FREEZING

Regulations and Guides to Good Practice

In the EC directives apply to the frozen food chain from initial manufacture to retailing and these directives may be used as a guide. They relate to the quality of foods labelled as "quick frozen" and require that foods labelled in this way should be brought through their zone of maximum ice crystallisation as quickly as possible. Thereafter, they must be maintained at -18°C or below. There are exemptions for local deliveries and frozen foods held in retail display cabinets. They also concern the monitoring of temperatures of quick frozen foods during transport, and storage and the sampling procedures and temperature measurement methodology to be used by enforcement authorities. Complying with these Directives requires an understanding of how different foods freeze, the effects of different freezing processes, and the ability to correctly measure the temperature of frozen foods. The recommendation that the fish should be reduced to the intended storage temperature is important and this should be included in all good codes of practice for quick freezing. These two basic requirements for freezing, that the fish be frozen quickly and be reduced to storage temperature, go together since it is likely that a freezer which can quick freeze fish also operates at a sufficiently low temperature to ensure that the recommended product storage temperature can be achieved. Some freezing codes and recommendations define freezing rate in terms of the thickness frozen in unit time. The freezing rate, however, is always quicker near the surface of the fish, where it is in contact with the cooling medium, and slower at the centre. Freezing rates are therefore, only average rates and they do not represent what happens in practice. Average freezing rates vary between 2 and 1000 mm/h.

Freezing rates

One exception to the general requirements tor quick freezing of fish requires special mention. Frozen tuna, which will eventually be eaten in its raw state as the Japanese product "Shasimi" seemingly requires to be reduced to a lower temperature than other fish products. Japanese fishing vessels catching fish for this product operate with freezers at -50° to -60°C. Tuna is a large fish and when frozen whole by immersion in sodium chloride brine at a temperature of -12 to -15°C takes up to three days to freeze. Air blast freezing has now replaced brine freezing for this purpose and operation with very low freezer temperatures can result in freezing times of about 24h or less. The exceptionally low temperatures used in these freezers of about -50 to -60°C have given rise to conditions which require special precautions to be taken to avoid low temperature brittle fracture of metal structures in the vessels.

The above current requirements for air blast freezing tuna is one special case where general rules for quick freezing are not applied and it should be kept in mind that local requirements for particular products may, in some countries, give rise to others.

Double freezing means freezing a product, thawing or partly thawing it, and refreezing. This practice is often necessary for the production of some frozen fish products made from fish previously frozen and stored in bulk. What must be remembered is that even quick freezing results in quality changes in the fish and double freezing will therefore result in further changes. Only fish that were initially very fresh could therefore be subjected to double freezing and still conform to good quality standards. Fish frozen quickly at sea immediately after catching, for instance, would be suitable for this purpose.

Freezing and cold storage is an efficient method of fish preservation but it must be emphasised that it does not improve product quality. The final quality depends on the quality of the fish at the time of freezing as well as other factors during freezing, cold storage and distribution. The important requirement is that the fish should at all times be kept in a cool condition before freezing, about 0°C, and the use of ice or other methods of chilling is recommended. The FAO document "Ice in Fisheries" FAO Fisheries Technical Paper No 331 describes in detail the methods of using ice or refrigerated sea water to cool fish.

Apart from keeping the product chilled, it is also essential to adopt a high standard of hygiene during handling and processing to prevent bacterial contamination and spoilage. The FAO/WHO Codex Alimentarius Commission "Recommended International Code of Practice for Fresh Fish", 1983 and "Code of Practice for Frozen Fish" 1984 give guidance on this aspect of quality control.

In some countries chemicals are currently used to treat fresh fish in order to assist with such things as colour retention and the retention, or even addition, of fluids. The treatment of food with chemicals is usually subject to national and local restrictions and it would be inappropriate to make any general comment on their use in this document.

Frozen Fish

Freezing and frozen storage of fish can give a storage life of more than one year, if properly carried out. It has enabled fishing vessels to rem ain at sea for long periods, and allowed the stockpiling of fish during periods of good fishing and high catching rates, as well as widened the market for fish products of high quality.

The mechanism by which frozen fish deteriorates is somewhat different from that causing spoilage of chilled fish. Provided the temperature is low enough - below -10°C bacterial action will be stopped by the freezing process. Chemical, biochemical and physical processes leading to irreversible changes will still occur, but at a very slow rate. Deterioration during frozen storage is inevitable, and in order to obtain satisfactory results, fish for freezing must be of good quality.

The proteins changes in fish frozen under poor conditions can be recognised in the thawed fish. The normally bright, firm and elastic product becomes dull and spongy. The flesh will tend to sag and break and there will be substantial losses of fluid, which can be squeezed out easily. When cooked the fish will be dry and fibrous. The rate at which protein denaturation takes place in frozen fish depends largely on the temperature and will slow down as the temperature is reduced.

Changes taking place in the lipids of the frozen fish will also slow down when the temperature is reduced. The oxidation of the fat leads to objectionable flavours and odours. This can be particularly serious in fish of high fat content and probably also accounts for most of the flavour changes in lean fish. Some substances, notably salt, and some processes, such as drying, can aggravate the problem. Smoked fish, for example, has a shorter storage life in frozen condition than the raw, frozen counterpart. The addition of chemicals to prevent oxidation has not been successful, except for some special types of products.

The rate of oxidation can be reduced by reducing the exposure to oxygen. This can be achieved by introducing a barrier at the surface of the fish. Thus fish in a block keep better than fish frozen individually, and the addition of an ice glaze is beneficial. Glazing is carried out after freezing by brushing or spraying chilled water onto the surface of the fish or by dipping in cold water. Packaging materials, impermeable to moisture and oxygen can be effective, especially if vacuum packaging is employed.

Some transfer of moisture from the product is unavoidable during freezing and frozen storage, which leads to dehydration of the fish. Good operating conditions are essential in order to keep dehydration to a minimum. It has been clearly established that fluctuating cold store temperatures are a major cause of dehydration. In practice the more severe cases of drying occur during frozen storage rather than during freezing. In extreme dehydration the frozen fish acquires a dry wrinkled look, tends to become pale or white in colour and the flesh become spongy. This characteristic appearance is called, inappropriately, 'freezerburn'. The weight loss is, of course, serious from an economic point of view and dehydration will accelerate the other important changes - protein denaturation, as well as oxidation. Glaze on the exposed surfaces of the fish before storage will however, evaporate over a period of time and drying of the fish itself will resume. Reglazing is therefore a common need. Paper wrappers can be used as a protection, but depending on the conditions some drying of the fish within the packing will still occur.

Source :

"So Easy to Preserve", 5th ed. 2006. Bulletin 989, Cooperative Extension Service, The University of Georgia, Athens. Revised by Elizabeth L. Andress. Ph.D. and Judy A. Harrison, Ph.D., Extension Foods Specialists.

Updated on : Feb 2015