By using ice and other methods of chilling, fish can be kept in a fresh condition for extended periods. Chilling should reduce the temperature of the fish to around 0°C. This slows down the rate of growth of bacteria and also the rate at which chemical reactions take place.
The temperatures used in chilling should not be low enough to freeze the fish.
The object of freezing the fish is to lower the temperature to a point where the spoilage almost stops. This means that when the product is thawed after being in cold storage, it is almost impossible to tell it from the fresh fish.
By reducing the temperature of the fish much further, to between - 20°C and - 30°C, the shelf life of the fish may be extended a lot. At these temperatures the fish is, of course, frozen. The shelf life may be counted in months rather than days.
In both chilling and freezing, the shelf life will depend on the species of the fish. Fatty fish, like herring, generally have a shorter shelf life. An extended shelf life is useful in any perishable food industry.
Sudden change in levels of available supply is a real problem in the fresh fish industry. Landings can vary enormously due to season, weather and quota restrictions amongst other things. These peaks and troughs in supply can be evened out by freezing, and storing surplus fish when landings are good. This also keeps the price to the customer more stable.
THE NEED FOR QUALITY Selecting Raw Materials The most important point to remember when selecting raw materials is that freezing cannot improve the quality of fish. Nothing can reverse the process of spoilage in fish.
The things which will affect the quality of the frozen fish product are:
The quality of fish changes during the year depending on things such as spawning times, feeding available and parasites. If the fish is of poor quality when it is caught it will not be suitable for freezing no matter how it has been handled.
Even fish which is of good quality when it is caught can be made unsuitable for freezing by bad handling. It can be damaged by being squashed, kept for too long or at too high temperature. Only fish of the highest quality should be used for freezing. Freshness is probably the most important aspect of quality in fish.
FREEZING What happens when fish freeze?
The flesh of fish contains approximately 80% water. Under normal conditions, pure water will change from a liquid to a solid at 0°C i.e. it will freeze.
If we add salts and other chemicals to water it has the effect of lowering the temperature at which the water will freeze.
An example in everyday life of how this works is putting salt on icy roads.
The water in fish flesh contains salts and it begins to freeze at about-1°C. As some of the water freezes, so the concentration of the salts in the remaining water increases. This has the effect of lowering the freezing point even further.
By the time we have cooled the fish to -5°C all but 20% of the water will have turned to ice, continue cooling until all the water in the fish is frozen. This will have happened by the time the fish is cooled to between -20°C and -30°C which is why it is recommended for storage at these temperatures. This period is called the Thermal Arrest Period and it is the critical stage in the freezing process. During this period a lot of heat has to be removed from the fish in order to change the water content into ice.
To make this change, about 80 times as much heat needs to be removed as when just dropping the temperature 1°C from say +1°C to 0°C. This is Latent Heat.
It is very important that the fish should pass through this period as quickly as possible for the following reasons:
• Slow freezing produces large ice crystals in the flesh of the fish which may damage it.
• Because the fish does not freeze instantly, we get a build-up of salts, or concentration as we called it earlier. Some water tries to flow through the flesh to reduce this build up. However, when the fish is eventually defrosted, this water does not flow back to its starting position and produces the wetness on the fish which we know as thaw drip. The result is a tougher product of lower quality.
• There are certain types of bacteria which are still active at temperatures around 0°C. This means that bacterial spoilage can still occur. It will be obvious that thaw drip also results in weight loss which will cost money. The quality changes which take place are not so noticeable unless the freezing time extends beyond 12 hours.
This can happen if bad freezing practice is employed, e.g. placing a pallet of fish in a cold store to freeze. In this case, the fish at the centre will freeze very slowly. Where freezing times of 24 hours or more are used, then the risk of bacterial spoilage making the fish unfit for human consumption is high.
FREEZING RATES 'Quick freezing' is defined in the U.K. as lowering the temperature of the fish from 0°C to -5°C (the thermal arrest period) in 2 hours or less. This should be followed by further temperature reduction to the recommended storage temperature of-30°C at the end of the freezing period. The second part of the process must always be coupled with the first. It is important that the fish should always be reduced in temperature to the intended storage temperature as well as frozen quickly. Equipment which can do the first can be expected to achieve the second. Now, the rate of freezing is always faster near the surface of the fish where it is in contact with the cooling medium. Naturally, the thicker the fish, the longer it will take to freeze. The time it takes to freeze different products depends on lots of things. Some of these are product shape and size, the area exposed to the refrigeration medium, the temperature of the refrigerant and the packaging if present.