Introductoin:
Fish Freezing is a process of preservation in which the temperature of fish and and fishery products is lowered at -40˚C or below with most of water inside of tissue turning into ice.
Icing and chilling can keep the fish for a few weeks only.Fish is required to kept for longer periods,for example to even out supplies during gluts,to enable fish to be distributed and solid in distant markets,and to stock pile products for lean seasons.If fish are properly frozen and stored at correct temperature,it is possible to provide a product which closely resembles fresh fish;in many cases consumers are unable to distinguish between a piece of fresh fish and a piece of frozen fish.
Mechanism of freezing of fish muscle tissue:
Clucas and Ward (1996)have described the freezing mechanism in fish muscle tissue in detail.Freezing may be defined as the processing of fish by lowering the temperature of fish body so that all most all of the water inside becomes frozen.Fresh fish contains approximately 80% water.At normal atmospheric pressure,pure water will change from liquid to solid (ice) at 0˚C,i.e.,it will freez.However,the water in flesh contains dissolved salts and chemicals which have the effects of lowering the temperature at which the tissue water begins to freez.The exact temperature of freezing varies between species but is usually between -1 and -2˚C.As the temperature drops below this critical temperature at which the remaining water freezes is lowered further as the solution becomes more concentrated.At -5˚C ,when it would appear that all the water is frozen,over 20% of the water in the muscle may still be unfrozen;at -30C,about 10% of the water remains unfrozen but at -40˚C almost all of the bound water is frozen.
In order to change the physical state of a substance from aliquid to a solid ,which is is that happens when fish are frozen,energy or latent heat has to be removed from the substance.To lower the temperature of 1g of water by 1˚C at temperatures above 0˚C,4.2 J of heat must be removed;this is known as the specific heat.However,to change liquid water at o˚C to solid ice at 0˚C,334.7J must be removed from each gram of water;this is known as the latent heat.
The specific heat of liquid water is therefore 4.2 and the latent heat of fusion for water is about 80 times as much,i.e.334.7 J.The specific heat of solid water (ice)at temperatures below o˚C is 2.1 J/g.For most practical purposes,it is assumed that fish(which contain 70-80% water)have the same specific and latent heat values as water.
If heat is removed from fish at a constant rate,there will be a period while the fish is freezing when the temperature will not drop.This period occurs while latent heat is being removed and the liquid water changing to solid;it lasts until approximately 75% of the water has been frozen,after which the temperature will begin to fall again.
The temperature profile of freezing fish has three stages:
i.During stage I:the temperature falls fairly rapidly,at a more or less constant rate,to just below 0˚C while the specific heat is being removed.
ii.During stage II:the temperature remains fairly constant at about -1˚Cto-5˚ while the latent heat is being removed and the liquid water is changing to ice;this is known as the thermal arrest period(TAP).
iii.During stage III:the temperatures drops rapidly again while the specific heat of ice is being removed and most of the remaining water freeze.
Using simple mathematics,the theoretical amount of energy required to freeze fish can be calculated.This is demonstrated in the following example.
Example: 1 kg of fish at 25˚C needs to be frozen to -30˚C(from Clucas and Ward,1996).
Step I
During this stage 4.2 J of energy needs to be extracted from each g of fish for each 1˚C drop in temperature.The of 1000g of fish will be lowered from +25˚C to -1˚C,i.e.,by 26˚c.The energy required to be removed will be
1000×26×4.2(specific heat of water)=109200 J or 109.2 KJ.
Step II
During this stage,334.7 J of energy for each g of material frozen needs to be extracted.In this example,1000 g of fish are to be frozen.The energy required will therefore be
1000×334.7(latent heat)=334700 J or 334.7 KJ.
Step III
During the last stage,2.1 J of energy for each g of material,for each 1˚C drop in temperature needs to be extracted.In the example the temperature of 1000 g fish will be lowered from -1˚C to -30˚C,i.e.,by 29˚C.The energy required will be equal to
1000×29×2.1(specific heat of ice)=60900 J or 60.9 KJ.
To summarize:
Step I 1000×26×4.2=109.2 KJ
Step II 1000×334.7 =334.7KJ
Step III 1000×29×2.1=60.9 KJ
Total =504.8KJ
Therefore,to freeze 1 kg of from 25˚Cto-30˚C requires the removal of 504.8 Kj heat.
Thermal arrest period (TAP)
From the example above,it is thus apparent that more than 65% of the energy extracted in freezing occurs during stage II,is called the TAP when little or no drop in temperature is taking place.This period is taking place.This period is critical if good quality frozen products are to be produced.Ideally,the fish or should pass through the TAP in as short a time as possible for the following reasons:
(a)show freezing produces large ice crystals in the cells of the fish which,as they can be larger than the cells themselves,can break the cell walls;
(b)as water begins to freeze in the flesh,it tends to freeze out as pure water;the remaining liquid water therefore contains a higher concentration of salts and enzymes,and these can accelerate autolysis;
(c)as it takes long time to freeze body fluid,drip-loss and as a result,thaw drip is pronounced in case of slow freezing that removes valuable nutrients.
The end result and apparent effect of slow freezing is the textural change caused by the break-up of the fish.In addition,water originally bound within the cells will be lost,and this will produce increased drip when the are thawed.Thaw drip can cause a considerable loss in weight and a poor dry texture when eaten,it can also be increased by fluctuating cold store temperatures.Even if a product is frozen quickly so that only small ice crystals are formed at this stage,the water may partically deforst if the product is held at fluctuating cold store temperatures.The small crystals will become larger and cause the same sort of effect on complete defrosting.
From a textural point of view,it is unlikely that a taste panel would detect any difference between fish passing through the TAP in 1 h and those frozen in 8 h;once the freezing times extend beyond of 12 h,however,the difference may become apparent.Freezing times of 24 h or more will almost certainly result in inferior products,and very long freezing times can result in the products being unfit for consumption because of bactrerial spoilage.
Types of freezing
1.Slow freezing:This is the process of gradual lowering of temperature of fish up to freezing.The temperature is usually -25˚C or lower but may vary from -15˚C to -29˚C and freezing may take 3 to 72 hours.
2.Quick freezing :This is the process of sudden decrease in the temperature of fish up to freezing.The temperature is usually -40˚C and freezing time is 30 min to 1 hour.In practical,quick freezing of fish is generally practiced as lowering the temperature from -1˚ to -5˚c (TAP) in around 30 min and further reducing the temperature at the end of the freezing period to the recommended storage temperature of -40˚C.This has two important parameters:
*fast passage through the arrest period; and
*reduction to low temperature.
Advantages of quick freezing:
i.smaller ice crystals are formed,hence less mechanical destruction of fish tissue cells;
ii.Shorter period of solidification and less time for diffusion of soluble materials and for separation of ice;
iii.More prompt prevention of microbial growth;
iv.More rapid slowing of enzyme action.
Conclusion:
It is possible to keep fish in a condition close to that of fresh fish using ice or chilling system for weeks at most.There are circumstances however,when fish need to kept for longer periods,for example to even out supplies during gluts,to enable fish to be distributed and solid in distant market.If fish are properly frozen and stored at the correct temperature,it is possible to provide a product which closely resembles fresh fish
Fish Freezing is a process of preservation in which the temperature of fish and and fishery products is lowered at -40˚C or below with most of water inside of tissue turning into ice.
Icing and chilling can keep the fish for a few weeks only.Fish is required to kept for longer periods,for example to even out supplies during gluts,to enable fish to be distributed and solid in distant markets,and to stock pile products for lean seasons.If fish are properly frozen and stored at correct temperature,it is possible to provide a product which closely resembles fresh fish;in many cases consumers are unable to distinguish between a piece of fresh fish and a piece of frozen fish.
Mechanism of freezing of fish muscle tissue:
Clucas and Ward (1996)have described the freezing mechanism in fish muscle tissue in detail.Freezing may be defined as the processing of fish by lowering the temperature of fish body so that all most all of the water inside becomes frozen.Fresh fish contains approximately 80% water.At normal atmospheric pressure,pure water will change from liquid to solid (ice) at 0˚C,i.e.,it will freez.However,the water in flesh contains dissolved salts and chemicals which have the effects of lowering the temperature at which the tissue water begins to freez.The exact temperature of freezing varies between species but is usually between -1 and -2˚C.As the temperature drops below this critical temperature at which the remaining water freezes is lowered further as the solution becomes more concentrated.At -5˚C ,when it would appear that all the water is frozen,over 20% of the water in the muscle may still be unfrozen;at -30C,about 10% of the water remains unfrozen but at -40˚C almost all of the bound water is frozen.
In order to change the physical state of a substance from aliquid to a solid ,which is is that happens when fish are frozen,energy or latent heat has to be removed from the substance.To lower the temperature of 1g of water by 1˚C at temperatures above 0˚C,4.2 J of heat must be removed;this is known as the specific heat.However,to change liquid water at o˚C to solid ice at 0˚C,334.7J must be removed from each gram of water;this is known as the latent heat.
The specific heat of liquid water is therefore 4.2 and the latent heat of fusion for water is about 80 times as much,i.e.334.7 J.The specific heat of solid water (ice)at temperatures below o˚C is 2.1 J/g.For most practical purposes,it is assumed that fish(which contain 70-80% water)have the same specific and latent heat values as water.
If heat is removed from fish at a constant rate,there will be a period while the fish is freezing when the temperature will not drop.This period occurs while latent heat is being removed and the liquid water changing to solid;it lasts until approximately 75% of the water has been frozen,after which the temperature will begin to fall again.
The temperature profile of freezing fish has three stages:
i.During stage I:the temperature falls fairly rapidly,at a more or less constant rate,to just below 0˚C while the specific heat is being removed.
ii.During stage II:the temperature remains fairly constant at about -1˚Cto-5˚ while the latent heat is being removed and the liquid water is changing to ice;this is known as the thermal arrest period(TAP).
iii.During stage III:the temperatures drops rapidly again while the specific heat of ice is being removed and most of the remaining water freeze.
Using simple mathematics,the theoretical amount of energy required to freeze fish can be calculated.This is demonstrated in the following example.
Example: 1 kg of fish at 25˚C needs to be frozen to -30˚C(from Clucas and Ward,1996).
Step I
During this stage 4.2 J of energy needs to be extracted from each g of fish for each 1˚C drop in temperature.The of 1000g of fish will be lowered from +25˚C to -1˚C,i.e.,by 26˚c.The energy required to be removed will be
1000×26×4.2(specific heat of water)=109200 J or 109.2 KJ.
Step II
During this stage,334.7 J of energy for each g of material frozen needs to be extracted.In this example,1000 g of fish are to be frozen.The energy required will therefore be
1000×334.7(latent heat)=334700 J or 334.7 KJ.
Step III
During the last stage,2.1 J of energy for each g of material,for each 1˚C drop in temperature needs to be extracted.In the example the temperature of 1000 g fish will be lowered from -1˚C to -30˚C,i.e.,by 29˚C.The energy required will be equal to
1000×29×2.1(specific heat of ice)=60900 J or 60.9 KJ.
To summarize:
Step I 1000×26×4.2=109.2 KJ
Step II 1000×334.7 =334.7KJ
Step III 1000×29×2.1=60.9 KJ
Total =504.8KJ
Therefore,to freeze 1 kg of from 25˚Cto-30˚C requires the removal of 504.8 Kj heat.
Thermal arrest period (TAP)
From the example above,it is thus apparent that more than 65% of the energy extracted in freezing occurs during stage II,is called the TAP when little or no drop in temperature is taking place.This period is taking place.This period is critical if good quality frozen products are to be produced.Ideally,the fish or should pass through the TAP in as short a time as possible for the following reasons:
(a)show freezing produces large ice crystals in the cells of the fish which,as they can be larger than the cells themselves,can break the cell walls;
(b)as water begins to freeze in the flesh,it tends to freeze out as pure water;the remaining liquid water therefore contains a higher concentration of salts and enzymes,and these can accelerate autolysis;
(c)as it takes long time to freeze body fluid,drip-loss and as a result,thaw drip is pronounced in case of slow freezing that removes valuable nutrients.
The end result and apparent effect of slow freezing is the textural change caused by the break-up of the fish.In addition,water originally bound within the cells will be lost,and this will produce increased drip when the are thawed.Thaw drip can cause a considerable loss in weight and a poor dry texture when eaten,it can also be increased by fluctuating cold store temperatures.Even if a product is frozen quickly so that only small ice crystals are formed at this stage,the water may partically deforst if the product is held at fluctuating cold store temperatures.The small crystals will become larger and cause the same sort of effect on complete defrosting.
From a textural point of view,it is unlikely that a taste panel would detect any difference between fish passing through the TAP in 1 h and those frozen in 8 h;once the freezing times extend beyond of 12 h,however,the difference may become apparent.Freezing times of 24 h or more will almost certainly result in inferior products,and very long freezing times can result in the products being unfit for consumption because of bactrerial spoilage.
Types of freezing
1.Slow freezing:This is the process of gradual lowering of temperature of fish up to freezing.The temperature is usually -25˚C or lower but may vary from -15˚C to -29˚C and freezing may take 3 to 72 hours.
2.Quick freezing :This is the process of sudden decrease in the temperature of fish up to freezing.The temperature is usually -40˚C and freezing time is 30 min to 1 hour.In practical,quick freezing of fish is generally practiced as lowering the temperature from -1˚ to -5˚c (TAP) in around 30 min and further reducing the temperature at the end of the freezing period to the recommended storage temperature of -40˚C.This has two important parameters:
*fast passage through the arrest period; and
*reduction to low temperature.
Advantages of quick freezing:
i.smaller ice crystals are formed,hence less mechanical destruction of fish tissue cells;
ii.Shorter period of solidification and less time for diffusion of soluble materials and for separation of ice;
iii.More prompt prevention of microbial growth;
iv.More rapid slowing of enzyme action.
Conclusion:
It is possible to keep fish in a condition close to that of fresh fish using ice or chilling system for weeks at most.There are circumstances however,when fish need to kept for longer periods,for example to even out supplies during gluts,to enable fish to be distributed and solid in distant market.If fish are properly frozen and stored at the correct temperature,it is possible to provide a product which closely resembles fresh fish