2.1 What happens during freezing
2.2 What is quick freezing
2.3 Double Freezing
2.4 Handling of fish before freezing
2.5 Frozen fish
2.6 Frozen fish products
2.7 Time-temperature tolerance
2.8 Codes of practice

Fish begins to spoil immediately after death. This is reflected in gradual developments of undesirable flavours, softening of the flesh and eventually substantial losses of fluid containing protein and fat. By lowering the temperature of the dead fish, spoilage can be retarded and, if the temperature is kept low enough, spoilage can be almost stopped.

Rigor mortis, over a period of hours or days soon after death, can have a bearing on handling and processing. In some species the reaction can be strong, especially if the fish has not been chilled. The muscles under strain tend to contract, therefore, some of the tissue may break, especially if the fish is roughly handled, leaving the flesh broken and falling apart. If the muscles are cut before or during rigor, they will contract and in this way fillets from fish can shrink and acquire a somewhat rubbery texture. In many species, however, rigor mortis is not strong enough to be of much significance.

The freezing process alone is not a method of preservation. It is merely the means of preparing the fish for storage at a suitably low temperature. In order to produce a good product, freezing must be accomplished quickly. A freezer requires to be specially designed for this purpose and thus freezing is a separate process from low temperature storage.

2.1 What happens during freezing

Fish is largely water, normally 60-80 percent depending on the species, and the freezing process converts most of this water into ice.

Freezing requires the removal of heat, and fish from which heat is removed falls in temperature in the manner shown in Figure 1. During the first stage of cooling, the temperature falls fairly rapidly to just below 0C, the freezing point of water. As more heat requires to be extracted during the second stage, in order to turn the bulk of the water to ice, the temperature changes by a few degrees and this stage is known as the period of "thermal arrest". When about 55% of the water is turned to ice, the temperature again begins to fall rapidly and during this third stage most of the remaining water freezes. A comparatively small amount of heat has to be removed during this third stage.

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Figure 1   Temperature-time graph for fish during freezing

As the water in fish freezes out as pure crystals of ice, the remaining unfrozen water contains an ever increasing concentration of salts and other compounds which are naturally present in fish flesh. The effect of this ever increasing concentration is to depress the freezing point of the unfrozen water. The result is that, unlike pure water, the complete change to ice is not accomplished at a fixed temperature of 0C, but proceeds over a range of temperature. The variation of the proportion of water (which is converted to ice) in the muscle tissue of fish against temperature is shown in Figure 2. The figure shows that by the time the fish temperature is reduced to -5C about 70% of the water is frozen. It also shows that even at temperatures as low at -30C, a proportion of the water in the fish muscle still remains in the unfrozen state.

Figure 2.    Freezing of fish muscle. The percentage of water frozen at different temperatures

Literature on the freezing of fish is confusing and often contradictory about what happens to fish as it freezes. This is particularly the case when reference is made to the difference between slow and quick freezing. One of the main reasons for this apparent confusion is that only in recent years has knowledge of the freezing process advanced sufficiently to explain these differences in freezing rates. The result is that much of the literature still in circulation is now outdated.

At first it was thought that rapid freezing was unsatisfactory since sudden cooling could disrupt and tear the muscle tissue. It was also thought that, since water expands on freezing, it might be reasonable to expect the cell walls to burst under the induced pressure. There is some justification for both of these theories but they do not fully explain the differences between slow and quick freezing.

For some time a widely held view was that slow freezing resulted in the formation of large ice crystals which damaged the walls of the cells. This would then result in a considerable loss of fluid when the fish was thawed. The smaller ice crystals formed, when fish is frozen quickly, were thought to do little damage to the cell walls and, as a result, little fluid was lost on thawing. Difference in size of ice crystal probably accounts for some of the differences between slow and quick freezing, but is has been shown that this still does not provide a full explanation. The walls of fish muscle cells are sufficiently elastic to accommodate the larger ice crystals without excessive damage. Also, most of the water in fish muscle is bound to the protein in the form of a gel, and little fluid would be lost even if damage of the above nature did occur.

Slow freezing, however, does result in an inferior quality product and this is now thought to be due mainly to denaturation of the protein. Changes take place in some fractions of the protein as a result of freezing and since they are altered from their "native" state they may be said to be "denaturated", hence the term "protein denaturation". This denaturation depends on temperature and as temperature is reduced the rate of denaturation is reduced. Denaturation also depends on the concentration of enzymes and other compounds present. Thus, as the water is frozen out as pure ice crystals, the higher concentration of compounds in the unfrozen portion will result in an increase in the rate of denaturation. These two factors, which determine the rate of denaturation, act in opposition to each other as temperature is reduced and it has been demonstrated that the temperature of maximum activity is in the region of -1 to -2C.

Slow freezing means that a longer time is spent in this zone of maximum activity and it is now thought that this factor accounts for the main difference in quality between slow and quick frozen fish.

2.2 What is quick freezing?

There is no widely accepted definition of quick freezing.

It is unlikely that even a trained taste panel could detect the difference between fish frozen in 1h and 8h, but once freezing times begin to extend beyond 12h the difference may well become apparent. Freezing times of up to 24h or even longer, achieved in some badly designed and operated freezers, will almost certainly result in an inferior product. Very long freezing times, for example, due to freezing fish by bulk stacking in a cold store, may even result in spoilage by bacterial action before the middle of the stack is sufficiently reduced in temperature.

Since the temperature just below 0C is the critical zone for spoilage by protein denaturation, an early UK definition of quick freezing recommended that all the fish should be reduced from a temperature of 0C to -5C in 2h or less. The fish should then be further reduced in temperature so that its average temperature at the end of the freezing process is equivalent to the recommended storage temperature of -30C. With normal freezing practice in the UK, this latter requirement is defined by stating that the warmest part of the fish is reduced to -20C at the completion of freezing. When this temperature is reached, the coldest parts of the fish will be at, or near, the refrigerant temperature of say -35C and the average temperature will then be near -30C. This is a rather elaborate definition of quick freezing and it is probably more strict than is necessary to ensure a good quality product.

The more widely used definitions of quick freezing do not specify a freezing time or even a freezing rate but merely state that the fish should be frozen quickly and reduced in the freezer to the intended storage temperature.

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 -18C 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 and, to give the reader some idea what these rates represent in practice, the range can be sub-divided as shown in Table 1.

Table 1 Freezing rates

2 mm/h Slow bulk freezing in a blast room.
5 to 30mm/h Quick freezing in a tunnel air blast or plate freezer.
50 to 100 mm/h Rapid freezing of small products.
100 to 1000 mm/h Ultrarapid freezing in liquefied gases such as nitrogen and carbon dioxide

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 -60C. Tuna is a large fish and when frozen whole by immersion in sodium chloride brine at a temperature of -12 to -15C 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 -60C 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.

2.3 Double freezing

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.

2.4 Handling of fish before freezing

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 0C, 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. Advice on handling fish before freezing at sea is given in Chapter 13.

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.

2.5 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 -10C 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.

2.6 Frozen fish products

The variety of species, processes, methods of presentation and packaging available provide scope for the preparation of numerous frozen fish products. These products, however, can be separated into two main groups; products intended for direct consumption and products intended for further processing.

Products for direct consumption

Individually quick frozen (IQF) products are frozen as single units which need not be thawed for sub-division or perhaps even for cooking purposes. IQF single fillets and shrimp are two products of this type.

The demand for IQF products has increased with the upsurge in the number of low temperature "freezer" cabinets both in catering establishments and in the home. IQF freezing allows for the purchase of a frozen product in bulk and the selection from storage of only sufficient quantities to meet immediate requirements.

Other products such as blocks of fish and fish portions usually packaged in cartons are also produced for direct consumption without the need for reprocessing. The consumer will purchase this type of product from the retailer, still in the frozen state, and either cook it in the frozen state or thaw it for immediate consumption.

The production of products for direct consumption may not yet be appropriate in many developing countries. This type of product requires the provision of an extensive network of refrigerated storage and transport. This facility, which is popularly known as the "cold chain", may not be developed enough to enable this system to operate.

Products for further processing

These products are produced for two purposes:

  1. Frozen in bulk and thawed after storing, to be used as newly caught, unfrozen fish.
  2. Frozen in bulk and after storage, further processed without thawing so that it may be presented as a retail pack.

Products frozen in bulk can be unprocessed, such as blocks of whole fish frozen in contact freezers. Blocks of frozen fish may weigh up to 50 kg; they are usually glazed or wrapped after freezing and are then stored until required for further processing.

In some cases, fish are bulk frozen, stored and finally thawed all in one place. This is usual when there is a short seasonal fishery and fish are preserved for processing over a longer period. Bulk frozen fish may also be distributed in the frozen state. This enables the fish to be sold to a larger home market and also allows the product to be exported. In this case there are additional requirements for low temperature transport and a more extensive cold chain.

Fish frozen in bulk may also be fully processed before freezing and only the skinless, boneless portion used. One particular process of this type worth special mentioning is the production of frozen fillet blocks. A frozen fillet block is a regular shaped block of fish flesh frozen in a horizontal plate freezer within a treated cardboard carton and a metal retaining frame (Figure 3) The filling process ensures that there are no voids in the block. After freezing, the blocks are stored in bulk and at a later date cut into smaller portions of different shapes. The fish portions may then be packaged and sold in this form or they may be coated with a flour batter and breadcrumbs. Coated fish portions should be returned to the freezer and rehardened before packaging and storing.

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The type of frozen fish product and the form in which it is produced in a particular country may well depend on the extent of the cold chain as well as on the demands of the consumer. It therefore seems likely that in most developing countries a bulk freezing process will be the initial development. This will enable the industry to cater for seasonal variations and allow a wider distribution of the fish catch. Other frozen products will follow later when the industry develops and the cold chain is extended.

2.7 Time-Temperature Tolerance

As in the case of iced fish the storage life for frozen fish varies considerably. Some typical data are given in Table 2.

Table 2 Practical storage life for fish. From IIR Guide to Refrigerated Storage (Appendix 1)

  Storage life, months
  -18C -25C -30C
Fatty fish, sardines, salmon,ocean perch 4 8 12
Lean fish, cod, haddock 8 18 24
Flat fish, flounder, plaice, sole 9 18 24
Lobster, crabs 6 12 15
Shrimp 6 12 12

From the table the importance of low temperature storage is clearly illustrated. It is, however, not only the length of storage life which is of importance, but the higher quality at any given moment during storage.

A number of scientific works have shown the importance of low temperature storage and for frozen foods the Time-Temperature Tolerance concept was introduced very early. The corner stones of the TTT theory are:

The storage life based on one or more of the chemical, biochemical and physical changes can be defined in many ways. A common definition is High Quality Life - HQL .

HQL is defined as the elapsed time between freezing of High Quality product and the moment when 70 percent of experienced tasters are able to distinguish the product from the control stored at very low temperature.

Other definitions of storage life are also used. Regardless of the definition the accumulated quality loss can be integrated from plots of 1/HQL against time, independent of the order of the exposures to different temperatures.

In Figure 4 quality loss during storage and transport of cod fillets at three different temperatures has been calculated.

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The distribution in this case includes 106 days at -30C, 30 days at -25C and 14 days at -18C. The total quality loss during the distribution of this particular fish fillet is 61 percent.

There is, in other words, 39 percent of the original quality left for the consumer. It is important to note that if the, storage and distribution had been carried out at -18C, the corresponding quality loss would have been obtained in 60 days. By keeping the product at -30C during the main part of the distribution, it has been possible to more than double the storage life for the same quality level.

As indicated above it must however, be observed that the quality changes in fish products are not only influenced by the storage temperature. Among the factors which are important are the original quality of the raw material, the processing method and the packaging material and method used for the final product. Those three factors are usually defined as the PPP factors - Product-Processing-Packaging.

2.8 Codes of practice

Most countries have legislation which relates to the handling and processing of foods in general and where appropriate, this legislation will apply when handling fish before, during and after freezing. However, additional recommendations are often made, usually in the form of codes of practice which, although not enforceable by law, can be rigidly applied by mutual agreement of all parties involved. Such codes of practice serve as a means of maintaining uniform standards based on good practice and take into consideration all relevant factors. In the absence of legislation, these codes of practice may also be quoted in cases of dispute as the minimum standards to be applied. Adoption of a code of practice is therefore an early step in the development of a freezing and cold storage industry.

For the wider aspects of freezing, codes of practice already exist which cover most of the likely requirements of a developing country with an expanding fish freezing industry. A number of these are listed below with a brief summary of their contents.

Codex Alimentarius Commission Joint FAO/WHO Food Standard Programme

The main aims of the Commission are to recommend product standards for international uniformity and to provide advice on how to meet such standards by issuing codes of practice. Relevant codes and standards should therefore be the starting point for all national and local codes and allowance made, if necessary, for differences that cannot be resolved due to legal or other factors. These codes and standards are often detailed and may refer to only one species or product. Until final acceptance by The Codex Alimentarius Commission, the codes are available as FAO Fisheries Circulars.

Code of Practice for Frozen Fish, FAO Fisheries Circular No. 145 (Revision 2) 1977

General advice in English, French and Spanish on the production, storage and distribution of frozen fish. The code covers the freezing of fish at sea and on shore and also deals with cold storage, packaging, transport and thawing of frozen fish and fish products. The code does not cover all the potential variations in freezing and cold storage practice but the information given can form the basis for more specialised codes which can take into account local and national requirements.

OECD/IIR Draft Code of Practice for Frozen Fish, 1969

Produced in an English-French edition it gives guidance on quality and handling at all stages of the processing of fish into a frozen product. The code covers a wide range without becoming too involved in details. (OECD = Organisation for Economic Co- operation and Development, Paris; IIR = International Institute for Refrigeration, Paris).

Recommendations for the Processing and Handling of Frozen Foods, IIR, 3rd Edition

Produced in a combined English-French edition, the document is concerned with all kinds of frozen foods including fish and fish products. It deals with principles and with basic and applied problems, and is intended as a guide for international and national organisations. In many ways, it is similar in content to this document but since it covers all frozen foods products, it has a wider application.

Guide to Refrigerated Storage IIR, 1976

Produced in a combined English-French edition, the document is a comprehensive and detailed guide covering all aspects of the design, construction and operation of cold stores. It is in a form which may be used for technical and practical study of cold storage and it can also be used commercially to make improvements in one of the most important links of the chain of refrigeration, namely refrigerated storage.

National codes of practice

The majority of developed countries with well established fisheries have codes of practice and guidelines for their own fishermen, processors, retailers and other interested groups involved in the handling and processing of frozen fish and fish products. It would be advisable for the authorities in developing countries to study these. They will give guidance for the formulation of new codes. In addition, a study of the codes will ensure that any new standards will be in accord with the standards of customers for frozen fish exports.

Most codes of this type are formulated and issued by the appropriate agricultural, food or fisheries division of national or state governments.