of the problem and take the appropriate action towardssolving the problem. If the problem causes a delay in processing it is advisable toturn off essential services such as steam and heating and cooling system to preventburn on in the heating section and a freeze up in the cooling system. The commonproblems and their remedial measures are given in practical exercises. These couldbe grouped in three broad areas: (i) inadequacy in achieving temperature, (ii)chocking of plates and (iii) leaking plant assembly. The broad reasons for these aregiven here.

(i) Inadequacy in achieving temperature : The possible reasons are: inadequate

35

steam supply, faulty temperature controllers, air in milk and improper assemblyof plates.

(ii) Chocking of plates : Fouling, high milk temperature, high milk acidity andinadequate filtering of milk could be the reasons for chocking of the plant.

(iii) Leaking plant assembly : The reasons are: damaged and worn gaskets,damaged plates and wrongly fitted plates.

vi. Preventive maintenance

Preventive maintenance will help to control damage, excessive wear and tear andoccurrence of accidents. Preventive maintenance can be divided into two areas, (i)avoiding damage to the plant and equipment and (ii) observations and inspection ofplant and equipment. Avoiding damage consists of basically the careful handling ofmachinery and equipment. The regular inspection of the plant and equipment isimportant as a part of preventive maintenance, and may include:

a) Periodical tests may be made to check the flow rates of heating medium,cooling medium and milk.

b) The recording instruments such as thermometers, etc must be periodically checkedfor accuracy.

c) Air operated instruments should be supplied with clean air.

d) The plate surfaces and gaskets must be checked during the manual cleaning ofplants.

e) Filter cloth/filters must be changed at regular intervals.

f) The faces of the plate bar and tightening spindle should be lightly coated withgrease.

5.7 TEST FOR PASTEURIZATION EFFICIENCY

Phosphatase Test: Phosphatase test is done to determine whether milk has beenproperly pasteurized or not immediately after pasteurization of milk. The test isbased on the principle that alkaline phosphatase, a natural enzyme present in rawmilk, is simultaneously deactivated by heat treatment as specified for pasteurization.When milk-containing phosphatase is incubated with p-nitro phenyl di-sodium orthophosphate, it hydrolyses the substrate and, as a result, para-nitro phenol is liberatedwhich gives a yellow colour under alkaline condition of the test. The amount of theyellow colour present is directly proportional to the amount of phosphatase presentin milk. The presence of yellow colour indicates inefficient pasteurization or post-pasteurization contamination of the milk. The intensity of the colour is comparedwith standard and lavibond comparator disc.


1. Name two methods for sterilizing the pasteurizer.

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2. Write the importance of phosphtase test.

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Pasteurization

Processing of Milk

36

5.8 LET US SUM UP

Pasteurization is a key process in dairy plant operations in which heat treatment isgiven to milk to destroy all pathogenic bacteria. It extends the keeping quality ofliquid milk by destroying most of the milk spoilage organisms. It safeguards publichealth and ensures good quality manufactured products. There are two methods ofpasteurization of milk (i) Low Temperature Long Time (LTLT) method and (ii)High Temperature Short Time Method (HTST). LTLT method is used in dairyprocessing less than 5000 litres of milk. The most commonly method adoptedconsists of heating the milk in continuous flow to a minimum temperature of 72°Cand maintaining it at this temperature for not less than 15 seconds after which themilk is rapidly cooled to 4°C. This is known as the High Temperature Short Time(HTST) process. The principal item of equipment required for HTST pasteurizationis a Plate Heat Exchanger (PHE) and usually this is of the plate type. The mainfunction of the PHE is to exchange or transfer of heat from a hot liquid to a coolerone. The bank of plates is grouped into different sections called as Regeneration,Heating and Cooling Section. The holding section can be either plate type or tube type.

Raw milk enters into Plant Heat exchange of the plate heat exchanger and itstemperature is increased from 4°C to 67°C. The heated raw milk is further heatedin heating section upto 72°C and kept for 15 seconds and then pass through FlowDiversion Valve (FDV) and milk is forwarded to regeneration section where properlypasteurized milk is cooled from 72°C to 10°C. The cooled milk enters the chillingsection and is further cooled to 4°C. The pasteurization plant is equipped withinstruments to indicate and control the temperature and also to performed the otherrelated functions for efficient pasteurization of milk.

Different operations like starting, sterilizing, cleaning and running of the plant shouldbe done in a correct way and the standardized procedure/steps should be followedas specified by the manufacturer of the plant.

The periodic inspection of the different components of the plant is essential toensure it’s proper functioning and trouble free service.

5.9 KEY WORDS

Corrugated : bent into regular curves, folds or grooves

Taint : objectionable foreign flavour

Thermal : determined, measured or operated by heat

Thermograph : a self-registering thermometer

Turbulence : violent commotion

Valve : a dense attached to a pipe to control thepassage of air, steam or gas

Organoleptic : sensory properties flavour (smell & taste),colour and texture

Pathogen : an agent that causes diseases.


Dairy Handbook. (1985). Alfa-Laval, Food Eng AB, PO Box 64, Lund, S-22100,Sweden.

De, Sukumar. (1980). Outlines of Dairy Technology, Oxford University Press Bombay

ICAR. (2002). Handbook of Animal Husbandry, Third Revised Edition – New

37

Delhi Chapter on Dairying contributed by B. N. Mathur & D. K.Thompkinson

Khan M. E. (1998) Milk Processing, Dairy Technology Textbook for Class XI.NCERT, Delhi.

NDDB. (1980). Milk Processing Manual, NDDB, PO Box – 40, Anand

Manual for Milk Pasteurizer Operators. Victoria Milk Distribution Association.

5.11 ANSWERS TO CHECK YOUR PROGRESS



1) i. To make safe for human consumption by destroying pathogenicmicroorganisms present in milk.

ii. To improve the keeping quality of milk.


2) i. Batch, holding or Low Temperature Long Time (LTLT):63°C for 30minutes.

ii. Continuous, High Temperature Short Time (HTST): 72° C for 15 seconds.


3) i. The components of a pasteurization plant: milk feed pump, constant headtank, flow controller, plate heat exchanger, filter, clarifier, homogenizer,flow diversion valve, instruments to record temperatures, systems forheating and cooling and piping system.


4) i. Two methods are (i) Hot water sterilization, and (ii) Sodium hypo chloridesterilization.

ii. Phosphatase test is done to determine whether milk has been properlypasteurized or not.

Pasteurization

Processing of Milk

38

UNIT 6 HOMOGENIZATION

Structure

6.0 Objectives

6.1 Introduction

6.2 Homogenization: Theories and Process Description

Definition of homogenized milk

Theories of homogenization

Advantages and disadvantages of homogenized milk

Viscolised milk

Design and operation of homogenizers

High pressure homogenization technology

Vacuum homogenization

Checking the efficiency of homogenization

6.3 Influence of process variables on the processing efficiency and productquality

Factors affecting homogenization efficiency

Effect of homogenization on milk properties

Problems/Defects associated with homogenized milk

6.4 Let Us Sum Up

6.5 Key Words



6.8 Some Questions to Check Your Progress

6.0 OBJECTIVES

After reading this unit we should be able to:

define homogenization;

explain the theories governing the homogenization process;

describe the homogenizer design and operations;

comprehend the innovations in homogenization technology;

specify factors that affect homogenization efficiency;

state the effect of homogenization on various physico-chemical properties ofmilk;

enumcrate various problems associated with the homogenized milk.

6.1 INTRODUCTION

Milk is an oil-in-water emulsion. Fat globules in the milk are dispersed in a continuouswater phase (skim milk) and normally vary in sizes ranging from 1 mm to 22 mm,with a mean size of approximately 3-4 mm. As the density of milk fat is less thanthat of skim milk, the fat globules tend to rise to the surface during storage and forma cream layer. The rise of fat globules follows Stoke’s law where the velocity ofrising fat globules is expressed as:

d2 (rs

- rf)

V a18 h

39

Where, d = diameter of the fat globule, rs= density of the serum phase, r

f= density

of milk fat and n = viscosity of milk serum.

Very small fat globules (<1 mm) remain suspended in the serum phase due tobrownian motion and adversely affect the creaming phenomenon. The presence ofcryoglobulins in the raw milk causes agglomeration of fat globules, which subsequentlyhave increased tendency to rise to the surface.

Homogenization is a mechanical process in which milk is forced through ahomogenization valve under very high pressure. The milk is thus deflected at rightangles through a narrow opening of about 0.1 nm (100mm). As the milk comes outof this valve opening, there is sudden drop in pressure and the milk is subjected toimpact against an impact ring. This complete process results in disruption of fatglobules leading to decrease in the average diameter (typically from 0.2 to 2 mm)and an increase in the number and surface area of fat globules.

Homogenization with reference to milk/ dairy applications thus refers to a mechanicalprocess that is used to reduce the size of fat globules such that milk fat does notrise to form a cream layer during storage of milk. Although homogenization rendersfat globules uniformly distributed in the body of the milk, upon prolonged storageit does not remain completely dispersed.

6.2 HOMOGENIZATION: THEORIES AND PROCESSDESCRIPTION

i. Definition of Homogenized Milk

United State Public Health Service has proposed one of the most comprehensivedefinitions for homogenized milk. This has been the most widely accepted andreferred definition. It states that “Homogenized milk is milk which has been treatedin such manner as to ensure break-up of the fat globules to such an extent thatafter 48 hours of quiescent storage no visible cream separation occurs in the milkand the fat percentage of the milk in the top 100 ml of milk in a quart bottle (946ml),or of the proportionate volumes in containers of other sizes, does not differ by morethan 10 per cent of itself from the fat percentage of the remaining milk as determinedafter thorough mixing.

ii. Theories of Homogenization

The principle underlying the process of homogenization is to subject the fat globuleto enough severe conditions, which disrupts it into smaller globules. The newlyformed fat globules are maintained in dispersion for sufficient time to allow milk fatglobule membrane (MFGM) to be formed at the fat-serum interface. The followingtheories have been proposed to be responsible for the entire phenomenon.

Shearing or Grinding: As milk is passed at high pressure (velocity ~ 200-300 ms-1) through the homogenizer valve (~ 100 mm gap), fat globules undergo shearingaction. The shear between fat globules and the surface of the homogenizer wallcoupled with wire drawing effect results in elongation of the fat globules whichprogressively becomes unstable. These phenomenon result in subdivision of the fatglobules. Furthermore, the difference in velocity of the faster moving serum phaseat the centre of the liquid stream as compared to the liquid near the edge of thestream causes the fat globules to grind against each other. The turbulence createdby the difference in velocity and eddy currents of the liquid add to the shear effectsand thus enhance the process of disruption of the fat globules.

Exploding: This theory suggests that during homogenization, there is build up oftremendous pressure. When this pressure is suddenly released, the internal pressures

Homogenization

Processing of Milk

40

within the fat globules pull the globule apart with exploding effect. This results indisintegration or subdivision of fat globules into smaller globules.

Splashing/Shattering: As the high homogenizing pressure is attained in thehomogenizer, the homogenizing valve releases the highly compressed milk at veryhigh velocity. The liquid suddenly strikes a retaining wall/ perpendicular surface.This causes splashing or shattering effect on the fat globules resulting in breakdown of globules into smaller sizes.

Acceleration and Deceleration: This theory relates sudden change in velocity ofmilk as it passes through homogenizer to the homogenization effect. When milkenters the homogenizer valve, velocity of milk changes from almost static to veryhigh velocity. As it comes out of the valve, there is sudden deceleration at a rateat which it was accelerated. This sudden change in velocity results in shatteringeffect leading to division of fat globules.

Cavitation: It is postulated that as the milk passes through the homogenizationvalve, the initial homogenization pressure decreases sharply due to sudden increasein the velocity of milk. Depending on the back pressure that exists outside thehomogenizer valve, the pressure can drop to as low as the saturated vapour pressureof liquid. This leads to formation of vapour bubbles due to cavitation. Cavitationgenerates shock waves, which could be in excess of 1600 kg/cm2 in intensity. Dueto overlapping of these shock waves, disintegration of the fat globules may occur.

iii. Advantages and Disadvantages of Homogenized Milk

Advantages

Prevents removal of fat/cream from milk

Homogenized milk results in softer curd and therefore easily digested by infants

Churning of fat does not occur during bulk transportation

Fat is uniformly distributed and therefore gives uniform consistency

Homogenized milk is comparatively resistant to development of oxidized flavourdefect

Disadvantages

Homogenization offers possibility of incorporation of foreign fat into milk

Homogenized milk is prone to development of ‘sunlight’ or ‘activated’ flavourdefect

Homogenized milk if returned unsold from the market is difficult to salvage ascentrifugal separation of fat is not possible

iv. Viscolised Milk

Viscolised milk refers to a product, which has unusually deep cream/fat layerresulting from admixing of homogenized cream, skim milk and/or whole milk. Thehomogenized fat forms very loose clumps with the unhomogenized fat globules andrise to the surface giving an appearance of deep cream layer. The unfair traderswho first separate the cream, homogenize it and then remix with the skim milksometimes practice it. This gives the remixed milk a very rich and creamy surfaceappearance and thus deceives the consumers.

v. Design and Operation of Homogenizers

There are several types of homogenizer valves and therefore designs of homogenizersvary depending on the manufacturers. However, many homogenizers used in thedairy industry have been developed based on the principles introduced by Gaulin.Homogenizers essentially consist of two components – a piston pump to generatehigh pressure and a homogenizing valve.

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The homogenizer pump is generally a positive displacement pump with at leastthree and sometimes five or seven pistons, which operate consecutively to generatesteady pressure. Single piston pumps generate pulsating output with fluctuatingpressure thereby resulting in poor homogenization. The pump block is generallymade of stainless steel but the piston seal rings are of a soft composite material.

Homogenizer valves, used for milk may be either a ‘poppet type’ or ‘ball type’. Apoppet design has relatively large contact surfaces and provides close fitting seal.If maintained properly, ‘poppet’ valves give better performance with low viscosityliquids like milk. ‘Ball’ valves can exert greater pressure on the much small sealarea and are therefore, suitable for high viscosity liquids or suspensions with smallerparticles.

Milk from high pressure manifold enters into the centre of the valve seat. Theinternal diameter of the valve seat is smaller than the manifold. As it passes intothe narrow gap between the fixed and the adjustable faces of this valve, milkvelocity gets accelerated. The gap is maintained against the feed pressure by acounter force exerted by an adjustable heavy duty spring. Shear effects result fromthe high velocity gradients between the liquid and the surface of the homogenizingvalve. Turbulence also results from the high velocity of the liquid in the valve,causing eddy currents within the flow. Liquid which passes across the valve atabout 200-300 m s-1 suddenly drops in pressure to below saturation vapour pressure.This permits microscopic bubbles to form for a few microseconds before collapsing.The high velocity jet of milk then impinges on a perpendicular impact ring. Theseeffects contribute to the disruption of the fat globules. Homogenizer valves aremade of very tough corrosion resisting alloys such as stellite. Better resistance tocorrosion can be achieved by using tungsten carbide and ceramic valves, which areused by many manufacturers in modern homogenizers.

As the fat globules are subdivided into smaller globules, there is increased surfacearea of the newly homogenized fat globules. The original milk fat globule membrane(MFGM) material is not sufficient to cover this. Proteins, particularly casein micellesmigrate from serum phase to form new membrane material with the existing MFGM.This may result in sharing of the casein micelles and therefore some aggregationof fat globules could take place thus defeating the purpose of homogenization. Asecond stage homogenization therefore, becomes essential at reduced pressure(almost 20% of the first stage pressure (175 kg/cm2) or upto 35 kg/cm2). Thisenables aggregated fat globules to be disrupted for formation of stable emulsion offinely dispersed fat globules.

Homogenization

B – Milkout

A

Handle

A – Milk enters into the centre of the valve seatB – Milk outC – Heavy Duty Spring

Sectional view of single-stage Gaulin type homogenizing valve

C

Processing of Milk

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vi. High Pressure Homogenization Technology

With significant improvement in understanding of machine design, material strengthand fluid mechanical knowledge, homogenizers with higher pressure capabilitieshave been developed. Such high pressure homogenizers could be operating basedon two principles: (1) Conventional valve type homogenizer operating at a farhigher pressure; (ii) Micro-fluidization based on the principle of collisions betweenhigh speed liquid jets.

Conventional Valve Type High Pressure Homogenizers (HPH): Such highpressure homogenizers work on the principle of conventional ball-and-seat typehomogenizer valve. Highly abrasive resistant and durable components of high pressurehomogenizers are made from best quality stainless steel, high alloy compositionsand new ceramic materials. This allows these systems to operate at pressures upto2550 kg/cm2 or more. Besides the regular use in emulsion formation, these highpressure homogenizers find applications in inactivation of enzymes and bacteriophagesand also in destruction of micro-organisms. Destruction of bacterial cells by HPHis due to several physical phenomenon viz., pressure drop, cavitation, shearing,turbulance and collision. These systems can be therefore, used as a combinedprocess for pasteurization and homogenization.

Microfluidization Technology: The microfluidizer operates under a differentprinciple as in this case the liquid being processed is divided into micro streams thatare so projected that these collide with each other. The essential design featuresof micro-fluidizers include a double acting intensifier pump and an interactionchamber. The intensifier pump, which is either air-driven or electric-hydraulic driven,forces milk/product at high pressure through the interaction chamber. The interactionchamber has fixed-geometry micro-channels, which divides the product into streams.These streams, which accelerate to a very high velocity, are made to collide againsteach other. Shear and impact that occur lead to homogenization effect. Thesemicro-fluidizers are capable of generating pressures upto 2800kg/cm2. As in caseof conventional valve homogenizers, microfluidizers too bring about changes in thefat and protein fractions of milk thereby altering some physico-chemical propertiesof milk.

vii. Vacuum Homogenization

This innovation in the homogenization technology is based on the discrete pulseenergy input theory. In vacuum homogenization, energy is introduced discretely intothe liquid (milk) through powerful short-time impulses. The homogenizer unit, placedin the HTST pasteurization line has two condensing chambers. The chilled milk isfirst pumped to condenser-I where it is heated to 20oC and subsequently to 30oCin condenser-II. Milk then enters regeneration section of the pasteurizer where itis heated to 65oC. It is then delivered to 1st stage vacuum (homogenization) chamberthrough a special nozzle. As a result of flashing effect, bubbles are formed in themilk as it falls in the vacuum chamber maintained at 0.15 to 0.2 kg/cm2. Due tothe pressure changes taking place, the bubbles either show high frequency pulsationand release energy or collapse producing shockwave effect in the product. Thebubbles therefore burst into smaller units and the fat globules are divided intosmaller globules. As the milk enters 2nd stage homogenization chamber, furtherbreakdown of fat globules takes place. The outgoing milk passes through regenerationsection followed by chilling section to finally attain 5oC temperature. Besides thegenerally accepted homogenization effect, the other major advantages offered bythis vacuum homogenizer are deodourization, reduced acidity and partial suppressionof microbial activity. These systems also claim to be economical as it consumesalmost 2.5 times less power than the valve type high pressure homogenizer.

viii. Checking the Efficiency of Homogenization

The method recommended by the United States Public Health Service has been the

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most widely used for checking the homogenization efficiency. It is performed bysubjecting a specified volume (one quart) of milk to quiescent storage for 48 hoursand then testing the fat in upper 100 ml and the remainder of milk. For properlyhomogenized milk, the percent difference in both the top 100 ml and the remaindermilk should not be more than 10 per cent.


1. What is stock’s law and how it is related to creaming phenomenon?

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2. How would you define homogenized milk?

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3. How would you relate shearing or grinding action with homogenization?

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4. What is the theory of cavitation?

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5. List out the different theories of homogenization

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6. What is viscolised milk ?

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7. How would you describe the homogenizer pump?

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8. Name the types of homogenizer valves and explain their suitability for milkprocessing

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Homogenization

Processing of Milk

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9. What are the materials of construction of homogenizer pumps and valves?

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10. Why is a two stage homogenization often recommended for milk?

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11. Describe the special features and applications of conventional valve type highpressure homogenizers.

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12. Describe the working of a micro-fluidizer.

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13. Describe the working of a vacuum homogenization system.

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14. How would you check the homogenization efficiency?

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6.3 INFLUENCE OF PROCESS VARIABLES ON THEPROCESSING EFFICIENCY AND PRODUCTQUALITY

i. Factors Affecting Homogenization Efficiency

Type of Homogenizing Valves: Design of homogenizer valve affectshomogenization efficiency. Grooved valves require less homogenization pressure toattain same degree of homogenization pressures as compared to either simple valvewith flat seat or needle valve.

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Homogenization Pressure: The recommended pressure ranges for homogenizationof milk is 140-175 kg/cm2. If the homogenizer is in perfect working condition i.e.the homogenizer valves are not worn out and are well seated, a homogenizerpressure of 175 kg/cm2 should give good homogenization efficiency. Some modernvalves may, however, give satisfactory performance at lower homogenization pressureas well. Higher pressure of homogenization however does not improve the efficiencyany further.

Single or Two Stage Homogenization: Two stage homogenization is oftenrecommended because broken fat globules after first stage homogenization (175 kg/cm2) may have a tendency to agglomerate. In order to re-disperse them,homogenization at reduced pressure (35 kg/cm2) may be thus necessary in thesecond stage. A homogenization process of two or more stages does not howeveraffect the mean particle size of the fat globules in any significant way. Modernhomogenizer designs permit two stage homogenization with a single machine.

Effect of Fat Content in Milk: Homogenization becomes less effective withincreasing fat content. When high fat milk is homogenized, the newly created totalfat globule surface becomes so large that materials required to form new membranesfor all the fat globules is not sufficiently availably in the serum phase. Thus thenewly formed fat globules may have a tendency to agglomerate and rise to thesurface during storage.

Effect of Temperature of Homogenization: Milk can be homogenized over awide range of temperature provided the homogenization temperature is above themelting point of milk fat (32OC). However, a temperature in excess of 50OC is oftenrecommended which is necessary to inactive nature lipases. It lipase is not inactivated;it acts as a surface active agent and becomes incorporated into the newly formedmembranes thereby causing hydrolytic rancidity in the product. Raw milk is thereforenot to be homogenized. The recommended temperatures for attaining high degreeof homogenization (80-90%) are therefore between 60 and 70OC. Higherhomogenization temperatures are also recommended for high fat milk. This is sobecause at higher temperatures, less protein is adsorbed during the formation of anew fat globule membrane. Furthermore, the membranes are formed more rapidlyand thus the tendency of the fat globules to agglomerate is significantly reduced.

ii. Effect of Homogenization on Physico-Chemical Properties of Milk

Effect on Fat: Homogenized milk drains more freely out of the glass containerleaving less milk sticking to the sides. This lack of adhesion is attributed to thereduction in size of the fat globules and the protection provided to these globulesby the adsorption of higher proportion of casein. Homogenized milk with normal fatcontent does not have marked clustering of fat globules. This lack of clustering isattributed to:

destruction of natural agglutinin of milk during homogenization.

resurfacing of the fat globules.

increased brownian movement resulting from greatly increased number of fatglobules.

Proper homogenization however, does not cause any change in important fat constantsor physico-chemical properties.

Effect on Protein: The fat globule membrane is composed of approximately 1/3phospholipids and 2/3 protein. The membrane acts as an emulsifier to keep theemulsion stable. During homogenization, the original membrane is destroyed and thesurface active agents in the serum phase get adsorbed on the fat globules to forma new membrane. The new membrane consists mainly of casein as well as serumproteins. While only 2% casein in milk is adsorbed on the fat globules in un-

Homogenization

Processing of Milk

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homogenized milk, in homogenized milk almost 25% of casein is adsorbed as partof fat globule membrane. Homogenization is often associated with destabilization ofproteins. This destabilization effect is reflected in reduced alcohol stability, increasedfeathering of cream in coffee and in coagulation during the manufacture of evaporatedmilk. This destabilization effect is partly attributed to adsorption of citrates andphosphates on the newly formed fat globule membrane, which lowers theirconcentration in the serum phase thereby adversely affecting the protein stability.

Colour of Milk: Homogenization results in more uniform, opaque and whiter milkwhich make the product more acceptable to the consumers. The increased whiteningis due to the increase in number and total surface area of fat globules, which reflectand scatter more light.

Emulsion Stability: It is practically not possible to churn homogenized milk.However, with increasing fat content, the emulsion stability decreases.

Curd Tension: Homogenized milk has greater tendency to form coagulum andrequires less coagulating agent. The resultant coagulum has lower curd tension anda soft, spongy body. Homogenization at recommended pressure of 175 kg/cm2

causes the curd tension to be lowered by more than 50%. The possible reason forthis effect of homogenization on curd tension is attributed to the increase in thenumber of fat globules, which serve as the points of weakness in the coagulum.Further, nearly 25% of the casein get adsorbed on the fat globules during theformation of new fat globule membranes as against only 2% of the total caseinadsorbed on the surface of the fat globules in un-homogenized milk. This resultsin lower casein concentration in the serum phase thereby lowering the curd tension.Fat losses in the cheese whey are however low as the finely divided fat globulesare retained in the curd due to adsorption of casein micelles on their surface.

Viscosity: Single stage homogenization causes increase in viscosity. This is broughtabout by formation of fat clusters, which results from membranes of newly formedfat globules joining together although fat itself is not in contact. When the milk issubjected to second stage homogenization, the fat globule clusters are disintegrated/broken down resulting in decrease in viscosity. The degree of clustering of the fatglobules is directly proportional to the viscosity. A high fat content, a highhomogenization pressure and a low homogenization temperature can significantlyincrease the fat clustering and hence the viscosity of milk. Preheating of milk attemperatures that promote whey protein denaturation also reduces membraneformation and hence increases agglomeration of fat globules.

iii. Problems/ Defects Associated with Homogenized Milk

Curdling During Cooking/Sterilization: Homogenized milk is some times moresusceptible to curdling when it is used in certain food preparations requiring cooking.This is in part related to reduced protein stability of homogenized milk as also tothe seasoning salts added as an ingredient in the new food formulation.

Recovery of Fat During Centrifugal Separation: Milk fat is difficult to separatefrom the homogenized milk. If the milk has been homogenized at the generallyaccepted homogenization pressure of 175 kg/cm2, a significant portion (50-90%) ofthe fat remain in the skim milk after centrifugal separation. Even addition ofhomogenized milk with un-homogenized milk and then centrifugal separation doesnot yield a satisfactory result. The recovery of fat from homogenized milk is aserious problem for commercial dairies which receive significant quantities ofprocessed milk as ‘returns’ from the market and need to salvage fat for economicoperation of the plant.

Formation of Cream Plug: Appearance of scum or buttery particles on thesurface of the homogenized milk is objectionable. Sometimes, fat rising in

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homogenized milk is to such an extent that a compact ring of creamy material isvisible under the container closure often referred as cream plug. Several factorssuch as worn out or poorly maintained homogenizer valve, improper homogenizationpressure, excessive foaming, improper cleaning of processing lines and failure torecycle the first few liters of milk coming out of the homogenizer lead to suchdefects in the product.

Sedimentation: Appearance of sediments in homogenized milk upon storage couldbe a serious problem. This defect is often ascribed to settling of the extraneousmatters such as body cells and dirt as also to destabilization of proteins duringhomogenization. However, clarification of milk before homogenization reduces theamount of deposits significantly whereas clarification after homogenization preventsthis defect entirely.

Foaming: Though not a serious problem, excessive foaming in homogenized milkposes handling difficulties. The two possible reasons for this could be inclusion ofair as a result of splashing or excessive agitation of the homogenized milk orhomogenizing the air into the milk during processing. However, improving the handlingprocedure during homogenization can largely eliminate this problem.

Flavour Defects of Homogenized Milk: The most important flavour defectassociated with homogenized milk is ‘sunlight flavour’, sometimes referred as‘tallowiness”, ‘burnt like’ or ‘activated flavour’. This develops due to oxidation offree methionine and formation of free SH compounds from sulfur containing aminoacids. Development of ‘sunlight flavour’ also requires riboflavin. Probably, all ofthese compounds are together responsible for ‘sunlight flavour’. The possible reasonsfor sensitivity of homogenized milk for development of these flavour defects couldbe the effect of the light upon the increased protein surface following homogenization.Homogenized milk is however resistant to development of oxidized flavour defect.This could be attributed to the formation of new fat globule membranes resultingin ‘dilution’ of catalytic metals viz., copper and iron, which are concentrated inthe native MFGM, thereby minimizing direct contact between the fat and themetal ions.


1. Why does high fat content in milk affect homogenization efficiency adversely?

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2. Why raw milk is not homogenized?

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3. Why relatively high heating temperatures are recommended for homogenizationof high fat milk?

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4. Why is homogenized milk sometimes less stable?

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5. Why homogenized milk is whiter?

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6. Why homogenized milk forms softer curd?

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7. What factors are responsible for increase in viscosity of homogenized milk?

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8. How does fat recovery from homogenized milk affect economic efficiency ofliquid milk processing plants?

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9. What factors contribute to formation of cream plug in homogenized milk?

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10. Describe the most common flavour defect associated with homogenized milk.

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6.4 LET US SUM UP

Milk is an oil-in-water emulsion, in which fat globules are dispersed in a continuousskim milk phase. As milk fat has less density than the plasma phase, it has a naturaltendency to rise to the surface and form cream layer. To overcome this problem,milk is subjected to a mechanical treatment referred as homogenization. Duringhomogenization, milk heated to a temperature of 60-70oC is passed through a tinyorifice under very high pressure such that as a result of shearing, turbulence,cavitation and impact there is decrease in the diameter and increase in the number

49

and surface area of the fat globules. Homogenizer essentially consists of two majorcomponents: a piston pump and the homogenizer valve. New generationhomogenization technologies such as microfluidization, however work based on adifferent principle involving collision of thin streams of liquid. Microfluidizers arecapable of operating at very high pressures and serve more functions than merehomogenization. Homogenization efficiency is determined by several factors includingthe valve design, the homogenization pressure, single or double stages ofhomogenization and the temperature of homogenization. Homogenization affectsseveral physico-chemical properties of milk including the plasma protein, the fatglobule membrane composition, colour, curd tension and viscosity. One of the majorproblems associated with the homogenized milk is it’s susceptibility to developmentof ‘sunlight’ or ‘activated’ flavour defect.

6.5 KEY WORDS

Alcohol stability : It refers to stability of milk to definiteconcentrations of alcohol and is a measure ofcolloidal stability of milk to heat. The alcoholtest is used as the initial classification of milk.

Brownian motion : It is zigzag, irregular motion exhibited by minuteparticles of matter when suspended in a fluid.It is named after the botanist Robert Brownwho observed (1827) the movement of plantspores floating in water. The effect, beingindependent of all external factors, is ascribedto the thermal motion of the molecules of thefluid. Brownian motion is observed for particlesabout 0.001 mm in diameter.

Cavitation : Cavitation is the formation of pockets of vaporin a liquid. This process is caused by lowpressures in the liquid. When the local ambientpressure at a point in the liquid falls below theliquid’s vapor pressure, the liquid undergoes aphase change to a gas, creating “bubbles,” or,more accurately, cavities, in the liquid.

Emulsion : It is a colloid in which both phases are liquids.Milk is an “oil-in-water emulsion”.

Feathering of cream : It refers to coagulation of cream forming smallflakes which is a body and texture defect offluid dairy products

Homogenizer Valve : It is the heart of homogenizer assembly. Itessentially comprises of a narrow openingthrough which the thin flow of milk passes.This creates conditions of high turbulence andshear, combined with compression, acceleration,pressure drop, and impact thereby resulting indisintegration of particles and dispersionthroughout the product.

Lipases : They are the group of enzymes that catalyzethe hydrolysis of fats into glycerol and fattyacids. Lipases are naturally present in milk andheat resistant lipases are liberated bypsychrotrophs.

Phospholipids : A class of molecules containing a polar headgroup that contain phosphorus atom and two

Homogenization

Processing of Milk

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non-polar hydrocarbon chains. There are manyphospholipids due to the various possible typesof head groups and hydrocarbon chains ofdifferent lengths. The major lipids in milk fatare trigylcerides, which are composed of threefatty acids covalently bound to a glycerolmolecule by ester bonds. The remainder of thelipid fraction (~2% of the total) is phospholipids,diglycerides and cholesterol. Milk fat globulemembrane is rich in phospholipids

Suspension : It is a mixture of two substances, one of whichis finely divided and dispersed in the other. Asuspension is different from a colloid or solution.Particles in a suspension are larger than thosein colloids or solutions; they are visible undera microscope, and some can be seen with thenaked eye. Particles in a suspension precipitateif the suspension is allowed to standundisturbed.

Viscosity : It is resistance of a fluid to flow. This resistanceacts against the motion of any solid objectthrough the fluid and also against motion of thefluid itself past stationary obstacles. Viscosityalso acts internally on the fluid between slowerand faster moving adjacent layers. All fluids,i.e., all liquids and gases, exhibit viscosity tosome degree.


Trout M. (1950) Homogenized Milk: A Review and Guide, Michigan State CollegePress, East Lansing, USA

Walstra P, Geurts T.J., Noomen A, Jellema A, Bookel M. A. J. S. van (1999).Dairy Technology: Principles of Milk Properties and Processes, Publisher:Marcel Dekker, Inc., New York, USA.

Kessler, H. G. (1981) Food Engineering and Dairy Chemistry, Publisher: Verlag A,Friesing, Germany.


Your answer should include following points:


1) i. This equation provides basis for explaining rise of fat globules in the un-homogenized milk. As the density of milk fat is less than that of the skimmilk, depending on the diameter/size, the fat globules rise to the surfaceduring storage and form cream layer.

2) i. Homogenized milk is milk which has been treated in such manner as toensure break-up of the fat globules to such an extent that after 48 hoursof quiescent storage no visible cream separation occurs on the milk andthe fat percentage of the milk in the top 100 ml of milk in a quart bottle(946ml), or of the proportionate volume in containers of other sizes, doesnot differ by more than 10 per cent of itself from the fat percentage ofthe remaining milk as determined after thorough mixing.

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3) i. As milk passes through the homogenizing valve at very high pressure,high velocity gradients between the liquid and the surface of thehomogenizer valve cause the fat globules to undergo shearing action.There is also wire drawing effect causing elongation and subsequentdivision of the fat globules. The difference in velocity of the liquid streamsat the centre and towards the edge result in grinding action therebyleading to sub-division of fat globules.

4) i. Milk attains very high velocity as it enters the homogenizer valve. As itcomes out there is sudden pressure drop and the pressure falls below thevapour pressure of the continuous phase. This leads to formation of smallvapour bubbles in the milk due to the cavitation. As the pressure increasesagain, these vapour bubbles collapse and sets up shock waves. Due tooverlapping of these shock waves, fat globules disintegrate.

5) i. The various theories that explain the phenomenon of homogenizationprimarily are: shearing or grinding, exploding, splashing/shattering,acceleration and deceleration and cavitation.

6) i. Viscolised milk is obtained by admixing homogenized cream with skimmilk and/or whole milk. The homogenized fat forms very loose clumpswith the unhomogenized fat globules and rise to the surface. The unfairtraders use this practice for giving their milk a very rich and creamysurface appearance and thus deceive the consumers.

7) i. The homogenizer pump is a positive displacement type pump with at leastthree and sometimes five or seven pistons. These pistons are so arrangedthat they operate consecutively to maintain an uniform feed pressure.

8) i. Commonly used homogenizer valves, for milk may be either a poppettype or ball type. A poppet design has relatively large contact surfacesand provide close fitting seat. This is suitable for milk. Ball valves havea small contact area with the valve face and are particularly advantageousfor viscous liquids and also when small particulates are present in thefeed.

9) i. The pump block is generally made of stainless steel but the piston sealrings are of a soft composite material. Homogenizer valves are made ofvery tough corrosion resisting alloys such as stellite. Modern homogenizersalso use valves machined from more corrosion resistant materials liketungsten carbide or ceramics.

10) i. During the first stage homogenization, new fat globule membranes areformed. Proteins, particularly casein from the serum phase are utilizedfor the purpose. Sharing of the casein micelles in the newly createdmembranes cause fat globules to form large aggregates, which have atendency to rise to the surface. A second stage homogenization at reducedpressure (20% of the first stage) disrupts these larger aggregates andforms stable emulsion.

11) i. Conventional ball-and-seat type valves in high pressure homogenizers aremade of high quality steel, special alloy or a range of ceramic materialsso that it can withstand high operating pressures which sometimes exceeds2550 kg/cm2. Besides in the formation of emulsions, these homogenizerscould also have applications in inactivation of enzymes, bacteriophagesand destruction of microorganisms. Therefore, much better processingsolutions could be sought for liquid milk industry by high pressurehomogenizers.

12) i. Microfluidizer is a high pressure homogenizer with a different workingprinciple. The two principal component of this homogenizer is (a) doubleacting intensifier pump (b) interaction chamber. The interaction chamberhas micro channels, which subdivide the liquid into very fine streams.These liquid streams at high velocity are made to collide with each other

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so that the shear and impact that is created results in homogenizationeffect. The micro-fluidizers can also operate at very high operatingpressures (upto 2800 kg/cm2).

13) i. The working of a vacuum homogenizer is based on the discrete pulseenergy input theory. In vacuum homogenization, milk is delivered into thevacuum (homogenization) chamber through a special nozzle. Due toflashing effect, bubbles are formed in the milk. As a result of the pressurechange, the bubbles either show high frequency pulsation and releaseenergy or collapse producing shockwave effect in the product. The bubblestherefore burst into smaller units and the fat globules are divided intosmaller globules.

14) i. A specified volume (one quart) of milk is subjected to quiescent storagefor 48 hours and then tested for the fat content in upper 100 ml and theremainder of milk. For properly homogenized milk, the percent differencein fat content of both the top 100 ml and the remainder milk should notbe more than 10 per cent.


1) i. When high fat milk is homogenized, surface area of newly created fatglobules becomes so high that there is not enough proteins available inthe serum phase for formation of new fat globules membranes. The fatglobules therefore form agglomerates and tend to rise to the surface.

2) i. If raw milk is homogenized, the natural lipases present in milk act assurface active agents and become a part of the newly formed membranes.During storage, they hydrolyse fat and cause rancidity in the product.

3) i. At higher homogenization temperature, less of protein from the serumphase is adsorbed during the formation of new fat globule membranes.Also the membranes are formed more rapidly. Therefore, the fat globulesremain uniformly dispersed and do not agglomerate.

4) i. When milk is homogenized, original fat globule membranes are destroyedand new membranes are created. This results in nearly 25% of thecasein from serum phase getting adsorbed as part of the new membranes,besides the citrates and phosphates. This lowers their concentration inthe serum phase and adversely affects the protein stability.

5) i. Homogenization results in increase in the number and surface area of fatglobules, which reflect more light. Milk after homogenization thereforebecomes whiter

6) i. Homogenization causes large increase in the number of fat globules,which serve as points of weakness in the coagulum when curd is formed.Furthermore, almost 25% of the casein from the serum phase is used upin the formation of new fat globule membranes. This results in lowercasein concentration in the serum thereby lowering the curd tension.

7) i. The factors responsible for increase in the viscosity of homogenized milkare: high fat content in milk, single stage homogenization, a highhomogenization pressure, a low homogenization temperature and pre-heating of milk at temperatures, which promote whey protein denaturation,agglomeration of fat and therefore viscosity of milk.

8) i. As centrifugal separation of fat from the homogenized milk is very difficult,utilization of processed milk as returns from the market, which is sizeableat times in many dairies is economically challenging.

9) i. The factors responsible for cream plug formation in homogenized milkare: worn out/damaged homogenizer valve, improper homogenizationpressure, excessive foaming, improper cleaning of processing lines etc.

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10) i. The most common flavour defect associated, with homogenized milk issunlight flavour also referred as ‘tallowy’, ‘burnt like’ or ‘activated flavour’.The mechanism for development of this flavour defect involves oxidationof free methionine and formation of free SH compounds from sulfurcontaining amino acids. Development of ‘sunlight flavour’ also requiresriboflavin. The effect of sun light upon the increased protein surfacefollowing homogenization increases the sensitivity of homogenized milk todevelopment of these flavour defects.


1. Explain the working principle of a valve type homogenizer.

2. Compare the valve-and-seat homogenizer with a microfluidizer.

3. Explain various factors affecting homogenization efficiency.

4. Explain various advantages and disadvantages of homogenized milk.

5. Explain why homogenized milk is resistant to development of oxidized flavourdefect.

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UNIT 7 STERILIZATION AND ULTRA-HIGH-TEMPERATUREPROCESSING

Structure

7.0 Objectives

7.1 Introduction

7.2 Sterilization

Definition

Theoretical Basis

Types of Sterilization Plants

Description of the Canning Process

Quality of Sterilized Milk

7.3 Ultra High Temperature Processing

Definition

Theoretical Basis for UHT Processing

Types of UHT Sterilization Plants

Changes in Milk during Processing

Changes in Milk during Storage

7.4 Aseptic Packaging

Types of Sterilizing Medium

Types of Packaging Materials

Description of Aseptic Packaging Systems

7.5 Let Us Sum Up

7.6 Key Words



7.9 Some Questions to Check Your Progress

7.0 OBJECTIVES


define sterilization.

describe the theoretical basis for conventional sterilization and UHT processing.

differentiate between in-package sterilization and UHT processing

state the different types of sterilization systems and how they compare againsteach other.

explain the changes in properties of milk that occur during sterilization andstorage

define aseptic packaging enumerate different packaging materials and sterilizingmediums available for aseptic packaging.

7.1 INTRODUCTION

We know milk is a highly perishable commodity. Its myriad nutrients makes itextremely favourable medium for the growth of microorganisms. It is, therefore,essential that milk is subjected to certain processing treatments for enhancing itskeeping quality and ensuring safety to consumers.

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Thermal processing is the most prevalent preservation process employed in thedairy and food industry. Starting from pasteurization, which is a mild heat processingtechnology, in-bottle/in-package sterilization emerged as a means of extending shelflife of milk for several weeks at room temperature. Considerable changes in nutritionaland sensory quality due to severity of heat treatment in this process, restrict itsapplication to only special milks. Ultra-high-temperature processing, a relativelynew processing know-how, became popular as it uses very high temperature (140OC)for short time (2 s) to sterilize milk. Such a time-temperature combination ensuresminimal change in the product quality. Sterilized milk is then packaged in sterilecontainer under aseptic conditions to prevent post-processing contamination. Theproduct thus obtained has very long storage life.

7.2 STERILIZATION

i. Definition

Sterilized milk refers to a product obtained by heating milk in a container in acommercial cooker/ retort to temperatures of 110-130OC for 10-30 min. The processis also referred as in-container sterilization. Sterilized milk is generally intended forprolonged storage at room temperature (up to 6 months). The major objective ofheat sterilization is to destroy microbial and enzymatic activity. The length of timeand magnitude of temperature employed during processing depend on the type ofthe product, number and heat resistance of microorganisms and enzymes presentin milk. The heat resistance of microorganisms or enzymes is generally evaluatedin terms of D-value or Z-value. Sterilization load or heat load for sterilization isgenerally expressed in terms of F

ovalue.

ii. Theoretical Basis

Clostridium botulinum is considered as the index organism for assessing thermalsterility in foods. Under anaerobic conditions, inside a sealed container, it canproduce botulin, a toxin, which can be 65% fatal to humans. Therefore, destructionof this organism is a minimum requirement of heat sterilization. As milk is a lowacid (pH>4.5) food, it is recommended to achieve 12 decimal reductions for C.botulinum. This can be achieved by heating the product at 121OC for 3 min(F

o= 3). However, this minimum treatment may produce milk that is safe but not

necessarily commercially sterile. This is so because there are more heat-resistantspores present in milk. There is B. stearothermophilus or B. sporothermodurans.These spores are not pathogenic. Their presence may require heat treatment equivalentto two (2) or more decimal reductions. This may correspond to an F

0value of 8.

Target spoilage rates should be less than one survivor in every 10,000 containers.

iii. Types of Sterilization Plants

Sterilizing retorts are either batch type or continuous in operation. Batch typesterilizers may be either vertical or horizontal. Horizontal retorts are easier to loador unload. They have facilities for agitating containers/cages. However, they requiremore floor space. Typically such horizontal retorts contain concentric cages. Cansare loaded horizontally into the annular space between the cages. When cages arefull, the retort is sealed. The cages are supported by guide rails, which slowly rotatethem. This stirring of the contents in cans facilitate proper heating. Continuousretorts are generally equipped with better controls. They cause very gradual changein pressure inside the cans. Thus products are heated more uniformly. Can seamsare also subjected to less strain in comparison to batch process.

Continuous sterilizers: They are mainly of three types: (a) cooker-coolers; (b)hydrostatic sterilizers; and (c) rotary sterilizers. Cooker-coolers carry cans on aconveyor which pass through three sections of a tunnel. These sections aremaintained at different pressures for preheating, sterilization and cooling. The

Sterilization and Ultra-High-Temperature

Processing

Processing of Milk

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hydrostatic sterilizer consists of a chamber equipped with provision for steaminjection. The chamber that is partially full of water is connected to two watercolumns (12 to 18 meter tall, barometric leg) which are used to adjust pressure inthe chamber. If the height of the water columns is changed, the steam pressure ischanged and therefore the maximum attainable temperature changes. For example,to get a temperature of 116oC, a difference in height between the two watercolumns should be 10.7 m while for attaining 121oC temperature in the chamber,the water column difference should be 13.7 m. A conveyor with provision toaccommodate cans of different sizes moves through the steam chamber carryingthe food cans. The heating time could be regulated by varying the speed of theconveyor. Hydrostatic sterilizers are very flexible and suitable for large capacityplants. However, size of the structure and high capital costs are the majordisadvantages of this system.

Continuous rotary sterilizer consists of several horizontal inter linked cylinders whichallow for preheating, heating, precooling and cooling in upto four continuous stages.The vessel has a spiral track on the inner wall. A spoke or reel within the centreof the cooker causes the cans to roll along the spiral track. Rotary valves used tointerconnect the shells, maintain pressure in the heating and cooling sections. Sealedcans are introduced directly from the sealing machines. The contents inside thecans are mixed as cans travel along the helix and therefore enhance heat transferand ensure less heat damage to the product. Cans coming out of the cooker aredirectly taken to labelling and palletizing machine. Rotary sterilizers are particularlysuitable for processing of milk and milk based products, which are extremely heatsensitive and susceptible to browning.

iv. Description of the Canning Process

Basic operations in conventional retorting/canning process include: preparation ofthe raw material, filling of the container, exhausting, sealing of container, sterilization,cooling of the cans, labelling and storage.

The preparation of raw materials refers to washing, peeling, cutting, blanching, pre-cooking, etc. in case of fruits, vegetables, meat, etc; and preheating, mixing,homogenization, etc; in case of milk. Filling of containers can be carried out eithermanually or mechanically. Correct and accurate filling is important from economicstandpoint as well as for prevention of entrapment of large volume of air/ gas insidethe can, which might decrease the intensity of heat treatment. Exhausting is anessential operation in the canning process and involves removal of air/ oxygen fromthe container before it is closed. Removal of air ensures minimum of strain on thecan seams or pouch seals through expansion of air during heat processing. Removalof oxygen is essential to prevent internal corrosion of the container through oxidationand creation of vacuum inside the container while cooling. Absence of oxygeninside the container also delays oxidative deterioration of the product besidesdestruction of ascorbic acid.

After exhaustion, containers are sealed. Depending on the type of containers (metalcans, glass bottles, flexible pouches); sealing machines are chosen. Glass jars arenormally vacuum-sealed while tins are closed with a double sealing on the seal sideand may also be vacuum-sealed. Flexible retortable pouches are sealed by fusionof two thermoplastic materials through application of heat by heated plates or jaws.

Product in the closed containers is heated in the sterilizer in an atmosphere ofsaturated steam or hot water or air-steam mixture. The sterilizing action of steamdepends on its latent heat of vaporization as it condenses on the surface of the can.Saturated steam condenses readily and is therefore an efficient sterilizing medium.Displacement of all air present in the retort by steam before the sterilizer is broughtto operating temperature is a very essential step. This is also known as venting.The purpose of this processing step is to maintain uniform steam-air mixture in the

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sterilizer and prevent under processing. Sterilization temperature – time combinationin retorts may vary from 110 – 130OC for 10-30 min. Sterilized containers are thencooled and brought to room temperature for labelling and storage. Turbidity testdeveloped by Aschaffenburg is conducted to ensure sterility of the product. Thisis an indirect test and it measures denatured whey proteins. Complete denaturationindicates that the milk is adequately sterilized.

v. Quality of Sterilized Milk

Sterilized milk has a rich creamy appearance and a distinct cooked flavour (rich,nutty, caramelized). It is considerably browner in colour than raw milk. The browncolour develops due to formation of coloured pigments resulting from interactionsbetween free amino groups of proteins and aldehyde group of lactose throughMaillard reactions. The intensity of cooked flavour and brown colour depends uponthe severity of heat treatment. In-container sterilization causes loss of nearly halfof the ascorbic acid (Vitamin C) and sizeable loss of thiamine (30-40%). VitaminB

12is almost completely destroyed. Fat soluble vitamin A, carotene, riboflavin and

nicotinic acid are not affected. Biological value of proteins is only marginally affected.Sterilized milk cannot be coagulated with rennet unless calcium chloride is addedexternally.


1. Define sterilized milk.

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2. Why Clostridium botulinum is so important in sterilization?

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3. How would you obtain commercially sterile milk?

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4. How does a batch type horizontal retort operate?

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5. What are the different types of continuous sterilizers? Why these are preferredover batch type system?

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6. How would you change the processing temperature in a hydrostatic sterilizer?

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58

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7. Why rotary sterilizers are suitable for processing of milk and milk products?

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8. Why exhausting is an essential step in canning process?

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9. What is the purpose of venting?

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10. What is Aschaffenburg test?

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11. What is the most undesirable physical change in milk after conventionalsterilization?

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7.3 UHT PROCESSING

i. Definition

UHT milk can be defined as a product obtained by heating milk in a continuousflow to a temperature in excess of 125ºC for not less than two seconds andimmediately packaging in sterile packages under aseptic conditions. In India, UHTmilk is generally processed at 140oC for 2 seconds.

ii. Theoretical Basis

Heating of milk results in death of microorganisms. While some bacteria aredestroyed by pasteurization (71.7OC/15 s) only, some survive this thermal treatment.Bacillus subtilis and Bacillus stearothermophillus spores are very heat resistant.Of the two, Bacillus stearothermophillus spores are most heat resistant. It istherefore, considered index organism for evaluating performance of UHT processing.

Heating of milk at higher temperatures also result in undesirable changes in chemicalquality. Browning reactions are particularly important. Higher thermal load results

59

in more browning and therefore loss of flavour and quality. In the temperaturerange of 100-120oC, time required for death of almost all B. stearothermophillusspores are more. This may therefore result in more browning in the product.However, if milk is treated in the UHT range i.e. 135-150oC for only few seconds,almost all spores may get killed and browning would be minimum. Loss of nutrientsand total quality also will be minimum. A product processed in this temperaturerange will be thus microbiologically safe and yet superior in terms of overall quality.

iii. Types of Sterilization Plants

There are two types of UHT plants: Direct type and Indirect type. In direct typeplants, heating is done by mixing product and steam. In indirect type plant, productis heated by steam or hot water without the two coming in direct contact. Heatingin direct type plant is very rapid particularly between 80-140oC and total heat loadis less. Changes in the product quality are therefore minimum. In indirect plant, risein temperature is very gradual. Therefore, heat load on the product is more. Changesin chemical quality are comparatively more in indirect type than in direct type plants.

(i) Direct Heating Plant: There are two types of direct heating plants (a) Injectiontype and (b) Infusion type.

Injection type: Processing is through steam-into-milk arrangement. Steam injectoris the heart of this plant. Preheated milk at 80-90OC enters the injector nozzles fromone side. Steam at slightly higher pressure enters the injector from the other side.As the steam mixes with milk, steam condenses and the product is rapidly heated.Rapid condensation of steam prevents entry of air in holding tube. Air in holdingtubes results in improper heating. Backpressure is maintained on the discharge side.Backpressure ensures that product does not boil in holding tube. Boiling may resultin fouling and improper heating of milk. Several designs of injector are available.

Infusion type: In this system, milk is heated by milk-into-steam arrangement. Theprocessing unit consists of a chamber filled with pressurized steam. Milk enters thechamber from the top. There are two alternative arrangements for distribution ofmilk. In the first type, milk flows to a hemispherical bowl with loose circular discclosing the top. When the bowl is full, milk overflows and falls in droplets throughthe steam environment. In an alternative arrangement, milk flows through a seriesof parallel and horizontal distribution tubes. These tubes have slits along the bottomand milk flows like a thin film through the chamber. As milk reaches the bottomof the chamber, it is heated to desired temperature. This system is particularlysuitable for thicker liquids and for liquids suspended with smaller chunks.

Advantages and Disadvantages of Direct Heating System: During processingin direct type heating systems condensing of steam coming into product contactresults in dilution of the product. To remove this excess of water from the product,cooling is done in an expansion cooling vessel. In expansion vessel, along with theevaporating water incondensable gases and undesirable flavour volatiles producedduring heating are also removed. The product therefore tastes better. Steam injectioninduces formation of casein aggregates, which give “chalky” or “astringent”mouthfeel to the product. Aseptic homogenizer, which can safely homogenize theproduct after final heating section, is generally preferred with direct heating systemsto overcome such defects in the product.

Rate of heating is very high (takes less than 1 sec to attain sterilization temperature).Thick/viscous liquid can also be easily processed. Deposit formation is minimum,hence plant can be operated for longer time without cleaning. Undesirable flavoursare removed during flash cooling. Oxygen is removed during cooling, hence oxidizedflavour defects are delayed during storage.

Cost of processing per unit volume of milk is high. Requires additional equipment


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60

(vacuum expansion chamber and aseptic homogenizer) – cost of plant is twice thatof indirect type plant. Heat energy requirement is very high. Water and electricity(25-50% more than in direct type) consumption are high. Requires culinary steamand hence special boiler. Creates greater noise during operation.

(ii) Indirect Type Heating System: There are three types of indirect heatingsystems: (a) Plate heat exchangers (b) Tubular heat exchanger (c) Scraped surfaceheat exchanger.

Plate heat exchanger: This resembles plate heat exchanger of HTST plants.Several rectangular stainless steel plates with corrugations are arranged in sequence.These plates are then mechanically tightened to hold together. Corrugations on theplates induce turbulence and therefore result in high heat transfer. High temperatureprocessing generates high internal pressure. The gaskets are therefore made ofheat resistant materials such as medium nitrile rubber or resin cured butylrubber. A major advantage of this plant is therefore simple design and comparativelyless cost. If deposit formation is more, plates can be removed and manually cleaned.

Tubular heat exchanger: There are two types of tubular heat exchangers – (a)concentric tube, (b) shell and tube type. Concentric tube type heat exchangerscomprise two or three stainless steel tube lengths put one inside another. Spaceris placed in each inner tube space to maintain them concentric. Several suchmultiple tubes are bound together and placed into an outer cylindrical housing. Twotube heat exchangers are used for simple cooling and heating. In triple tube heatexchanger, available heat transfer area is doubled. It is generally used in finalcooling section. It is also suitable for processing of thick liquids, which generallyreduces heat transfer rate. Product flows through the middle annular space. Heatingor cooling medium passes through inner tube and outer annular space. In shell andtube type heat exchangers, 5-7 straight lengths of smaller tubes (10-15 mm internaldiameter) are assembled in an outer tube. The smaller tubes are connected to largeouter tube at both ends by a manifold. Product passes through the smaller tubes.Heating or cooling medium passes through the space around them in a countercurrent flow. Tubular heat exchangers are mechanically very strong and canwithstand even very high internal pressure generated during homogenization (200-300 bar). Therefore the need for acquiring an aseptic homogenizer to be placedafter heating section is totally eliminated. Instead, the high pressure reciprocatingpump of an ordinary homogenizer can be placed before the sterile section. Thehomogenizing valve can be put at any point on the downstream side (even afterfinal heating section). The problem of product contamination arises from thehomogenization pump and not the valve. Therefore, with tubular heat exchangers,the product can be homogenized before sterilization, after sterilization or on both theoccasions. Fat rich products like cream require homogenization after final heatingto prevent re-association of fat globules due to high temperature processing afterhomogenization.

Scraped Surface Heat Exchanger (SSWE): It is a very specialized type of heatexchanger. It consists of a jacketed cylinder. A shaft passes along the axis of thecylinder. The shaft is supported by bearings at both ends of the cylinder. The shaftalso carries several scrapper blades. As shaft rotates, scrapper blades provideturbulence and physically remove the product from the surface of the wall. Thecolder product subsequently replaces the heated product and the cycle continues.SSHE is used only for heating very thick liquids. SSHE units are very expensiveand have poor energy conversion efficiency. The cost of processing is thereforevery high.

Advantages and Disadvantages of Indirect Heating System: It is simple indesign and requires less pumps and controls. It can regenerate 90% of the thermalenergy requirement. It does not require aseptic homogenizer, which is very costly.

61

It does not require culinary steam and therefore special type of boiler. The indirecttype plant is less noisy. It requires low initial capital and operational cost is alsocomparatively less.

In indirect type heat exchanger, rate of heat transfer is low. More heat load resultsin less acceptable product quality. Deposit formation is more and therefore plantrequires frequent cleaning. For removal of dissolved oxygen from milk, additionalequipment ‘deaerator’ is required.

iv. Changes in Milk during Processing

UHT processing does not cause reduction in biological value of proteins. There isonly small loss of available lysine (6-7%). UHT processing changes the caseinmicelle structure. This slows rennet action during cheese manufacture. Serumproteins are denatured (direct processing – upto 50-75%, indirect processing upto70-90%). Denatured serum proteins interact with casein and increase casein micellesize. This reflects more light and UHT milk appears whiter. Aggregates of denaturedserum proteins and casein also give ‘chalky’ mouth feel to the product.

There is no physical or chemical change in milk fat. The total mineral content alsodoes not change during UHT processing. The vitamin content of UHT milk iscomparable to pasteurized milk. Losses in B-complex vitamins are not more than10%. Folic acid and ascorbic acid are destroyed up to 15% and 25%, respectively.Fat soluble vitamins A, D, E and K are not affected by UHT processing. FreshUHT milk has slightly cooked flavour. The cooked flavour is due to oxidotion of theSH (Sulphydryl) groups from the denatured serum proteins.

v. Changes in UHT Milk during Storage

Chemical, physical or sensory changes in stored UHT milk are dependent onstorage temperature. Changes are rapid if storage temperature exceeds 30OC.Browning reactions between protein and lactose progress during storage. At higherstorage temperature (>30OC) UHT milk may become little brown after 3-4 months.Refrigerated storage of raw milk before UHT processing favours growth ofpsychrotrophs. They liberate heat resistant proteases and lipases. Proteases thatsurvive UHT treatment act on proteins during storage. Bitter peptides are releasedcausing bitterness in the product. Extensive proteolysis and other physico-chemicalchanges occurring as a result of interaction of proteins and salts during storage maycause thickening or sweet curdling also referred as age thickening after longerstorage (more than 6 months). Lipases surviving ultra-high-temperature treatmentact on lipid fraction. Short and medium chain free fatty acids are released. Shortchain fatty acids particularly butyric acids contribute to development of rancidflavour in the product. Air in the product or in the packet reacts with unsaturatedfatty acids. This auto oxidation reaction causes formation of aldehydes and ketones.These compounds cause oxidative rancidity (flavour defect) in the product. Thecooked flavour in UHT milk disappears in first few days and milk tastes best afterthis period. Few weeks after this, depending on the temperature of storage, oxidizedflavour defects appear which becomes more pronounced with progressive storage.In milk stored for considerable period of time, which could be 3-4 months at >30OC,stale flavour is a common defect. Several compounds that form during the progressof Maillard reactions in stored milk are associated with the appearance of thisdefect. Sometimes coconut like flavour defect also appears in UHT milk stored forlonger period. Compounds such as d–dodelactone and d–dodecalactones areresponsible for this.


1. Describe UHT milk.

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62

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2. Why heating of milk above 130OC is desirable?

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3. Why UHT processing is recommended for liquid milk marketing in India?

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4. Why quality of milk processed in direct type UHT plant is better?

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5. Why direct type UHT systems are commercially less successful?

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6. What types of special gaskets are required in plate heat exchanger of indirectheating systems and why?

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7. List the major reasons for commercial success of tubular heat exchanger inUHT plant.

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8. When would you like to use SSHE?

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9. Why UHT milk is whiter?

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10. What is age thickening and how it occurs?

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11. How rancid and oxidized flavour defects develop in UHT milk?

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7.4 ASEPTIC PACKAGING

Aseptic packaging can be defined as the process in which UHT processed orsterilized milk is filled in pre-sterilized containers under aseptic/sterile environment.This ensures that there is no post processing contamination of the milk so that theproduct has longer shelf life. Since aseptic packaging systems are complex, greatcare is needed to prevent contamination. Before the start of product packaging, trialruns are routinely conducted with sterile water. Critical parts of the filling machineand carton forming systems are thoroughly checked. The seal integrity of thepackage and overall microbial quality of the packaging material are monitoredproperly. Generally, for a good processing plant permissible spoilage rate is one inevery 5000 sterilized, filled and sealed package of one litre carton.

i. Types of Sterilizing Medium

Sterilizing mediums to be used in aseptic packaging systems could be broadlyclassified under two categories: physical sterilization mediums and chemicalsterilization mediums.

Physical sterilization mediums: Steam under pressure or hot water is the mostsimple and reliable sterilant for high sterilization efficiency in short time. In asepticpackaging, its use is however restricted to sterilization of the milk tubes and valveand fittings coming into product contact.

Dry heat/ super heated steam: Hot air is generally used to sterilize the closedspace where the filling of milk takes place. Air heated to 300OC may be taken tothe areas surrounding electric resistances used for sealing the packages. Dry airat 330-350OC is also used for sterilizing the milk filling tubes. Sterilized air (180-200OC) is used for evaporating residual H

2O

2(chemical sterilant) from the package.

Ultraviolet radiation: UV rays (optimum wavelength 250 nm) alone are not avery effective sterilizing medium for aseptic packaging units. Two major reasonsfor this are: (i) intensity of radiation is not uniform on the entire package surface(ii) bacteria adhering to packages could be protected by dirt/ dust particles presenton the surface. UV radiations are therefore used as a complementary sterilizing medium.

Ionizing radiations: Gamma rays are often used for sterilizing packaging materialsunable to withstand high temperature. Usually 2.5 Mrad intensity is suitable forsterilizing plastic laminates used in aseptic bag-in-box.

ii. Chemical Sterilization Mediums

Ethylene oxide: Ethylene oxide has slow sporicidal action. It is sometimes usedas a pre-sterilization agent to reduce microbial load on packaging films so that ashorter time is required for final sterilization.


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Hydrogen Peroxide (H2O

2): H

2O

2has poor sporicidal effect at room temperature.

However, with increasing application temperature and concentration, sterilizationperformance improves. H

2O

2is the most popular sterilant for aseptic packaging

system. H2O

2is applied on the package surface by either dipping or spraying. As

its boiling temperature is slightly above 100OC, supply of heat by either sterilizedhot air or infrared elements can evaporate the residual H

2O

2from the package

surface. Thus there is little H2O

2left for contaminating the product. Safety regulation

recommended by IDF requires that atmospheric concentration of H2O

2in the

packaging hall should not exceed 1 ppm. Further more, residual concentration inmilk immediately after filling should not exceed 100 ppb and should reduce to 1 ppbwithin 24 hours. The most successful combination of sterilizing medium being usedin commercial aseptic packaging units are H

2O

2coupled with heat supplied by

radiant heating element. Some packaging systems also use a combination of H2O

2

and UV radiation.

Other sterilizing agents which are rarely used in such applications are sodiumhypochlorite and per acetic acid. These agents leave the residues of chloride andacetic acid on the package, which may finally contaminate the product.

ii. Type of Packaging Materials

Metal container: Cans made of tin plate or drawn aluminium are generally usedfor packaging of condensed milk, viscous liquids and chunk-in-gravy type of products.These are expensive and unsuitable for low cost products like liquid milk. They arebulky and require large storage and shipment space. The empty cans are carriedin a conveyor to a tunnel for sterilization with steam super heated with gas flameat atmospheric pressure and require about 40-45s. The cans then move to fillingchamber for product filling. The can lids are separately sterilized, placed on thecans and seams sealed. The can sterilizing, filling and sealing zones are sterilizedbefore the filling begins with the same mixture of superheated steam and flue gas,which fills them during operation. Cans have been used for in-package sterilizationfor a long time. Manufacturers of UHT milk who want to impress the consumerswith the advantages of the new technology therefore do not prefer to use canswhich are so identified with a old technology

Laminates/cartons: Different layers of flexible films of different materials viz.paper, polyethylene and aluminum foil are co-extruded to form a laminate. Thesematerials have specific properties viz. water vapour transmission, burst strength,etc; and hence when co-extruded form an ideal packaging film. Such laminatescould be 3, 4 or 5 ply and are generally used for products like, milk, cream, fruitjuices, soups, etc. These laminates are supplied as film rolls, which can be mountedon FFS (form-fill-seal) machines. Alternatively, cartons made of laminates aresupplied as preformed blanks, which are assembled into cartons for filling andsealing at the top.

Plastic films: Black and transparent polyethylene films are co-extruded forpackaging of UHT processed milk intended for 2-3 weeks shelf life. The co-extruded film protects the product against light but not oxygen. The packagingmachines also need to operate at not more than 45-50OC filling environment. Aco-extrusion of polyvinylidine chloride (PVDC) or ethylene vinyl alcohol (EVOH)with black or white polyethylene film is also used as packaging film. Such acombination imparts protection against oxygen as well as light and shelf life of milkcan be extended upto 3 months.

Other forms of packaging materials : Preformed packages of different shapesand sizes are also used for aseptic packaging of value added dairy products. Blow-moulded plastic bottles of polyethylene or polypropylene are used as cheap substitutes.However, these are transparent and permeable to oxygen. Multilayer materials withbetter light and oxygen barrier properties have also been developed. Pre-formed

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plastic cups of polypropylene (PP) or polystyrene (PS) are now gaining popularity.Bulk filling bags are made of laminates of 3 or 4 layers of which one will be barriermaterial such as metalized polyester (polyester with a coating of aluminium particles)or ethyl vinyl alcohol (EVOH). The bag with filling valve is sterilized by r-radiation(2.5 Mrad dose) before shipping. Bags remain sealed and internal surface thereforeremains sterile. At the filling station, the sterilized bags are opened, filled and sealedunder aseptic condition. All product contact surfaces in the filler however need tobe sterilized with steam before the filling operation begins.

iii. Description of the Packaging System

Most of the aseptic packaging machines being used in the country are of form-fill-seal (FFS) type. Packaging material used generally is laminate of polyethylene –paper – polyethylene – Al foil – polyethylene. Packaging film in the form of a rollis mounted on the packaging machine. The film moves continuously downward inthe form of a strip and a shaping roll gives it a cylindrical shape. Heat sealing formsan overlapping longitudinal seal. Simultaneously extra polythene strip is heat bondedalong inside of longitudinal seam. This is done to prevent filled product penetratingthe paper layer. As this continuous cylinder moves downward, jaws at the bottommake transverse heat seal. The product is filled instantly and another jaw seals thepackage at the top. Depending on the type of machine, different shapes can begiven to the package. The most popular is brick shaped package. Tetrahedronshapes were also being used some times back. Some new innovations that are nowbeing used for packaging of fruit juices are Fino packs. To cut down on costs somedairies have introduced pillow packs for packaging of milk.


1. What is aseptic packaging?

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2. Why UV rays alone are not effective medium for package sterilization?

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3. When should ionizing radiations be used in aseptic packaging?

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4. What are the regulatory requirements for H2O

2levels during aseptic packaging?

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5. Why cans are not suitable for packaging of UHT processed milk?

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6. What types of laminates are generally used for packaging of UHT milk?

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7. What are the cheaper films available for packaging of UHT milk?

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8. What materials bulk filling bags are made of?

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9. How seals are formed on the FFS type machine?

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7.5 LET US SUM UP

Liquid milk is either subjected to pasteurization or sterilization for preservation andconsumer safety. While pasteurization extends storage life of milk by a few daysat refrigerated temperature, sterilization offers a far longer life. Conventionalsterilization, which involves heating milk to 110-130oC for 10-30 min, is generallyemployed for the manufacture of special milks like flavoured or chocolate milk.Equipments available are both batch and continuous type. Of the available types ofsterilizers rotary sterilizer is preferred for processing of milk. However the technologyis not suitable for plain liquid milk processing as considerable changes in nutritionalquality takes place. Furthermore, milk becomes visibly brown due to higher intensityof Maillard reactions at conditions employed during in-package sterilization. Ultrahigh temperature (UHT) processing, a relatively new technology is a betteralternative for plain liquid milk processing. It requires milk to be heated to 140oCfor 2 sec. Both direct type and indirect type systems are commercially available.Tubular heat exchangers are generally preferred for its manifold advantages.Processed milk is subsequently packaged in sterile containers under asepticconditions. Form-fill-seal aseptic packaging machines, which are more popular useco extruded 3-5 ply laminates for packaging of liquid milk. UHT processing isslowly gaining popularity in our country and holds much promise for the future ofIndian dairy industry.

7.6 KEY WORDS

Blow molding : It is a manufacturing process by which hollowplastic or glass objects are formed. In generalthere are three types of blow moldingprocesses: extrusion, injection and stretch.

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Canning : It refers to a preservation method wherebyprocessed foods are put into metal cans orglass bottles, hermetically sealed to keep outair and then heated to specified temperaturefor definite time to destroy disease causingorganisms and prevent spoilage.

Caramalization : It is the oxidation of sugar; a process that isextensively used in food processing fordeveloping nutty flavours and brown colour. Itis therefore a type of non-enzymatic reaction.

Culinary steam : It is super heated water under pressure suitablefor direct food contact and require food gradeequipment, clean water and sanitary conditionfor its production

D-value : It is time in minutes required for one log or90% reduction of specific microbial populationunder specified lethal condition viz. constanttemperature.

Fo- Value : It is sterilization value. One minute at 121.1o C

or an equivalent amount of heat is defined asone unit of F

o. Other equivalent temperature-

time combinations are: 11.1oC/10 min or 101.1/100 min.

Homogenization : It refers to a process in which milk is subjectedto high shear at a temperature, which is abovethe melting point of the fat.

Milk fat, which varies from 1 to 10 micron insize, are broken down into smaller particlesand remain dispersed so that they don’t rise tothe top.

Latent heat : It is the quantity of heat absorbed or releasedby a subsance undergoing a change of statesuch as steam to water at constant temperatureand pressure.

Lipases : They are the group of enzymes that catalyzethe hydrolysis of fats into glycerol and fattyacids. Lipases are naturally present in milk andheat resistant lipases are liberated bypsychrotrophs.

Maillard : It is a chemical reaction between an aminoacid and a reducing sugar and requires additionof heat. The reactive carbonyl group of thesugar interacts with the free amino group ofthe amino acid and give rise to formation ofcompound, called melanoidins that impart browncolour.

Pasteurization : It is a processing treatment named after theinventor Luis Pasteur that requires milk to beheated to either 63oC/30 min or 71.7O/15 secso that all pathogenic organisms are destroyed.

Minimum time and temperature conditions arebased on the requirements for the destructionof the most heat resistant pathogenicmicroorganism present in milk i.e. Coxelliae


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burnettii and Mycobacterium tuberculosis.

Proteases : Any of the various enzymes that catalyzehydrolytic breakdown of proteins into peptidesor amino acids. Proteases are naturallyoccurring in milk and heat resistant proteasesare liberated by psychrotrophs.

Rennet : It is a substance containing rennin, an enzymehaving the property of clotting or curdling milk.It is generally used in the manufacture ofcheeses. Rennin is obtained from the stomach(abomasums) of milk fed calves.

Saturated steam : It is steam at temperature of the boiling point,which corresponds to its pressure.

Sterilization : It is a process by which all microorganismspresent in milk (both vegetative and spores)are destroyed or are rendered incapable ofgrowth so that milk can be stored for longerperiod without refrigeration.

Tetrahedron : It refers to a polyhedral shape composed offour triangle faces three of which meet at eachvortex. It looks like a pyramid.

Turbidity : Having sediments or foreign particles stirredup or suspended, which give the liquid cloudyor unclear appearance.

Z-value : The number of degree of temperature changenecessary to change the D-value by a factorof 10.


Ashton, T.R. and Romney, A.J.D (1981). In-container sterilization. In: Factorsaffecting the keeping quality of heat treated milk, IDF Bulletin, Doc.

Burton, H. (1988). Ultra-high-temperature processing of milk and milk products.Elsevier Applied Science, London.

Cerf, O. (1981). Aseptic packaging. In: New Monograph on UHT Milk, IDFBulletin, Doc.

Lewis, M.J (2000). Improvements in the pasteurization and sterilization of milk. In:Dairy Processing: Improving quality, Gerrit Smit (ed.) Woodhouse PublishingLtd., Cambridge, England.




1) i. Sterilized milk refers to a product obtained by heating milk in a sealedcontainer in a commercial retort at temperatures of 110-130oC for 10-30min. The sterilized product can be stored for 4-6 months at roomtemperature without spoilage.

2) i. Clostridium botulinum can produce botulin, a toxin that is fatal to human.For ensuring safety of low acid foods like milk, 12 decimal reduction of

69

this organism equivalent to Fo

of 3 is required which can be achieved byheating milk at 121oC for 3 min or such equivalent time-temperaturecombinations.

3) i. For obtaining commercially sterile milk a minimum of two (2) decimalreductions in counts of heat resistant B. stearothermophilus or B.sporothermodurans is necessary which may require a corresponding F

o

value of 8.

4) i. Horizontal retorts are equipped with concentric cages, which are loadedwith sealed cans. Guide rails provided in the retort help the cages rotatethereby ensuring stirring of the contents in can for proper heating.

5) i. Continuous sterilizers are broadly classified under three categories namelycooker-coolers, hydrostatic sterilizers and rotary sterilizers. Continuoussterilizers are preferred over batch types as these have better controls.As these sterilizers cause gradual change in pressure inside the cans,heating is more uniform which results in better quality product.

6) i. A hydrostatic sterilizer is connected to two water columns (12 to 18meter tall barometric leg). Depending on the operating temperaturerequired, the water levels in the two legs are changed to change thesteam pressure inside the sterilizer chamber. Typical sterilizationtemperature of 121oC is attained if the water column difference is 13.7m.

7) i. Rotary sterilizers have a spiral track on the inner wall of the vessel. Aspoke within the vessel facilitate cans to roll along the track and thecontents inside the cans are thoroughly mixed during heating. This ensuresrapid heat transfer and less heat damage (particularly browning) to theproducts.

8) i. Exhausting involves removal of air/ oxygen from the can before sealingof cans. It ensures minimum strains on can seams through expansion ofair during heating and therefore little chance of leakage of cans. It alsoprevents corrosion of containers through oxidation and delays oxidativedeterioration of the product during storage.

9) i. Venting refers to displacement of air from the heating chamber of retortbefore the heating begins. This helps in maintaining uniform steam-airmixture in the sterilizer for efficient processing which is essential forgood product quality.

10) i. Aschaffen burg developed a turbidity test to assess efficiency ofsterilization. It measures the amount of denatured whey proteins. Completedenaturation indicates adequate sterilization.

11) i. The most undesirable physical change in milk after in-bottle sterilizationis visible browning which occurs as a result of high intensity of Maillardreactions at time-temperature conditions followed during in-bottlesterilization.


1) i. UHT milk refers to a product obtained by heating milk in a continuousflow to a temperature in excess of 125oC for not less than 2 sec andsubsequent packaging in sterile containers under aseptic conditions.Generally, UHT milk in India is processed at 140oC/ 2s.

2) i. Heating milk to temperature beyond 130oC results in several fold increasein the rate of destruction of spores like B. Stearothermophilus. At thesame time, reactions responsible for browning or nutrient loss areaccelerated to a lesser extent. Therefore at temperatures higher than130oC, while spore destruction is maximum, change in total quality isminimum.


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70

3) i. Tropical climatic conditions prevailing in India require refrigerated conditionsduring storage, transport and marketing of pasteurized milk, which isdifficult to maintain due to frequent electricity failure. UHT milk can overcome these problems and the product can be marketed to even placeswhere there is no electricity.

4) i. In direct type UHT systems, milk is instantly heated (takes only 1 secto reach 140oC) and therefore heat damage to the products’ quality isminimal. Furthermore, removal of oxygen and volatile compoundsresponsible for heated flavour result in improved taste and delayedoxidation during storage.

5) i. The direct type plant costs twice that of indirect type plant. Water andelectricity requirements are more and therefore, cost of processing perunit volume of milk is high.

6) i. High temperature processing creates high internal pressures. Therefore,wherever plate type heat exchangers are used, the gaskets used aremade of heat resistant materials like medium nitrile rubber or resin curedbutyl rubber.

7) i. Tubular heat exchangers made of SS tubes have high mechanical strength,do not require gaskets and can withstand high internal pressures.Regeneration is possible up to 90% of the thermal energy requirementsand fouling is less frequent. It is possible to use ordinary homogenizer byplacing the high pressure pump before the sterilization section andhomogenizing valve either before or after the final heating section.Therefore it offers versatility in operation and the need for acquiringexpensive aseptic homogenizer is totally eliminated.

8) i. SSHE unit is very expensive and has poor energy conversion efficiency.It is therefore, to be used only for UHT processing of thick liquids, whichcannot be otherwise, processed successfully in other heating systems.

9) i. During high temperature heating, serum proteins get denatured and formlarge size complexes. Denatured serum proteins also interact with caseinand increase casein micelle size. They thus reflect more light and appearwhiter.

10) i. Age thickening refers to progressive increase in viscosity of UHT milkduring storage leading to formation of gel. The probable reasons areproteolysis by residual heat resistant proteases and physico-chemicalchanges involving interaction of proteins and salts in the stored milk.

11) i. Heat resistant lipases surviving UHT treatment act on lipid fraction andrelease short and medium chain fatty solids during storage of UHT milk.Therefore the released fatty acids particularly butyric acids impart rancidflavour. The residual oxygen in the milk and packet react with unsaturatedfatty acids and forms different types of aldehydes and ketones, whichlead to development of oxidative rancidity (flavour defect) in the product.


1) i. Aseptic packaging refers to packaging of a sterile product (read UHTmilk) in pre-sterilized containers under aseptic environment so as to preventpost-processing contamination of the product and thus ensure long shelflife.

2) i. During exposure of the package surface to UV radiations the intensityof radiation is not uniform. The bacteria adhering to the package surfacecould also be protected from the radiations due to hindrances offered bydust and dirt particles present.

3) i. Ionizing radiations like g-rays are used for sterilizing packaging materials,which may not withstand high temperature exposure. It is particularly

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suitable for pre-sterilization of plastic laminates used in bag-in-box packagessuitable for bulk packaging of UHT processed products.

4) i. As per IDF requirements, residual H2O

2in the freshly packaged UHT

milk should not exceed 100 ppb and should subsequently reduce to 1 ppbafter 24 hours. The atmospheric concentration of H

2O

2in the aseptic

packaging hall must not be more than 1 pm.

5) i. Cans are expensive, bulky and require large storage and shipment space.Furthermore, they are so identified with in package sterilization that UHTprocessing plants do not find it attractive from marketing point of view.

6) i. Different packaging materials viz. paper, polyethylene and aluminium foilwith specific properties such as water vapour transmission, burst strengthetc. are co-extruded together into 3, 4 or 5 ply laminates. These laminatessupplied in the form of film rolls are generally used in form-fill-seal (FFS)types of aseptic packaging machines.

7) i. Black and transparent polyethylene films are co-extruded and used forpackaging of UHT milk intended for relatively short shelf life of 2-3weeks. Other alternatives are films obtained by co-extrusion ofpolyvinylidine chloride (PVDC) or ethylene vinyl alcohol (EVOH) withblack or white polyethylene films. These are generally used for products,which offer 2-3 months of shelf life.

8) i. Bulk filling bags are generally made of 3 or 4 layers of packaging materialsof which one should be barrier materials such as metallized polyester(polyester with a coating of aluminium particles) or ethylene vinyl alcohol(EVOH).

9) i. The downward moving film of multi-layer laminate is given a cylindricalshape by a shaping roll. Heat sealing forms an overlapping longitudinalseal.

An extra polythene strip is heat bonded along inside of longitudinal seam.As the cylinder moves further, transverse heat seal is made by jaws firstat the bottom and instantly after filling, at the top.


1. Define D, Z and Fo

values

2. Draw a comparison between direct and indirect type UHT heating systems.

3. Explain the storage induced changes in flavour profile of UHT milk

4. What routine care is taken before starting the aseptic packaging of the product?

5. What are the different types of pre-formed packages and their materials ofconstruction?


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UNIT 8 PREPARATION OF DESIGNATEDAND SPECIAL MILK

Structure

8.0 Objectives

8.1 Introduction

8.2 Full Cream Milk

Definition and Standards

8.3 Toned Milk and Double Toned Milk

Definition

History

Preparation

8.4 Standardized Milk

Definition

Advantages

Preparation

8.5 Skim Milk

Composition

Utilization of Skim Milk for making different dairy products

8.6 Recombined Milk

Definition

Advantages

Preparation

8.7 Reconstituted Milk

Definition

Advantages

Preparation

8.8 Flavoured Milk

Definition

Types of Flavoured Milk

Preparation of Chocolate Milk

Preparation of Fruit Flavoured Milk

Preparation of Sterilized Flavoured Milk

8.9 Let Us Sum Up

8.10 Key Words



8.0 OBJECTIVES


define various types of special milk

outline advantages/ of manufacturing special types of milk

specify the requirements for preparation of special types of milks

prepare various types of special milks

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8.1 INTRODUCTION

When natural constituents of whole milks have been altered by addition, removal,exchange and/or treatment, the resultant milk is designated as special milk. Recentyears have witnessed a large increase in the market penetration of special typesof milk into the total fluid milk market. In India, there is great seasonal fluctuationin the production of milk on account of which many milk plants have to run belowtheir installed capacity, particularly during the lean season. Besides, the cost ofwhole milk generally remains very high throughout the year. Production ofrecombined milk and low fat toned milk have greatly helped in extending themarket supply and reducing the cost of milk to the consumers. The machinery andmanpower of a market milk plant can be fully utilized all the year round by suchdiversifications.

8.2 FULL CREAM MILK

i. Definition and Standards

Full cream milk means milk, or a combination of cow or buffalo milk or a productwhich has been prepared by a combination of both, which has been standardizedto fat percentage of 6.0 and solids-not-fat (SNF) percentage of 9.0 by adjustmentor addition of milk solids. Full cream milk must be pasteurized. It should show anegative phosphatase test. Upon pasteurization, it should be packaged in clean andsanitary containers and should be properly sealed in order to prevent subsequentcontamination.

8.3 TONED AND DOUBLE TONED MILK

i. Definition

Toned milk means the milk obtained by the addition of water and skim milk powderto whole milk. In practice, whole buffalo milk is admixed with reconstituted spraydried skim milk for its production.

Under the PFA Rules (1976), toned milk should contain a minimum of 3.0 per centfat and 8.5 per cent solids-not-fat throughout the country, whereas double tonedmilk should contain a minimum of 1.5 per cent fat and 9.0 per cent solids-not-fatthroughout India.

ii. History

Toned milk is the brainchild of D. N. Khurody (an Indian Dairy Pioneer), who isalso credited with coining its name. Under his auspices, it was first produced in1946 in the Central Dairy of the Aarey Milk Colony and marketed in Bombay city.Soon other cities, notably Calcutta, Madras and Delhi started producing and marketingtoned milk.

iii. Preparation

The calculated amount of potable water is received in the pasteurizing vat/tankequipped with an agitator. The water is heated while the agitator is kept in motionto 38 – 43OC. Then a proportionate amount of spray dried skim milk is slowly addedat the point of agitation and the mixture thoroughly agitated till it dissolves completely.A calculated amount of whole buffalo milk is now added and the mixture againagitated thoroughly till a homogenous mixture is obtained. The mixture is thenfiltered, pasteurized at 63OC for 30 min, rapidly cooled to 5OC, packaged and keptat 5OC or below until distribution. The detailed flow diagram for manufacture oftoned and double toned milk is given below:

Preparation of Designatedand Special Milk

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74

Receiving water in pasteurizing vat

Pre-heating (38-43oC)

Addition of skim milk powder and mixing,and addition of whole buffalo milk and mixing

Filtration

Pasteurization (63oC/ 30 min)

Cooling (5oC)

Packaging and Storage (5oC)


1. Define special milk.

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2. List out quality parameters for full cream milk.

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3. Who invented toned milk? Give PFA requirements for toned and double tonedmilk.

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4. How do we prepare toned milk.

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8.4 STANDARDIZED MILK

i. Definition

Standardized milk is a product, whose fat and/or solids-not-fat (SNF) content havebeen adjusted to a certain pre-determined level. Under the PFA Rules (1976), thestandardized milk for liquid consumption should contain a minimum of 4.5% fat and8.5% SNF throughout the country. The standardization can be done either by

75

partially skimming the fat in the milk with a cream separator, or by admixture withfresh or reconstituted skim milk in proper proportions.

ii. Advantages

Standardized milk offers several advantages such as:

It ensures a milk of practically uniform and constant composition and nutritivevalue to the consumers.

The surplus fat can be converted into butter and ghee.

It becomes possible to supply cheaper milk as compared to the full cream milk.

It is more easily digestible because of less fat content as compared to full creammilk.

iii. Preparation

The detailed step-wise method of manufacture of standardized milk is given below:First of all milk should be received, and tested for fat and SNF levels. It is to bepre-heated to 35-40OC, followed by filtration/clarification. Milk should be standardizedto 4.5% fat and 8.5% SNF levels after calculation of required quantity of skim milkor cream to be added. Upon standardization, milk should be homogenized (2500 psi/65OC) and then it must be pasteurized (72OC/15 sec). After pasteurization, milkmust be packaged either in glass bottles or polypacks and then stored below 5OCtill distribution. The detailed flow diagram for preparation of standardized milk isgiven below:

Milk(Testing of fat & SNF levels)


Filtration/ clarification

Standardization (4.5% fat/ 8.5% SNF)

Homogenization 2500 psi/ 65oC)

Pasteurization (72oC/ 15 sec)

Packaging

Storage (<5oC)

8.5 SKIM MILK

i. Composition

The average percentage composition of skim milk is given in the following table:Constituent % (Average)Water 90.6Fat 0.1Protein 3.6Lactose 5.0Ash 0.7


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ii. Utilization of Skim Milk for Making Different Dairy Products

Skim milk is a by-product of cream separation process. It is mainly used forstandardization of milk and cream. The broad principles for utilization of skim milk,together with the names of commonly made dairy products is given in followingtable:

Principle of Utilization Dairy Products Made

1. Pasteurization Flavoured milks

2. Sterilization Sterilized flavoured milk

3. Fermentation Cultured butter milk (Lassi)

Acidophilus milk

Bulgarian butter milk

4. Fermentation and concentration Concentrated sour skim milk

5. Concentration Plain condensed skim milk

Sweetened condensed skim milk

Low-lactose condensed skim milk

Frozen condensed skim milk

6. Drying Dried skim milk

7. Coagulation Cottage cheese

Bakers’ cheese

Quarg cheese

Casein (edible)

8.6 RECOMBINED MILK

i. Definition

Recombined milk refers to the product obtained when butter oil (also called dry/anhydrous milk fat), skim milk powder and water are combined in the correctproportion to yield fluid milk. The milk fat may also be obtained from other sources,such as unsalted butter or plastic cream. However, production of recombined milkis currently not in practice.

Under PFA rules, recombined milk should contain a minimum of 3.0 per cent fatand 8.5 per cent solids-not-fat throughout the country.

ii. Advantages

It helps in making up the shortage of fresh milk supplies in developing countries.

Helps prevent price rise of liquid milk in cities.

iii. Preparation

A stepwise process for preparation of recombined milk is given below:

A calculated amount of potable water is received in pasteurization tank and it isheated to a temperature of 38O-43OC, while the agitator is kept in motion. Aproportionate amount of dried skim milk is slowly added at the point of agitation.When the water reaches a temperature of 43-49OC, proportionate amount of butteroil is added. The mixture is thoroughly mixed, filtered and pasteurized at 63OC for30 min. It is then homogenized at 2500 psi pressure and cooled to 5OC.

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The detailed flow diagram for manufacture of recombined milk is given below:

Receiving water in pasteurization vat

Pre-heating (38-49OC)

Addition of skim milk powder and mixing (38-49OC)

Addition of butter oil and mixing (42-49OC)

Filtration

Pasteurization (63OC/ 30 min)

Homogenization (2500 psi/ 63OC)

Cooling (5OC)

Packaging and storage


1. What are the advantages of manufacturing standardized milk? Define standardizedmilks according to PFA.

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2. What is skim milk? Give its average chemical composition.

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3. Define recombined milk and briefly describe the method of manufacturing thesame.

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8.7 RECONSTITUTED MILK

i. Definition

Reconstituted milk refers to milk prepared by dispersing whole milk powder inwater (approximately in the proportion of 1 part powder to 7-8 parts water). Duringthe lean season, reconstituted milk is the main source of milk supply in cities.


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ii. Advantages

It helps in making up the shortage of fresh milk supplies.

It is used by the military forces

iii. Preparation

The calculated amount of potable water is received in pasteurization tank equippedwith agitator. The water, is heated to 38-43oC and then calculated amount of spraydried whole milk is slowly added at the point of agitation. The mixture is thoroughlymixed, filtered and pasteurized at 63oC/30 min. and promptly cooled to 5oC orbelow until distribution. Detailed flow diagram of the process is given below:

Receiving water in pasteurization vat


Addition of skim milk powder and mixing (38-43oC)

Filtration

Pasteurization (63oC/30 min.)

Cooling (5oC)

Packaging and storage (5oC)

8.8 FLAVOURED MILK

i. Definition

Flavoured milk is milk to which some flavour has been added. When the ‘milk’ isused, the product should contain a milk fat percentage at least equal to the minimumlegal requirement for market milk. But when the fat level is lower (1-2 per cent),the term ‘drink’ is used.

ii. Types of Flavoured Milk

The main types of flavoured milk are as follows:

chocolate milk/drink

fruit flavoured milk/drink

sterilized flavoured milk/drink

iii. Preparation of Chocolate Milk Drink

The milk on receipt is standardized to 2% fat level for preparation of drink.Standardized milk is then pre-heated to 35-40OC and filtered; alternatively, afterstandardization it is pre-heated to 60OC, homogenized at 2500 psi and then clarified.To the warm milk, cocoa powder (1 to 1.5%), sugar (5 to 7%) and stabilizer(sodium alginate – 0.2%) are slowly added and stirred to dissolve them properly.The mixture is then pasteurized at 71OC/30 min., cooled rapidly to 5OC, bottled andkept under refrigeration (5OC) until used. The detailed flow diagram for themanufacture of chocolate milk/ drink is given below:

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Receiving milk

Standardization

Pre-heating (60OC)

Pre-heating (35-40OC)

Homogenization (2500 psi)

Mixing cocoa powder, sugar and stabilizer orMixing flavour/ essence, colour and sugar

Pasteurization (71OC/ 30min)

Cooling

Bottling and storage (5OC)

iv. Preparation of Fruit Flavoured Milk

The method of preparation of fruit flavoured milk is similar to that used for chocolatemilk/drink. Instead of cocoa powder, permitted fruit flavours/essence, together withpermitted (matching) colours and sugar are used. The common flavours used arestrawberry, orange, lemon, pineapple, banana, vanilla, etc. In order to obtain goodresults, the following precautions should be taken:

No acid (citric or tartaric) should be added to the fruit syrup, as this may causecurdling of milk.

Excessive sweet syrup should be avoided. The best sugar content of the syrupis 45-55 per cent.

Add 1 part of fruit syrup to 5 parts of milk.

Care should be taken to see that there is a pleasant blend of sweet, fruity andmilky flavours (together with an appealing colour)

v. Preparation of Sterilized Flavour Milk

These combine the advantages of both sterilized and flavoured milk/drinks. Themethod of preparation is given below:

Receiving milk

Cooling to 5OC and bulk storage

Pre-heating to 35-40OC

Filtration/Clarification

Mixing flavour/essence, colour and sugar


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Filling and capping(in cleaned and sanitized bottles)

Sterilization (108-111OC/ 25-30 min)

Cooling (room temperature)

Storage (room temperature)

The raw milk, upon receiving, should be strictly examined by the prescribed physico-chemical and bacteriological tests and only high quality milk should be used forproduction of sterilized milk. The incoming milk should be promptly cooled to 5OCfor bulk storage in order to check any bacterial growth. Next, it should be pre-heated to 35-40OC for filtration, so as to remove visible dirt, etc. Flavour/ essence,permitted (matching) colour and sugar (syrup) are added to clarified milk and mixedwell. The fruit flavoured milk is now filled in cleaned and sterilized bottles and thencapped properly. The filled bottles are then sterilized at 108-111oC for 25-30 min.The sterilized milk bottles should be gradually cooled to room temperature. Finally,the sterilized milk is stored in a cool place.


1. How reconstituted milk is different from recombined milk?

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2. Define flavoured milk and list out different types of flavoured milk.

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3. Briefly describe method of manufacturing chocolate milk.

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4. What precaution should be taken during manufacturing of fruit flavoured milk?

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8.9 LET US SUM UP

Special milk physically resembles and behaves like liquid milk. Special types of milkare prepared by altering natural constituents of milk by addition, removal, exchangeand/ or treatment. Different types of special milk are full cream milk, toned milk,

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double toned milk, standardized milk, skim milk, recombined reconstituted milk andflavoured milk such as chocolate and fruit flavoured milk.

All types of special milk should fulfill certain requirements laid down in the PFArules. Also there are certain merits of manufacturing such special milk.

Different manufacturing procedures and ingredients are required in the productionof special types of milk.

8.10 KEY WORDS

Pasteurization : Pasteurization of milk is done by heating milkto at least 63OC for 30 min, or 72OC for 15sec. After pasteurization milk is cooled to 5OCor below.

Pre-heating : Pre-heating of milk refers to heating beforethe operation which follows immediately. Theusual temperature of pre-heating is 35-40OC,and the equipment used may be a plate ortubular heater.

Filtration : Filtration removes dirt, dust, suspended andforeign particles by the straining process.

Cooling : Milk is generally chilled to 5OC or below andstored cool till it is used, to prevent deteriorationin its bacteriological quality.

Standardization : Standardization of milk refers to adjustment offat and solid-not-fat of milk to fulfill the legalrequirements before sale.

Homogenization : Homogenization refers to the process ofbreaking fat globules in order to preventformation of cream layer upon storage andenables easy digestion of milk

Sterilization : Sterilization of milk refers to subjecting milk toheat treatment at more than 100OC forsufficient period of time, so as to destroy almostall spoilage causing microorganisms.


Ahmed, T. (1999). Dairy Plant Engineering and Management, Kitab Mahal,Allahabad.

Aneja, R.P., Mathur, B.N., Chandan, R.C., Banerjee, A.K. (2002). Technology ofIndian milk products, Dairy India Publication, Delhi.

De, S. (1999). Outlines of Dairy Technology, Oxford University Press, New Delhi.

Mathur, M.P., Datta, Roy, D., Dinakar, P. (1999). Textbook of Dairy Chemistry,Indian Council of Agricultural Research, New Delhi.

Rangappa, K.S., Acharya, K.T. (1974). Indian Dairy Products, Asia PublishingHouse, Bombay.


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1) i. Definition of special milk.

2) i. Types of milk or milk products used for manufacture of full cream milk.

ii. Fat and solid-not-fat per cent

iii. Pasteurization, packaging, storage

3) i. Name of person who invented toned milk.

ii. Fat and solid-not-fat requirements for toned and double toned milkaccording to PFA.


1) i. Merits of manufacturing standardized milk

ii. PFA requirements for standardized milk – fat and solid-not-fat per cent.

2) i. Skim milk – a by product of cream separation

ii. Chemical composition of skim milk

3) i. Definition

ii. Mixing of potable water and skim milk powder – addition of butter oil –mixing – pasteurization – homogenization


1) i. Differentiate between ingredients used

ii. Homogenization (Recombined milk) vs. non-homogenized (reconstitutedmilk)

2) i. Definition of flavoured milk

ii. Different types of flavoured milk

3) i. Milk reception – standardization – pre-heating – homogenization – Additionof ingredients – Pasteurization – Cooling – Packaging – Storage.

4) i. Avoid addition of acids.

ii. Optimum levels of sugar, fruit, flavour and colour, etc.

BPVI-13-02

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Transcript of BPVI-13-02