The use of chemical testing systems that allow

verifying milk quality and, if necessary, modifying the production procedures, is essential.

The need for specific and reliable systems for the control of milk production opens the search for new analytical approaches with biological markers and electronic analyzers that are ever easier and faster to use.

TESTS TO DETECT ADULTERATION IN MILK

There are many methods known for detection of adulteration in milk but the methods discussed below are simple but rapid and sensitive methods to detect adulteration.

I. Detection of Neutralizers in milk

1) Rosalic acid test (Soda Test)

In milk neutralizers like hydrated lime, sodium hydroxide, sodium carbonate or sodium bicarbonate are added which are generally prohibited.

How to detect?

Take 5 ml of milk in a test tube and add 5 ml alcohol followed by 4-5 drops of rosalic acid. If the colour of milk changes to pinkish red, then it is inferred that the milk is adulterated with sodium carbonate / sodium bicarbonate and hence unfit for human consumption.

This test will be effective only if the neutralizers are present in milk. If the added neutralizers are nullified by the developed acidity, then this test will be negative. In that case, the alkaline condition of the milk for the presence of soda ash has to be estimated.

How to proceed?

2) Take 20 ml of milk in a silica crucible and then the water is evaporated and the contents are burnt in a muffle furnace. The ash is dispersed in 10 ml distilled water and it is titrated against decinormal (N/10) hydrochloric acid using phenolphthalein as an indicator. If the

titre value exceeds 1.2 ml, then it is construed that the milk is adulterated with neutralizers.

II. Test for detection of hydrogen peroxide

Take 5 ml milk in a test tube and then add 5 drops of paraphenylene diamine and shake it well. Change of the colour of milk to blue confirms that the milk is added with hydrogen peroxide.

III. Test for detection of formalin

Formalin (40%) is poisonous though it can preserve milk for a long time.

How to detect?

Take 10 ml of milk in test tube and 5 ml of conc. sulphuric acid is added on the sides of the test tube with out shaking. If a violet or blue ring appears at the intersection of the two layers, then it shows the presence of formalin.

IV. Test for detection of sugar in milk

Generally sugar is mixed in the milk to increase the solids not fat content of milk i.e. to increase the lactometer reading of milk, which was already diluted with water.

How to detect?

Take 10 ml of milk in a test tube and add 5 ml of hydrochloric acid along with 0.1 g of resorcinol. Then shake the test tube well and place the test tube in a boiling water bath for 5 min. Appearance of red colour indicates the presence of added sugar in milk.

V. Test for detection of starch

Addition of starch also increases the SNF content of milk. Apart from the starch, wheat flour, arrowroot, rice flour are also added.

How to detect?

Take 3 ml milk in a test tube and boil it thoroughly. Then milk is cooled to room temperature and added with 2 to 3 drops of 1% iodine solution. Change of colour to blue indicates that the milk is adulteratedstarch.

VI. Test for detection of glucose

Usually poor quality glucose is added to milk to increase the lactometer reading. There are two tests available to detect the adulteration of milk with glucose.

How to proceed?

1. Phosphomolybdic or Barford Test

Take 3 ml of milk in a test tube and add 3 ml Barford’s reagent and mix it thoroughly. Then keep it in a boiling water bath for 3 min and then cool it for 2 min by immersing in tap water with out disturbance. Then add 1 ml of phosphomolybdic acid and shake. If blue colour is visible, then glucose is present in the milk sample.

2. Diacetic test

Take a strip of diacetic strip and dip it in the milk for 30 sec to 1 min. If the strip changes colour, then it shows that the sample of milk contains glucose. If there is no change in the colour of the strip, then glucose is absent. In this method the presence of glucose in milk can be quantified by comparing the colour developed with the chart strip.

VII. Test for detection of urea

1. Urea is generally added in the preparation of synthetic milk to raise the SNF value.

Five ml of milk is mixed well with 5 ml paradimethyl amino benzaldehyde (16%). If the solution turns yellow in colour, then the given sample of milk is added with urea.

2. Take 5 ml of milk in a test tube and add 0.2 ml of urease (20 mg / ml). Shake well at room temperature and then add 0.1 ml of bromothymol blue solution (0.5%). Appearance of blue colour after 10-15 min indicates the adulteration milk with urea.

VIII. Test for detection of ammonium sulphate

The presence of sulphate in milk increases the lactometer reading.

How to proceed?

5 ml of hot milk is taken in a test tube and added with a suitable acid for e.g. citric acid and the whey thus separated is filtered. Collect the whey in another test tube and add 0.5 ml of 5% barium chloride. Appearance of precipitate indicates the presence of ammonium sulphate in milk.

IX. Test for detection of salt

Addition of salt in milk is mainly resorted to with the aim of increasing the corrected lactometer reading.

How to detect?

Five ml of silver nitrate (0.8%) is taken in a test tube and added with 2 to 3 drops of 1% potassium dichromate and 1 ml of milk and thoroughly mixed. If the contents of the test tube turn yellow in colour, then milk contains salt in it. If it is chocolate coloured, then the milk is free from salt.

X. Test for detection of pulverized soap

Take 10 ml of milk in a test tube and dilute it with equal quantity of hot water and then add 1 – 2 drops of phenolphthalein indicator. Development of pink colour indicates that the milk is adulterated with soap.

XI. Detection of detergents in milk

Take 5 ml of milk in a test tube and add 0.1 ml of bromocresol purple solution. Appearance of violet colour indicates the presence of detergent in milk. Unadulterated milk samples show a faint violet colour.

XII. Detection of water in milk

Though the adulteration of milk with water can be checked by lactometer reading, other adulterations too affect the lactometer reading. Hence freezing point depression, recognized by AOAC, is usually adopted.

Percentage of water added = Normal freezing point – Observed freezing point   X 100                                                                        Normal freezing point

Normal freezing point of milk is taken as –0.55°C. A tolerance level of 3% is given which is equivalent to specifying a minimum freezing point depression for authentic milk of –0.55°C.

XIII. Detection of skim milk powder in milk

If the addition of nitric acid drop by drop in to the test milk sample results in the development of orange colour, it indicates the milk is adulterated with skim milk powder. Samples with out skim milk powder shows yellow colour.

XIV. Detection of vegetable fat in milk

The characteristic feature of milk is its fatty acid composition, which mainly consists of short chain fatty acids such as butyric, caproic, caprylic acid; whereas the vegetable fats consist mainly of long chain fatty acids and hence adulteration of vegetable fat in milk can be easily found out by analyzing the fatty acid profile by gas chromatography.

XV. Detection of buffalo milk in cow milk

The presence of buffalo milk in cow milk is tested by Hansa test. It is based on immunological assay. One ml of milk is diluted with 4 ml of water and then it is treated with 1 ml of antiserum. The characteristic precipitation reaction indicates the presence of buffalo milk in the sample taken. (The antiserum is developed by injecting buffalo milk proteins into rabbits).  

XVI. Detection of benzoic and salicylic acid in milk

Five ml of milk is taken in a test tube and acidified with concentrated sulphuric acid. 0.5% ferric chloride solution is added drop by drop and mixed well. Development of buff colour indicates presence of benzoic acid and violet colour indicates salicylic acid.

XVII. Detection of borax and boric acid in milk

Five ml of milk is taken in a test tube to which 1 ml of concentrated hydrochloric acid is added and mixed well. Tip of a turmeric paper is dipped into the acidified milk and it is dried in a watch glass at 100°C or over a small flame. If the turmeric paper turns red, it indicates the presence of borax or boric acid.

Confirmation can be made by adding a drop of ammonia solution on the turmeric paper and if the red colour changes to green, it shows the presence of boric acid. 

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Testing Bulk Tank Milk Samples

404-405

G.M. Jones, Professor of Dairy Science and Extension Dairy Scientist, Milk Quality & Milking Management; and Susan Sumner, Associate Professor of Food Science and Technology and Extension Food Scientist; Virginia Tech

Samples of bulk tank milk are collected regularly and milk quality tests are performed by milk coops, plants, or Virginia Department of Agriculture and Consumer Services. Some milk coops offer bulk tank profiles which involve several tests related to milk quality. The various tests are briefly described below, as well as a list of goals for high quality milk and conditions which adversely affect test results. Hopefully, an explanation of the use of these tests will help Virginia dairy farmers produce high quality milk, which has been defined by the Extension Milk Quality Leadership Council as milk with:

Somatic cell count equal to or less than 200,000 Standard plate count equal to or less than 5,000 Preliminary incubation count equal to or less than 10,000 Drug residue free Sediment count equal to or less than 1.0 Cryoscope reading equal to or less than -.530° H

Standard Plate Count (SPC) determines the number of visible colony-forming units (numbers of individual or tightly associated clumps of bacteria) in 1 ml. of milk incubated at 90°F for 48 hours. If SPC exceeds 5,000 cfu/ml., there is usually a reason. SPC is usually less than 5,000 cfu/ml. if sanitation is good and cooling is adequate. A SPC of 10,000 should be achievable by most farms (Murphy, 1997). The Extension Milk Quality Leadership Council has recommended that quality milk should have a SPC of 5,000 and less. In a test of 10,662 Virginia bulk tank samples by Virginia Department of Agriculture and Consumer Services (VDACS) between December 1, 1997, and November 16, 1998, 59% were below 5,000, 76% were below 10,000, and 87% were below 20,000. In New York State, of 855 bulk tank samples, 30% were below 5,400 cfu/ml and 50% were below 10,000 (Boor et al., 1998). The regulatory accepted limit is 100,000, although industry standards are often 50,000. Most high raw milk bacteria counts involve improper cleaning and sanitizing of dairy equipment, poor production practices (especially dirty

01392002671650 FORID:10 UTF-8

pubs.ext.vt.edu/4

cows), inadequate cooling, mastitis problems, or improper handling of sample by hauler or carrier of sample from plant to lab.

Preliminary Incubation Count (PICount)- A milk sample is incubated at 55° F for 18 hours, followed by the SPC procedure. PICount is considered the best measure of raw milk keeping quality and sanitation practices on farms. These organisms are usually associated with off-flavors, milk spoiling, and short shelf-life (Dairy Practices Council). PICount should not exceed 100,000 cfu/ml. Desirable PICount is less than 25,000 cfu/ml. The Extension Milk Quality Leadership Council has recommended that quality milk should have a PICount of 10,000 and less. Of 855 bulk tanks tested in New York, only 30% were less than 26,000 and 10% were below 8,000 cfu/ml. VDACS does not conduct PICounts but several milk coops do and use them in their quality premium payment programs.

Listed below are possible causes of high PICounts. These also could apply to high SPC.

Failure to thoroughly clean equipment after each use or neglecting to sanitize equipment before using (a major cause)

Slow cooling farm bulk tank or temperature above 40° F (bulk tanks should be less than 40° F within two hours of milking and kept below 45° F during subsequent milkings). This points out the importance of the recording thermometer and its use.

Problems with debris buildup in plate coolers and chillers. When milking fresh cows and problem cows in bucket milkers, hoses need to be keep

clean Dirty animals; may need clipping Poor udder sanitation practices (dirty, excessive water used to wash teats & udders);

teats need to be clean, sanitized, and dry Contaminated water supply, especially coliforms or other spoilage-causing bacteria Wash water temperature should start at 155-170°F and drain at above 120° F Gaskets and rubber parts need to be clean, free of cracks and deposits, and replaced

when necessary Improperly drained milking equipment Teatcup liners should be clean and free of cracks and changed on schedule Pulsator and main vacuum supply lines need cleaning on a regular basis and especially

whenever milk enters the line Streptococcus agalactiae or environmental streptococcus mastitis infections

Lab Pasteurized Count (LPC) measures the number of bacteria that survive lab pasteurization at 145° F for 30 minutes. Levels in raw milk should be less than 250-300 cfu/ml. Bacteria may be Bacillus species; usually it does not include mastitis-causing bacteria. Of 855 bulk tank samples analyzed in New York State, 60% were below 200 and 70% were less than 300 cfm/ml. These thermoduric (heat loving) bacteria may cause spoilage in pasteurized milk. High LPC can be caused by unclean equipment and/or improper sanitizing practices, as well as leaky pumps, old pipeline gaskets, inflations and other rubber parts, and milkstone deposits (Murphy, 1997). VDACS does not perform this test. Some milk coops include LPC as a part of the bulk tank profile.

Coliform Count provides an indication of unsanitary production practices and/or mastitis infection. A count of less than 100 cfu/ml. is considered acceptable. Counts of 10 cfu/ml. or less are achievable and desirable. Of the NY bulk tank samples, 10% were less than 10 and 70% were below 66 cfu/ml. Udders and teats may be dirty and premilking hygiene may be inadequate. This test is not a part of VDACS' regulatory testing program.

Contamination from the exterior of the udder can influence LPCs, PICounts, and coliform counts (Murphy, 1997). The degree of cleanliness of the milking system probably influences the total bulk milk bacteria as much as, if not more, than any other factor. Milk residue left on equipment contact surfaces supports the growth of a variety of microorganisms. Environmental contaminants (i.e. from bedding, manure, feeds) are more likely to grow on soiled equipment surfaces. Less efficient cleaning, using lower temperatures and/or the absence of sanitizers tends

to select for the faster growing, less resistant organisms, principally Gram-negative rods (coliforms and Pseudomonads) and lactic streptococci. This will result in high PICounts and in some cases elevated LPCs.

Cryoscope or Freezing Point- Over -.530° Horvet suggests milk contains some added water or composition has been altered somehow. In New York, less than 3% of bulk tanks had freezing points greater than -.530. From December, 1997, through May, 1998, VDACS found only 16 violative samples in 5,631 tests or 0.28%.

Possible causes of high cryoscope readings include:

Intentional or accidental addition of water o Chasing milk from receiver jar o Poor milking system drainage o "Wet cow" milking or use of excessive water o Backflushing units with vacuum on o Bulk tank valve should be open to drain o Rinsing top of tank

Faulty gaskets in plate coolers (some brands) Freezing of milk in bulk tank Thin or under-conditioned cows because of grazing or low concentrate or energy intakes

Sediment- Acceptable levels are less than 1.5 mg./gal. milk. Cows’ udders should be clipped, teats and udders need to be clean, and tea#tcups properly attached and removed. Large herds need to stop during milking and change filters. Although this test isn't conducted by VDACS nor by all milk coops, the Extension Milk Quality Leadership Council recommends that high quality milk should contain 1.0 or less.

Rancidity causes objectionable flavor with sharp, unclean, astringent taste that lingers as an unpleasant aftertaste. Psychrotrophic organisms such as Pseudomonas, Aerobacter, and Bacillus are harbored in crevices of pipelines, gaskets, etc., as a result of poor cleaning and incomplete sanitation, and they produce lipases which may not be destroyed by pasteurization. Lipases hydrolyze (break down) fats to glycerol and free fatty acids. It is expressed as acid degree value (ADV) which should not exceed 1.0. This test is conducted by some processors, but it's not done by all milk coops.

Possible causes of high ADV:

Violent agitation, such as excessive pumping, or excessive agitation caused by slow cooling in farm bulk tank, pipeline obstructions, or freezing in bulk tank The elimination of risers in milk lines and the use of vacuum tanks have removed some of this problem.

Airleaks in pipelines Leaky valves Flooded milklines, high milklines Teatcup liner slippage or excessive air admission Late stage of lactation weakens fat globule membrane’s protective lecithin-protein layer

Milk Cultures- Culturing of bulk tank milk samples can be useful to determine the presence or absence of certain bacteria and may be conducted by milk plants, VDACS, veterinary services, or the Virginia-Maryland Regional College of Veterinary Medicine. Changes in bulk tank cultures will monitor the impact of management changes upon milk quality and infection status. Samples taken over consecutive days or weeks give a clearer picture of a herd’s problem than a sample from one bulk tank of milk. Agitate the milk in the tank for 5 minutes prior to collection (National Mastitis Council or NMC). Use a clean, sanitized dipper to collect the milk sample from the top of the tank (never the outlet). Pour the first sample back into the tank to rinse sanitizer off the

dipper. Sample two consecutive bulk tanks and freeze the samples. Sample the third bulk tank load and refrigerate it until all three samples can be delivered to a laboratory. The presence of Staphylococcus aureus or Streptococcus agalactiae almost always indicates the presence of these kind of mastitis infections in the herd.

Staphylococcus aureus is shed in low numbers by infected quarters and generally does not cause high bacteria counts (SPC) in bulk tank milk, but bulk tank somatic cell count (SCC) would be expected to be elevated. NMC states that it should be possible to attain a bulk tank Staphylococcus aureus count of less than 50 cfm/ml.

The presence of Streptococcus agalactiae in bulk tank milk is due only to infected udders. Both bacteria count (SPC) and SCC could be elevated by the presence of this infection in the herd. Eradication of this pathogen from the herd is possible.

Environmental streptococci and coliform (usually E. coli) counts above 1,000 and 500 cfm/ml., respectively, indicate poor hygiene either during equipment cleaning and sanitation, during milking, or between milkings. Common contaminants are bedding, manure, soil, or water. According to NMC, milking wet udders, organic soil buildup in milklines, cracked gaskets and inflations, inadequately heated wash water, inadequate cooling of milk, and udder infections all contribute to high environmental streptococcal or coliform counts in bulk milk. Udder infections should be highly correlated to cases of clinical mastitis in the herd. Bacteria counts (SPC) may be high while SCC are low.

Once results of bulk tank cultures are known, milk samples should be collected from individual cows and cultured. Take aseptic samples from cows with high DHI SCC or positive CMT, cows with clinical mastitis, and fresh cows, especially heifers. Samples can be stored in the freezer for as long as 6 weeks. Individual cow sampling can sometimes be disappointing because no growth can occur with milk from problem cows. According to the NMC (1987), 25-40 percent of all clinical samples can be negative on routine culturing. The reasons include:

Numbers of certain organisms, such as mycoplasma, Staphylococcus aureus, and coliforms, can vary greatly in infected quarters, and may occasionally be less than the minimum detection limit of the assay. The minimum detection limit when plating 0.01 ml of milk is about 100 colony forming units per ml.

The organism may no longer be present and the clinical signs are due to by-products such as endotoxins.

Somatic cells may have phagocytized the organisms. Antibiotics may have killed or suppressed organism numbers to unrecoverable levels. Storage may have reduced numbers of viable organisms to undetectable levels. The organism may require cultural conditions other than those used for isolation (i.e.

reduced temperature, prolonged incubation, special media, anaerobic conditions, etc.).

Three bulk tank samples were collected from a herd where the DHI SCC score averaged 3.36, with an actual SCC average of 537,000 and 28% of the cows had high SCC (score 5 and above or actual SCC above 284,000). The herd was treating 1-2 cows per week for clinical mastitis. At the previous month's test, the average SCC score was 3.12, or actual SCC averaged 477,000 and 20% of cows had high SCC. This suggests that the herd's subclinical mastitis is increasing. The bulk tank cultures had low coliform and streptococci counts accompanied by low bacteria counts. Two bulk tank samples had Staphylococcus aureus counts higher than desired. Staph infected herds often have high SCC but low bacteria counts. The dairy producer felt that the herd had symptoms characteristic of a herd with Staph infections. However, if bulk tank sample C had been the only sample collected, results would have been inconclusive and perhaps misleading.

Bulk tank Streptococci per Coliform per ml Staphylococcus Bacteria count

ml aureus per ml per ml

Desired < 1,000 < 500 < 50 < 5,000

A 320 340 80 740

B 840 40 120 1,000

C 760 0 40 800

 

Microorganisms Found in Bulk Tank and Individual Cow Milk Samples

Type of Bacteria Source CommentsHigh SCC

High SPC

Staph. aureus (coagulase positive)

Infected quarters,Teat lesions, new additions to herd

Contagious: hands, wash rags, teatcup liners

**  

Staph species (coagulase neg.)

Normal on teat skin, bedding, low SCC

Dry treat all cows, predip & postdip with germicidal teat dip

   

Str. agalactiae Infected quarters, new additions to herd

Contagious: hands, wash rags, teatcup liners

** **

Environmental streptococci (non-agalactiae, Str. uberis or Str. dysgalactiae)

Manure, bedding, wet & dirty teats, liner slips

Wet & dirty conditions that expose teat ends to bacteria: dirty housing and calving environments

* **

Coliforms (E. coli, Klebsiella)

Manure, bedding, wet & dirty teats

Clipped, clean, and dry udders & teats: freestalls

* *

Mycoplasma Infected quarters, new additions to herd

Contagious: hands, wash rags, teatcup liners

**  

Pseudomonas Water, soil, manure Found in dirty milking equipment, contaminated water, contaminated

   

antibiotics

Prototheca Water, manure, infected quarters

No treatment; eliminate stagnant water & manure

   

Serratia Contaminated water, bedding or litter

Eliminate washing of udders & teats

   

Nocardia Infected quarters, Found in contaminated treatments & multi-dose

   

Yeast & molds Infected quarters Found in contaminated treatments & multi-dose

   

 

References:

Boor, K.J., D.P. Brown, S.C. Murphy, S.M. Kozlowski, and D.K. Bandler. 1998. Microbial and chemical quality of raw milk in New York State. J. Dairy Sci. 81:1743-1748.

Dairy Practices Council. 1991. Guidelines for raw milk quality tests, Publication DPC 21, Barre, VT.

Dairy Practices Council. 1997. Guidelines for troubleshooting on-farm bacteria counts in raw milk, Publication 24, Barre, VT.

Murphy, S.C. 1997. Raw milk bacteria tests: Standard plate count, preliminary incubation count, lab pasteurization count and coliform count - What do they mean for your farm? page 34-42 in Proc. National Mastitis Council Regional Meeting, Syracuse, NY.

National Mastitis Council. Using Bulk Tank Milk Cultures in a Dairy Practice. http://www.nmconline.org/bulktank.htm.

National Mastitis Council. 1987. Laboratory and Field Handbook on Bovine Mastitis, Madison, WI.

Reinemann, D.J., G.A. Mein, D.R. Bray, D. Reid, and J.S. Britt. 1997. Troubleshooting high bacteria counts in farm milk. p. 65-79 in Proc. 36th Annual Meeting of National Mastitis Council, Madison, WI.

 

Reviewed by Christina Petersson-Wolfe, Extension Specialist, Dairy Science

Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences, and Interim Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg;  Wondi Mersie, Interim Administrator,1890 Extension Program, Virginia State, Petersburg.

May 1, 2009

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Back to Facts for Farmers

Milk testing and Quality Control

Milk Processing Guide SeriesVolume 2

Published by:FAO/TCP/KEN/6611 Project

Training Programme for Small Scale Dairy Sector and Dairy Training Institute - Naivasha

TABLE OF CONTENTS

1. INTRODUCTION *

2. MILK TESTING AND QUALITY CONTROL *

2.1 WHAT IS MILK QUALITY CONTROL? *

2.2 WHY HAVE MILK QUALITY CONTROL? *

2.3 QUALITY CONTROL IN THE MILK MARKETING CHAIN IN KENYA *

2.4 TECHNIQUES USED IN MILK TESTING AND QUALITY CONTROL *

2.4.1 Milk sampling *

2.4.2 Sampling milk for bacteriological testing *

2.4.3 Preservation of sample *

2.4.4. Labelling and records keeping *

2.4.5 Common testing of milk. *

3. QUALITY CONTROL OF PASTEURISED MILK *

1. INTRODUCTION

 

Milk testing and quality control is an essential component of any milk processing industry whether small, medium or large scale. Milk being made up of 87% water is prone to adulteration by unscrupulous middlemen and unfaithful farm workers. Moreover, its high nutritive value makes it an ideal medium for the rapid multiplication of bacteria, particularly under unhygienic production and storage at ambient temperatures. We know that, in order for any processor to make good dairy products, good quality raw materials are essential. A milk processor or handler will only be assured of the quality of raw milk if certain basic quality tests are carried out at various stages of transportation of milk from the producer to the processor and finally to the consumer.

There are a number of standard manuals and text books on milk quality control. However these may not be easily available to the emerging small scale to medium scale processors in Kenya.

For these reasons, the Training Programme for the Small Scale Dairy Sector under project GOK/FAO/TCP/KEN/6611, has assembled this guide on Milk Testing and Quality Control so that it may be used for training and by the private small scale dairy processors. The methods selected are simple and basic and will suffice the requirements of most milk quality control laboratories of small scale processing units. For the larger plants with bigger laboratories more tests are to be found in the bibliography at the end of this booklet.

2. MILK TESTING AND QUALITY CONTROL

2.1 WHAT IS MILK QUALITY CONTROL?

Milk quality control is the use of approved tests to ensure the application of approved practices, standards and regulations concerning the milk and milk products. The tests are designed to ensure that milk products meet accepted standards for CHEMICAL COMPOSITION AND PURITY AS WELL AS LEVELS OF DIFFERENT MICRO-ORGANISMS.

2.2 WHY HAVE MILK QUALITY CONTROL?

Testing milk and milk products for quality and monitoring that MILK PRODUCTS, PROCESSORS and MARKETING AGENCIES adhere to accepted codes of practices costs money. There must be good reasons why we have to have a quality control system for the dairy industry in Kenya.

The reasons are:

i)To the Milk Producer.

The milk producer expects a fair price in accordance with the quality of milk she/he produces.

ii) The Milk Processor.

The milk processor who pays the producer must assure himself/herself that the milk received for processing is of normal composition and is suitable for processing into various dairy products.

iii) The Consumer.

The consumer expects to pay a fair price for milk and milk products of acceptable to excellent quality.

iv) The Public and Government Agencies.

These have to ensure that the health and nutritional status of the people is protected from consumption of contaminated and sub-standard foodstuffs and that prices paid are fair to the milk producers, the milk processor and the final consumer.

All the above-is only possible through institution of a workable quality testing and assurance system conforms to national or internationally acceptable standards.

2.3 QUALITY CONTROL IN THE MILK MARKETING CHAIN IN KENYA

i) At the farm

Quality control and assurance must begin at the farm. This is achieved through farmers using approved practices of milk production and handling; and observation of laid down regulations regarding, use of veterinary drugs on lactating animals, regulations against adulterations of milk etc.

ii) At Milk collection Centres

All milk from different farmers or bulked milk from various collecting centres must be checked for wholesomeness, bacteriological, and chemical quality.

iii) At the Dairy Factories

Milk from individual farmers or bulked milk from various collecting centres

iv) Within the Dairy Factory

Once the dairy factor has accepted the farmer milk it has the responsibility of ensuring that the milk is handled hygienically during processing. It must carry out quality assurance test to ensure that the products produced conform to specified standards as to the adequacy of effect of processes applied and the keeping quality of manufactured products. A good example is the phosphatase test used on pasteurised milk and the acidity development test done on U.H.T milk.

v) During marketing of processed products

Public Health authorities are employed by law to check the quality of food stuffs sold for public consumption and may impound substandard or contaminated foodstuffs including possible prosecution of culprits. This is done in order to protect the interest of the milk consuming public.

2.4 TECHNIQUES USED IN MILK TESTING AND QUALITY CONTROL

2.4.1 Milk sampling

Accurate sampling is the first pre-requisite for fair and just quality control system. Liquid milk in cans and bulk tanks should be thoroughly mixed to disperse the milk fat before a milk sample is taken for any chemical control tests. Representative samples of packed products must be taken for any investigation on quality. Plungers and dippers me used in sampling milk from milk cans.

2.4.2 Sampling milk for bacteriological testing

Sampling milk for bacteriological tests require a lot of care. Dippers used must have been sterilised in an autoclave or pressure cooker for at least 15mm at 120° C before hand in order not to contaminate the sample. On the spot sterilisation may be employed using 70% Alcohol swab and flaming or scaling in hot steam or boiling water for 1 minute.

Fig. 1: Equipment used for taking milk samples

2.4.3 Preservation of sample

Milk samples for chemical tests.

Milk samples for butterfat testing may be preserved with chemicals like Potassium dichromate(1 Tablet or ½ ml 14% solution in a ¼ litre sample bottle is adequate.) Milk samples that have been kept cooling a refrigerator or ice-box must first be warmed in water bath at 40 ºC, cooled to 20ºC, mixed and a sample then taken for butterfat determination. Other preservative chemicals include Sodium azid at the rate of 0.08% and Bronopol (2-bromo-2-nitro-1,3-propanediol) used at the rate of 0.02%.

If the laboratory cannot start work on a sample immediately after sampling, the sample must be cooled to near freezing point quickly and be kept cool till the work can start. If samples are to be taken in the field e.g. at a milk cooling centre, ice boxes with ice pecks are useful.

2.4.4. Labelling and records keeping

Samples must be clearly labelled with name of farmer or code number and records of dates, and places included in standard data sheets. Good records must be kept neat and in a dry place. It is desirable that milk producers should see their milk being tested, and the records should be made available to them if they so require.

2.4.5 Common testing of milk.

2.4.5.1 Organoleptic tests

The organoleptic test permits rapid segregation of poor quality milk at the milk receiving platform. No equipment is required, but the milk grader must have good sense of sight, smell and taste. The result of the test is obtained instantly, and the cost of the test are low. Milk which cannot be adequately judged organoleptically must be subjected to other more sensitive and objective tests.

Procedure:

Open a can of milk. Immediately smell the milk. Observe the appearance of the milk.

If still unable to make a clear judgement, taste the milk, but do not swallow it. Spit the milk sample into a bucket provided for that purpose or into a drain basin, flush with water.

Look at the can lid and the milk can to check cleanliness.

Judgement:

Abnormal smell and taste may be caused by:

Atmospheric taint (e.g. barny/cowy odour). Physiological taints (hormonal imbalance, cows in late lactation- spontaneous rancidity). Bacterial taints. Chemical taints or discolouring. Advanced acidification (pH < 6.4).

2.4.5.2 Clot on Boiling (C.O.B) Test

The test is quick and simple. It is one of the old tests for too acid milk(pH<5.8) or abnormal milk (e.g. colostral or mastitis milk ). If a milk sample fails in the test, the milk must contain many acid or rennet producing microrganisms or the milk has an abnormal high percentage of proteins like colostral milk. Such milk cannot stand the heat treatment in milk processing and must therefore be rejected.

Procedure:

Boil a small amount of milk in a spoon, test tube or other suitable container. If there is clotting, coagulation or precipitation, the milk has failed the test. Heavy contamination in freshly drawn milk cannot be detected, when the acidity is below 0.20-0.26% Lactic acid.

Fig 2. Equipment used in C.O.B. test

2.4.5.3. The Alcohol Test

The test is quick and simple. It is besed on instability of the proteins when the levels of acid and/or rennet are increased and acted upon by the alcohol. Also increased levels of albumen (colostrum milk) and salt concentrates (mastitis) results in a positive test.

Procedure:

The test is done by mixing equal amounts of milk and 68% of ethanol solution in a small bottle or test tube. (68 % Ethanol solution is prepared from 68 mls 96%(absolute) alcohol and 28 mls distilled water). If the tested milk is of good quality, there will be no coagulation, clotting or precipitation, but it is necessary to look for small lumps. The first clotting due to acid development can first be seen at 0.21-0.23% Lactic acid. For routine

testing 2 mls milk is mixed with 2 mls 68% alcohol.

Fig. 3. Equipment used in alcohol test

2.4.5.4. The Alcohol-Alizarin test

The procedure for carrying out the test is the same as for alcohol test but this test is more informative. Alizarin is a colour indicator changing colour according to the acidity. The Alcohol Alizarin solution can be bought ready made or be prepared by adding 0.4 grammes alizarin powder to 1 litre of 61% alcohol solution.

RESULTS OF THE TEST

Parameter Normal milk Slightly acid Milk Acid milk Alkaline Milk

PH 6.6 – 6.7 6.4 – 6.6 6.3 or lower 6.8 or higher

Colour Red brown Yellowish-brown Yellowish Lilac

Appearance of milk No coagulation no lumps

No coagulation Coagulation * No coagulation **

Note:

* = Sour milk looks yellowish with small lumps or completely coagulated.

** = Alkaline milk looks like lilac and it may be mastitis milk. Clots and flakes too, indicate mastitis milk.

2.4.5.5 Acidity test

Bacteria that normally develop in raw milk produce more or less of lactic acid. In the acidity test the acid is neutralised with 0.1 N Sodium hydroxide and the amount of alkaline is measured. From this, the percentage of lactic acid can be calculated. Fresh milk contains in this test also "natural acidity" which is due to the natural ability to resist pH changes .The natural acidity of milk is 0.16 - 0.18%. Figures higher than this signifies developed acidity due to the action of bacteria on milk sugar.

Apparatus:

A porcelain dish or small conical flask 10 ml pipette, graduated 1 ml pipette A Burette, 0.1 ml graduations A glass rod for stirring the milk in the dish

A Phenophtalein indicator solution, 0.5%in 50% Alcohol N Sodium hydroxide solution.

Fig. 4. Apparatus used be acidity test

Procedure:

9 ml of the milk measured into the porcelain dish/conical flask,1 ml Phenopthalein is added and then slowly from the burret, 0.1 N Sodium hydroxide under continuous mixing, until a faint pink colour appears.

The number of mls of Sodium hydroxide solution divided by 10 expresses the percentage of lactic acid.

2.4.5.6 Resazurin test.

Resazurin test is the most widely used test for hygiene and the potential keeping quality of raw milk. Resazurin is a dye indicator. Under specified conditions Resazurin is dissolved in distilled boiled water. The Resazurin solution can later be used to test the microbial activity in a given milk sample.

Resazurin can be carried out as:

i. 10 min test. ii. 1 hr test.

iii. 3 hr test.

The 10 min Resazurin test is useful and rapid, screening test used at the milk platform.

The 1 hr test and 3 hr tests provide more accurate information about the milk quality, but after a fairy long time . They are usually carried out in the laboratory.

Apparatus and reagents:

Resazurin tablets Test tubes with 10 mls mark 1 ml pipette or dispenser for Resazurin solution. Water bath thermostatically controlled Lovibond comparator with Resazurin disc 4/9

Fig. 5. Apparatus used in 10 min. Resazurin Test

Procedure:

The solution of Resazurin as prepared by adding one tablet to 50 mIs of distilled sterile water. Rasazurin solution must not be exposed to sunlight, and it should not be used for more than eight hours because it losses strength.

Mix the milk and with a sanitized dipper put 10 mls milk into a sterile test tube.

Add one ml of Resazurin solution, stopper with a sterile stopper, mix gently the dye into the milk and mark the tube before the incubation in a water bath, place the test tube in a Lovibond comparator with Resazurin disk and compare it colourimetrically with a test tube containing 10 ml milk of the same sample, but without the dye (Blank).

READINGS AND RESULTS (10 MINUTE RESAZURIN TEST)

Resazurin disc No. Colour Grade of milk Action

6 Blue Excellent Accept

5 Light blue v. good Accept

4 Purple Good Accept

3 Purple pink Fair Separate

2 Light pink Poor Separate

1 Pink Bad Reject

0 white Very bad Reject

2.4.5.7 The Gerber Butterfat test

The fat content of milk and cream is the most important single factor in determining the price to be paid for milk supplied by farmers in many countries.

Also, in order to calculate the correct amount of feed ration for high yielding dairy cows, it is important to know the butterfat percentage as well as well as the yield of the milk produced. Further more the butterfat percentage in the milk of individual animals must be known in many breeding programmes.

Butterfat tests are also done on milk and milk products in order to make accurate adjustments of the butterfat percentage in standardised milk and milk products.

Fig. 6. Equipment used in Gerber Butterfat test

Apparatus for DF test:

Gerber butyrameters, 0-6% or 0-8% BF Rubber stoppers for butyrometers 10.94 or 11 ml pipettes for milk 10 mls pippetes or dispensers for Gerber Acid 1 mls pippetes or dispensers for Amyl alcohol stands for butyrometers

Gerber water bath Reagents:

Gerber sulphuric acid,(1.82 g/cc) Amyl alcohol

Treatment of samples.

Fresh milk at approximately 20ºC should be mixed well. Samples kept cool for some days should be warmed to 40ºC, mixed gently and cooled to 20ºC before the testing.

Procedure:

Add 10 mIs sulphuric acid to the butyrometer followed by 10.94 or 11 mls of well mixed milk. Avoid wetting of the neck of the butyrometer.

Next add 1 ml of Amyl alcohol, insert stopper and shake the butyrometer carefully until the curd dissolves and no white particles can be seen. Place the butyrometer in the water bath at 65ºC and keep it there until a set is ready for centrifuging. The butyrometer must be placed in the centrifuge with the stem (scale) pointing towards the centre of the centrifuge.

Spin for 5 min. at ll00 rpm.

Remove the butyrometers from the centrifuge.

Put the butyrometers in a water bath maintained at 65ºC for 3 min. before taking the reading.

(Note: When transferring the butyrometers from the centrifuge into the water bath make sure that the butyrometers are all the time held with the NECK POINTING UP).

The fat column should be read from the lowest point of the meniscus of the interface of the acid-fat to the 0-mark of the scale and read the butterfat percentage.

The butyrometers should be emptied into a special container for the very corrosive liquid of acid-milk, and the butyrometers should be washed in warm water and dried before the next use.

APPEARANCE OF THE TEST

The colour of the fat column should be straw yellow.

The ends of the fat column should be clearly and sharply defined.

The fat column should be free from specks and sediment.

The water just below the fat column should be perfectly clear.

The fat should be within the graduation.

PROBLEMS IN TEST RESULTS

Curdy tests:

Too lightly coloured or curdy fat column can be due to: Temperature at milk or acid or both too low. Acid too weak. Insufficient acid. Milk and acid not mixed thoroughly.

Charred tests:

Darkened fat column containing black speck at the base is due to: Temperature of milk-acid mixture too high. Acid too strong. Milk and acid mixed too slowly. Too much acid used. Acid dropped through the milk.

2.4.5.8 The Lactometer test

Addition of water to milk can be a big problem where we have unfaithful farm workers, milk transporters and greedy milk hawkers. A few farmers may also fall victim of this illegal practice. Any buyer of milk should therefore assure himself/herself that the milk he/she purchases is wholesome and has not been adulterated. Milk has a specific gravity. When its adultered with water or other materials are added or both misdeeds are committed, the density of milk change from its normal value to abnormal. The lactometer test is designed to detect the change in density of such adulterated milk. Carried out together with the Gerber butterfat test, it enables the milk processor to calculate the milk total solids (% TS ) and solids not fat (SNF). In normal milk SNF should not be below 8.5% according to Kenya Standards(KBS No 05-l0:-1976).

Procedure:

Mix the milk sample gently and pour it gently into a measuring cylinder (300-500). Let the Lactometer sink slowly into the milk. Read and record the last Lactometer degree (ºL) just above the surface of the milk. If the temperature of the milk is different from the calibration temperature (Calibration temperature may be=20 0C ) of the lactometer, calculate the temperature correction. For each ºC above the calibration temperature add 0.2ºL; for each ºC below calibration temperature subtract 0.2 ºL from the recorded lactometer reading.

EXAMPLE: Calibration temperature of lactometer 20ºC.

Fig 7. Equipment used for determination of milk density

Sample Milk temperature Lactometer reading

Correction True reading

No.1 17 ºC 30.6 ºL - 0.6 ºL 30.0 ºL

No.2 20 ºC 30.0 ºL Nil 30.0 ºL

No.3 23 ºC 29.4 ºL + 0.6 ºL 30.0 ºL

For the calculations, use lactometer degrees, and for the conversion to density write 1.0 in front of the true lactometer reading ,i.e. 1.030 g/ml. Clever people may try to adulterate milk in such a way that the lactometer cannot show the adulteration. But look to see if there is an unusual sediment from the milk at the bottom of the milk can and taste to find out if the milk is too sweet or salty to be normal. Samples of milk from individual cows often have lactometer reading outside the range of average milk, while samples of milk from herds should have readings hear the average milk, but wrong feeding, may result in low readings. Kenyan standards expects milk to have specific gravity of 1.026 -1.032 g/ml which implies a Lactometer reading range of 26.0 -32.0 ºL. If the reading is consistently lower than expected and the milk supplier disputes any wrong doing arrange to take a genuine sample from the supplier (i.e. inspect milk right from source).

2.4.5.9 Freezing Point Determination

The freezing point of milk is regarded to be the most constant of all measurable properties of milk. A small adulteration of milk with water will cause a detectable elevation of the freezing point of milk from its normal values of -0.54ºC. Since the test is accurate and sensitive to added water in milk, it is used to detect whether milk is of normal composition and adulterated.

Fig. 8. A Cryoscope is used for determination of freezing point of milk.

2.4.5.10 Inhibitor test.

Milk collected from producers may contain drugs and/or pesticides residues. These when present in significant amounts in milk may inhibit the growth of lactic acid bacteria used in the manufacture of fermented milk such as Mala, cheese and Yoghurt, besides being a health hazard.

Principle of the method: The suspected milk sample is subjected to a fermentation test with starter culture and the acidity checked after three (3) hours. The values of the titratable acidity obtained is compared with titratable acidity of a similarly treated sample which is free from any inhibitory substances.

Materials:

test tubes Starter culture lml pipette water bath material for determination of titratable acidity (Fig.9)

Fig. 9. Materials used to test inhibitory substances in milk

Procedure:

Three test tubes are filled with l0 ml of sample to be tested and three test tubes filled with normal milk.

All tubes are heated to 90 0C by putting them in boiling water for 3 - 5 minutes.

After cooling to optimum temperature of the starter culture (30,37, or 42ºC), 1 ml of starter culture is added to each test tube, mixed and incubated for 3 hours.

After each hour, one test tube is from the test sample and the control sample is determined.

Assessment of results:

If acid production in suspected sample is the same as the normal sample, then the suspect sample does not contain any inhibitory substances;

If acid production as suspect sample is less than in the normal milk sample, then, the suspect sample contains antibiotics or other inhibitory substances.

3. QUALITY CONTROL OF PASTEURISED MILK

When milk is pasteurised at 63ºC for 30 min in batch pasteuriser or 72ºC for 15 seconds in heat exchanger, continuous flow pasteurisers, ALL PATHOGENIC BACTERIA ARE DESTROYED, there by rendering milk safe for human consumption. Simultaneously various enzymes present in milk, and which might affect its flavour, are destroyed.

In order to determine whether or not milk has been adequately pasteurised, one of the enzymes normally present in milk phosphatase, is measured. A negative phosphatase result indicates that the enzyme and any pathogenic bacteria have been destroyed during pasteursation. If it is positive, it means the pasteurisation process was inadequate and the milk may not be safe for human consumption and will have a short shelf life.

Test tubes 5 mls pipettes 1 ml pipettes l00 ml volumetric flask 500 ml volumetric flask water bath at 37ºC

Note: All glassware must be rinsed, cleaned, rinsed in chromic acid solution and boiled in water for 30 min.

Reagent:

Buffer solution:

Is mixed by 0.75g anhydrous sodium carbonate and l.75g Sodium bicarbonate in 500 ml distilled water.

Buffer-substrate solution:

Place 0.l5 g of di-sodium paranitrophenylphosphate(the substrate)into a clean 100ml measuring cylinder.

Add the buffer solution to make to 100 ml mark.

Store this buffer-substrate solution in a refrigerator and protected against light. It should not be used after one week. Prepare a fresh stock.

Procedure:

Pipette 5mls buffer-substrate solution into a test tube, stopper and warm the solution in the water bath at 37ºC. Add to the test tube 1ml of the milk to be tested, stopper and mix well and place in water bath at 37ºC.

Prepare a blank sample from boiled milk of the same type as that undergoing the test. Incubate both the test samples and the blank sample at 37ºC for 2hrs. After incubation, remove the tubes and mix them thoroughly.

Place one sample against the blank in a Lovibond comparator" ALL PURPOSES" using A.P.T.W. disc and rotate the disc until the colour of the test sample is matched and read the disc number.

Interpretation:

Disc Reading after 2 hrs incubation at 37ºC Remarks

0-10 Properly pasteurised

10-18 Slightly under pasteurised

18-42 UNDER PASTEURISED

> 42 NOT PASTEURISED

REFERENCES:

ILCA Manual No.4, Rural Dairy Technology. Experiences from Ethiopia.

IDF Doc. No.9002, Handbook on Milk collection in Warm Developing Countries. International Dairy Federation, Brussels, Belgium.

Marshall, R.T. 1992 .Standard Methods for the determination of Dairy Products. 16th ed. Publ. American Public Health Association.

HINT ON MAKING THE PROJECT REPORT BODY

when doing the introduction be specific tell about what is milk, kind of milk, able to differentiate the condensed and evaporated milk, fresh milk and any source of milk ( cow or soya ) the benefit and function of milk in our body, industrial application and many others...another also is that you tell what enzymes are present in that milk and how our body react with that.when we are doing this in the lab we do the following test:lactic acid ( quantitative approach )pH ( titration method, pH meter method )preservative test ( formaldehyde test and sodium benzoate test)calcium ( quantitative test)potassium ( qualitative and quantitative approach )percent ash testlactobacilli ( quantitativre test using the modern method )sugar test ( reducing or non-reducing )