Freezing Class

8/7/2019 Freezing Class

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Fish is a highly perishable commodity & spoilage of fish begins as soon as fish dies.

In tropical conditions, fish spoils quite rapidly i.e. within few hours of landing if not

properly cooled.

The spoilage rate can be reduced by good handling practices and effective

temperature control from the very beginning.

Increasing the temperature from 0o to 5oC doubles the spoilage rate.

In raw fish spoilage takes place mainly due to three reasons Enzymatic action Microbial action

Chemical action oxidation of fish lipids

Enzymes and bacteria do not cause any deterioration in living cell because of the

natural defensive mechanism but in dead fish enzyme are responsible in autolytic

changes. Fish gut is rich in proteolytic enzymes and in dead fish it digest the gut and

belly region and making fish very soft.

Bacteria present on the surface, gills & gut of fish invade the dead fish anddecompose the tissue resulting in undesirable changes e.g. off odour, off flavour,

slime production, gas production, discoloration, soft texture etc.

Enzymes are protein catalysts which can change the substances without being

changed. They are also able to continue working even after death. This process by

which postmortem changes take place due to the action of intrinsic enzymes, iscalled autolysis or self digestion


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Autolytic spoilage which causes due to enzymatic action First enzymatic changes in fish muscle is hydrolysis of glycogen to lactic acid

called glycolysis. As Lactic acid accumulates pH falls down which accomplished

by the natural post-mortem, stiffening of body called rigor mortis. Rigor mortis

begins from 0-8 hrs after death and last up to 10-120 hrs. depending on the

exhaustion, temperature, handling (physical damage), size of fish etc. e.g. rigormortis is very rapid in active fish.

Nucleotide degradation causes flavour change

Adenosine Tri Phosphate (ATP)

Adenosine Di Phosphate (ADP)

Adenosine Mono Phosphate (AMP)

Inosine Mono Phosphate (IMP) In fresh fish gives characteristic fish flavour

Inosine

RiboseHypoxanthine Bitter taste

Belly Bursting

If large content of enzyme in digestive tract gut is dissolved and

bacteria proliferate and produce gases like CO2, H2 which leads to belly bursting

Colour changes1. Fat oxidation and 2. Pigment formation


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Refrigeration:Refrigeration is the technique of producing cold

and removing heat from the substance or source and

lowering its temperature below those of the immediatesurroundings.Refrigerant:

Refrigerant is the volatile liquid substance which

boils under pressure and absorbs heat from the

surrounding objects. They have low boiling points andhigh latent heats of evaporation.e.g. CO2, SO2, Methyl

bromide, Ammonia (R-717), Chloro Fluoro Carbons(CFC or R-12), Hydro Chloro Fluoro carbon (HCFC or R-

2) etc.Simple Refrigeration System:Refrigeration is accomplished by the evaporation

of a liquid refrigerant thereby extracting heat from themedium to be cooled.


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Refrigeration Cycle:It involves the steps to remove heat from the

evaporating refrigerant by again converting it into the

liquid state in order that it may be used repeatedly in a

continuous process. For this it is necessary that the gas

be compressed, permitting condensation at the normal

temperatures. This is done by compressors requiring

mechanical energy.It consists of

1. Compressor

2. Condenser

3. Receiver

4. Expansion or regulator valve

5. Evaporator


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Receiver:

It is a container containing the refrigerant maintained asa liquid under high pressure. It receive the refrigerantfrom the condenser.

Expansion Valve:

It maintains the necessary pressure differencecorresponding to the required difference of temperaturebetween condenser and evaporator. It regulates the flowof liquid refrigerant.

Evaporator:

Due to fall in pressure liquid refrigerant startsevaporating by taking up the latent heat from thesurroundings of the vapours and leaves the evaporator in

vapour form.Compressor:

The vapour coming out of the evaporator is compressedunder pressure. The compressor is normally driven byelectric motor.

Condenser:


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VAPOUR-COMPRESSION REFRIGERATION SYSTEM


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Freezing is one of the best methods of fish preservation. It giveslong term preservation with the quality almost near to freshproduct.

Fish contains about 75-80% by weight of water. The process offreezing converts most of the water into ice.

At normal atmospheric pressure, pure water will change fromliquid to solid (ice) at 0oC i.e. it will freeze.

Water in fish contains several dissolved and colloidalsubstances like sugars, salts etc. whereas proteins are presentin colloidal suspension. These all depress the freezing pointbelow 0C and the extent of depression of freezing point isproportional to the concentration of the solute.

About 75-80% of the water is frozen between -1C to -5C. Thistemperature range is known as the critical zone or freezing zone.

During freezing, water gradually converted to ice and theconcentration of dissolved organic and inorganic saltsincreases, depressing the freezing point continuously.

Freezing of fish and

Biochemical changes during freezing


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Even at a temperature of -25C only 90-95% of water is actuallyfrozen. Remaining unfrozen water is called bound wateras it isbound to the carbonyl and amino groups of proteins.

In order to change the water from liquid to solid (freezing)

energy or latent heat has to be removed from the muscle. To lower the temperature of 1 gram of water by 1oC at

temperature above 0oC, 4.2 joule of heat needs to be removed.

This is known as specific heat. To change liquid water at 0oC to solid ice, 334.7 joule must be

removed from each gram of water. This is know as latent heat.1 calorie = 4.184 4.2 joules

Specific heat of liquid water = 4.2 joule or 1 calorie Latent heat of fusion for water = 334.7 joule or 80 calories

(80 times higher)

Specific heat of ice = 2.1 joule or calorie During freezing the temperature falls rapidly to below 0C and

then falls very slowly until most of the water is changed to ice.

Further the temperature drops quickly. Since spoilage

continues at temperatures just below 0C, it is important to

pass the critical zone quickly.


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Tempe

rature

Freezing Time

TYPICAL FISH FREEZING CURVE


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The temperature profile of freezing fish is divided into three

stages:-

Stage 1

The temperature falls rapidly up to just below 0oC while

the specific heat of water is being removed. In many casessome super cooling occurs.

Stage 2

The temperature remains more or less constant at about

1OC to 5OC while the latent heat is being removed and

liquid water is changing to ice, this is known as the thermal

arrest period or zone of maximum crystallization. In this stage

bulk of water changes to ice

Stage 3

During this stage temperature drops rapidly again whilethe specific heat of ice is being removed and most of the

remaining water freezes.

Super cooling is a condition attained by a liquid where its

temperature falls below its freezing point without crystallizationtaking place.


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Freezing process Heat is transferred from one medium to another

medium Heat is removed from the product by conduction and by convection

Heat is transferred from the center of the product to its outsidesurface - by conduction

Heat transfer from the outside surface to the cooling medium

by convection or radiation

Freezing point :

Freezing point of a liquid is the temperature at which its vapourpressure is in equilibrium with that of the solid.

Freezing begins in the fish usually at 1oC to 2oC

Eutectic or Cryohydric temperature:

The highest temperature at which maximum solidification of the system

can be achieved. It vary widely depending on the solutes involved.

Temperature % water frozen- 1OC Crystallization begins- 3OC 70 % water frozen max. water freezes- 5OC 80 % water frozen

- 25O

C about 90 % water frozen- 50OC to - 60OC almost all water frozen


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About 90 % of bacteria present in the fish are destroyed during

freezing as they belongs to the mesophilic group. The

psychrophilic bacteria remain inactive as long as fish is in frozen

condition because of low temperature and non availability of free

water for their activity. The activity of the enzymes is considerablyretarded at frozen temperature.

Bacteria: Optimum temp. for growth

Psycrophillic 13 to 15oC ( -15 to + 10oC)

Mesophillic 30 to 40oC ( +10 to + 50oC)Thermophillic 55oC ( +50 to + 90oC)

Effect of temperature on fish spoilage

A drop in temperature of 5oC can half the rate of spoilage

Reduction in temp. Rate of spoilage Extension of shelf life0 100 -

5 50 x 2

10 25 x 4

15 12.5 x 8

20 6.25 x 16


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Freezing Rate:-

Rate of freezing is faster near the surface and slower in the

centre. Freezing rate depends on various factors :- Method of heat transfer

Size, shape and thermal properties of fish Packaging material used and method of packaging etc.

- packages should be tight and air pockets should be avoided- Rate of cooling is inversely proportional to the thickness i.e. if the

thickness is double, it will take four times as long for centre tocool.

Quick Freezing

The freezing process in which the fish passes through the zone of

maximum crystallization ( 1o

C to 5o

C) in 30 min or less is referredas quick freezing.

Slow freezing : 2 5 mm/ hr.

Quick freezing : 5 50 mm/ hr.

Rapid freezing : 50 100 mm/ hr.

Ultra rapid freezing : 100 1000 mm/ hr.


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FAST FREEZING & SLOW FREEZING

Fast freezing will control the extent of protein

denaturation by limiting the exposure of tissues to the

salts concentrated by ice formation.

The longer the exposure, the larger is the damage to the

proteins and to other enzyme systems and their

substrate

DOUBLE FREEZING:

It is a common practice to preserve fish by freezing

onboard vessels when the voyage lasts for few weeks.

On reaching the shore, fish is thawed and reprocessed.This practice is also adopted for making consumer

packs and value added products like IQF shrimps, fish

fillets sea food mix etc..


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GLAZING :

The main problem of storage of frozen products

due to long term exposure to air are dehydration and fat

oxidation. In order to prevent both dehydration and fatoxidation, the fish and fish products are glazed by

dipping in or spraying wit ice cold chilled water. This

provides a uniform protective thin coating of ice.

In case of block frozen products, glaze water is

added to the pack before the freezing and also after the

freezing by dipping in ice cold glaze water, whereas for

IQF products it is done after freezing by spraying or

dipping in ice cold glaze water.

THAWING :

Thawing is the reverse of the freezing process.

When heat energy is added to the frozen fish, the ice is

converted into water and fish takes up the original state.


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FREEZING METHODS AND EQUIPMENTS

There are three basis methods of freezing fish and fish

products

Those in which a continuous stream of cold air blowover the fish and absorbs heat energy from the product

as it passes over them, thereby reducing their

temperature. These are generally known as air blast

freezers.

Those in which heat is absorbed when the fish are

placed in contact with a refrigerated surface. These are

generally known as contact freezers or plate freezers.

Those in which heat is rapidly absorbed from the fish

products immersed in or spraying with a refrigerated

liquid. These are generally known as immersion

freezers or spray freezers.

Cryogenic freezers

FREEZING IN AIR


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FREEZING IN AIR1. Freezing in still air

2. Air blast freezer

3. Continuous air blast freezer

Belt freezer

Batch freezer

1. Fluidized bed freezer

Freezing in still air : Freezer consists of an insulated room or cabinet

maintained at 30oC to 40oC.

Fish is placed in aluminum trays and the trays are kept on

shelves made of pipes or coils though which refrigerant is

circulated. Time taken for freezing may be 12 hrs. or more which is

influenced by presence or absence of packaging, freezer

temperature, size and shape of fish, load on freezer etc..

This is the least expensive method of freezing, however isthe slowest method as well.


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AIR BLAST FREEZERS These freezers are generally constructed in the form of

insulated rooms or tunnels. They are used for freezing fishery

products such as shrimp, fish fillets, breaded precooked

products packed in packages and small whole fishes. Air is cooled by blowing by a fan through the finned cooling

coil. A stream of cold air vigorously move over the fish and

absorbs heat energy from the product as it passes over them,

thereby reducing their temperature..

Air temperature maintained at 35oC to 40oC is forced around

the product at speeds ranging from 5 to 7 m/s.

Freezing time varies from 6 to 8 hours depending upon the

thickness of the product.


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AIR BLAST FREEZERS

Advantages:

A versatile method for freezing applicable to any size

and shape of fish.

Suitable for operation as batch or continuous process.

Advantageous for producing IQF and coated products.

Disadvantages:

The main disadvantages of freezer is that freezing time

tend to be longer.

Consume more energy mainly because of the relative

poor heat transfer properties of air.

Occupy more space compared to other types of freezers

Unpackaged fish will undergo excessive dehydration


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Batch continuous freezer or Tunnel freezer:

Fish placed on trolleys or hang singly if the fish is large and move

through the freezer on rails or wheels.

Loaded trolleys are pushed into the freezers at one end and whenthe product is fully frozen, they are removed from the other end to

make room for a fresh batch at the entry point.


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Continuous Belt freezer: An improvement in air blast freezing is to use a conveyer to

move the fish continuously through the room or the tunnel.

They are used for small individual products which can befrozen within 30 minutes. Longer freezing time would

require longer belt which would be cumbersome and costly. Double or treble belt may be installed or spiral belt freezers

in which a long belt can be compacted into a relatively

limited space, are now available. In these type freezers the air flow is from top to bottom or

across the belt. The speed of conveyor belt can be varied as per the type of

product being frozen. Individual Quick freezers are the recent advancement of

these type of freezers. Belt freezers use a belt generally made up of stainless

steel wire mesh or HD-PVC and it moves inside the

chamber and the fish to be frozen is kept on the belt.


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Vertical plate freezers:

The principle is the same as for Horizontal plate freezer.

This type of freezer was developed primarily for freezing of whole fish at sea

on distant water vessels, now also these are most commonly used for

freezing in factory trawlers at sea. The freezer consists of a series of vertical plates with spaces in between

them, forming partitions in a container. The spaces between the plates are

known as stations.

Fish are dropped between the plates until each station is full. The plates are

then closed together to form fish blocks. It is useful for bulk freezing of fish in large blocks.

Temperature ranges between at 35oC to 40oC and fish freeze in 2 3

hours.

Advantages : Freezing trays are not required

Operation is simple

Require less space

Easyto install


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Twenty-station vertical plate freezer with top unloading arrangement


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ROTARY DRUM FREEZER Rotary drum freezer is a refrigerated stainless steel drum

revolving at a preset speed.

Freezing time can be adjusted by adjusting the speed ofrevolution. Unpacked products like fish fillets, shrimp etc. can be

rapidly frozen using rotary drum freezer. The material to be frozen is fed on the external surface of

the drum through a conveyor. It adheres immediately tothe surface of the drum by freezing the water on the

surface of the material. At the end of one revolution the frozen product is scraped

off and is passed through an automatic glazer beforepackaging.

As there is no movement of air and freezing is very rapid,

there will be little or no loss of weight during freezing.


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IMMERSION FREEZERS

In this method, freezing is achieved by immersion in or

spraying with a refrigerant that remains liquid throughout

the process. Refrigerated aqueous solutions of propylene glycol,

glycerol, sodium chloride, calcium chloride and mixture of

sugars and salts can be used as the medium for freezing.

Immersion freezing allows direct contact of every surfaceof the material with the freezing medium and thus ensures

very efficient heat transfer.

In brine spray freezing, super cooled brine is sprayed on

the fish for freezing. Some times material is taken in metallic containers / cans

and keep immersed in calcium chloride brine solution like

in case of ice making plants.


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CRYOGENIC FREEZER:


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CRYOGENIC FREEZER: The principle Involved here is to freeze the material with

liquefied gases at extremely low temperatures, such as Liquid

Nitrogen (-196C), Liquid carbon dioxide (-71C).

The liquid is carefully sprayed in required quantities on thematerial passing on a conveyor belt.

Advantages: Dehydration loss is negligible. Freezing damage is minimum.

Attain individually quick freezing of the product. Equipment is simple and adaptable to various production rate

and product sizes. Suitable for continuous operation and high output can be

achieved with minimum space and time. Initial investment is low.

Disadvantages : High operation cost as refrigerant is not reused. Not suitable for big size fishes.

Chances of freezer burn are more due to extremely low temp

Freezing Class

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