Method 14: Freezing of fish

Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish Method 14: Freezing of fish

Freezing of fish Introduction This note sets out the general principles to be observed when freezing fish It defines the purpose of freezing, and describes what happens when the temperature of a fish is lowered below freezing point Advice is given on selection and handling of the raw material before freezing, the correct use of freezers, and the handling of frozen fish between freezer and cold store Some hints are also given on the freezing of smoked fish Cold storage of fish is dealt with in Advisory Note 28 The purpose of freezing The purpose of freezing fish is to lower the temperature and thus slow down spoilage so much that when the product is thawed after cold storage it is virtually indistinguishable from fresh fish A fish goes bad principally from two causes, self digestion and the action of bacteria Enzymes present in the living fish remain active after a fish dies and cause breakdown of the flesh by self digestion; enzyme activity in the dead fish can be reduced by lowering the temperature Bacteria are present in the gut and on the skin and gills of living fish; most of them no harm, and some can even be beneficial But when a fish dies they increase in number and begin to invade the flesh, which they use as food They break down the complex chemical substances of the flesh and produce some simpler objectionable compounds such as ammonia; the spoilage process continues until the flesh becomes putrid and inedible Bacterial action is also slowed when the temperature is reduced Thus, by lowering the temperature of the fish, spoilage can be retarded and, if the temperature is low enough, spoilage can be almost stopped Freezing is the means of preparing fish for storage at low temperature but is not of itself a method of preservation What happens during freezing Fish are largely water, 60-80 per cent depending on species, and the process of freezing converts most of the water into ice Freezing requires the removal of heat, and the temperature of fish flesh from which heat is steadily removed first falls rapidly to just below 0°C, the freezing point of water The temperature then remains almost stationary until most of the water turns to ice, when it again begins to fall rapidly as the frozen flesh is further cooled The manner in which the temperature changes during freezing is shown in Figure At -5°C most of the water in the fish is frozen The ice in a frozen fish is in the form of myriads of tiny needles or chips, the Size and number of which depend on the speed of freezing; when a fish is frozen slowly the particles are comparatively large and few in number, although usually too small to see clearly, perhaps mm across The appearance of the flesh is affected by the size of the ice particles; a thin piece frozen quickly can have a chalky white appearance because the tiny ice particles scatter the light, but this effect, which is indicative of quick freezing, should not be confused with freezer burn, a term used to describe the dull, matt appearance of frozen fish that have dried during cold storage A fish frozen slowly has a colour similar to that of unfrozen fish, but a fish frozen extremely slowly, in days rather than hours, takes on a dark glassy look FIG How temperature changes during freezing Different sizes of ice particle not affect the quality of the thawed product; fish frozen quickly within an hour or two, or slowly over several hours, look the same when they are thawed immediately after freezing The different sizes of ice needle simply produce different optical effects Quick freezing is nevertheless important Spoilage changes continue fairly rapidly at temperatures just below 0°C, and it is therefore desirable to remove heat from the fish quickly to avoid loss of quality The faster the freezing process, the more quickly spoilage is stopped Furthermore, slow freezing can have an adverse effect on the quality of the thawed product after cold storage, and on the suitability of fish for subsequent smoking For these reasons, and to maintain a high output from the freezing plant, freezing should be completed quickly In practice this means that the time to pass through the temperature range to -5°C should be not more than a few hours; most freezers nowadays are capable of achieving a freezing time of or hours Slow freezing over a day or more causes poor quality in most fish products and should be avoided Such conditions are obtained when bulk produce is frozen by placing it still unfrozen in a cold store The temperature of the fish should be lowered to a safe level; the recommended storage temperature for frozen fish is -30°C, for reasons that are fully discussed in Advisory Note 28 It is advisable to lower the average temperature of the fish to this value during the freezing process; in practice, in a freezer operating at about -40°C, this means lowering the temperature of the warmest part of the fish, usually the centre, to -20°C so that the average temperature of the product entering the cold store is then at or below the desired -30°C How to make a good product Fresh fish can be prepared, packed and frozen so quickly that there is almost no loss of quality during the process, but carelessness during preparation can turn fresh fish into stale fish even before they reach the freezer Figure shows some of the points at which loss of quality can occur during preparation Care must be exercised at each stage from receipt of raw material to deposit of the frozen fish in cold store The raw material Freezing and cold storage cannot improve the quality of fish; at best the process maintains the fish in the same condition as they were when frozen It is therefore essential that the raw material be fresh White fish such as cod and haddock spoil fairly rapidly at 0°C and reach condemnation level about 16 days after catching When stored at higher temperatures they spoil faster; for instance at 5°C they become inedible after about days, and at 16°C they last only days It is therefore important to avoid delay before freezing, but when fish have to be held back they should be well iced to keep them at 0°C When fish are cheap at the port market, processors sometimes buy quantities in excess of a day’s freezing capacity; this means the surplus remains in the factory for a day or two awaiting freezing Overnight storage causes a measurable amount of spoilage, and fish kept longer than this can deteriorate badly before they are frozen A shovelful of ice on top of a box or kit of fish in a chillroom is not good enough for overnight storage; plenty of ice should be distributed uniformly throughout the fish Here is an example of industrial practice that resulted in loss of quality A processor bought good quality cod for freezing that were stored without ice in a chillroom operating at 2-4°C for 24 hours These fish spoiled three times as quickly as they would have done had they been properly mixed with ice; in other words they lost the equivalent of days’ storage life, so that when they were finally frozen they were of only moderate quality The staler the fish to begin with, the worse the effect of bad practice of this kind will be Whole cod should be frozen within three days of capture, and be kept in ice while awaiting freezing, to give a product that when thawed can be treated like fresh iced fish Flatfish can be kept longer in ice before they are frozen; for example plaice can be kept 5-6 days after catching and still give a first class product Haddock on the other hand should be not more than days in ice before they are frozen, and hake are sometimes too soft to stand up to the effects of freezing after only day in ice FIG Some of the causes of poor quality frozen fish These limits of storage in ice before freezing have been fixed on the assumption that the thawed whole fish must be suitable for filleting and smoking, but when fillets are frozen, the limits are somewhat wider, particularly when there is no intention of smoking the fillets after thawing Acceptable products can be obtained by freezing fillets cut from fish that have been kept well iced for up to days after capture It must again be emphasized however that the frozen product when thawed will be similar in quality to the original raw material; freezing cannot improve the product When the period in ice is extended beyond days there is a marked fall in the quality of the frozen product; fish that have reached the limit of acceptability for marketing as wet fish are certainly not suitable for freezing, even as fillets Clearly, therefore, care must be taken to choose raw material for freezing that will give an acceptable end product The consumer should never be confronted with frozen fish made from wet fish of low quality FIG Icing fish awaiting freezing Special problems arise when white fish are filleted for freezing so soon after catching that the natural process of rigor, that is stiffening after death, has not been completed This is likely to occur only when fish are frozen at sea The effects of rigor are described in detail in Advisory Note 36, and it is sufficient to state here that fillets taken from a fish before rigor sets in can shrink or become distorted during freezing, and can have a tough texture after thawing and cooking Fatty fish such as herring and mackerel are not normally gutted when caught, and as a result the digestive juices in the gut start to break down the belly wall very soon after a fish dies For this reason fatty fish must be iced immediately and be frozen within 24 hours of capture, preferably even sooner for fish that are full of food Care during filleting Fish that are to be filleted before freezing should be kept chilled throughout the process; quality and yield are improved, and the demand on the freezer is reduced Water in the filleting trough should be kept chilled, especially in summer, by the addition of ice so that whole fish awaiting filleting remain cool It is extremely difficult to cool a mass of fillets once it has become warm; therefore fillets that are likely to be held up, even for only an hour or two, before packing and freezing should be iced top and bottom Wet strength paper can be used between ice and fillets to prevent the fillets becoming waterlogged, and meltwater should be free to drain away, clear of fillets in boxes below There should be as little delay as possible between filleting and packing, but when fillets have to be kept overnight they should be stored in thin layers with ice between The containers should be clearly marked to make sure these fillets are the first to be packed and frozen the following day Here is another example of industrial practice that resulted in serious loss of quality Several boxes of medium quality cod fillets were stored without ice for 24 hours in a chillroom running at 1-3°C On inspection they were found to have lost the equivalent of days’ storage in ice and were only just acceptable Fillets are sometimes dipped in a polyphosphate solution before freezing them in order to reduce the amount of drip when they are thawed after cold storage The manner of applying polyphosphates and the effectiveness of the treatment are discussed in detail in Advisory Note 31, but since there is very little drip loss from fresh fillets that have been properly frozen, cold stored and thawed, treatment with phosphates is not usually necessary The treatment has more effect on staler raw material or when freezing and cold storage are less than ideal When fillets are dipped prior to freezing, they should be allowed ample time to drain thoroughly; otherwise there will be additional weight loss later that could be wrongly attributed to drip loss White fish fillets can be dipped for about a minute in a chilled brine containing 10-15 per cent salt prior to freezing as an alternative means of reducing drip The fillets must be properly drained before freezing Fatty fish should never be brined before freezing Monosodium glutamate is sometimes added to fish before freezing; it contributes practically no flavour of its own, but can enhance the fish flavours originally present About 1·5 g to kg of fish is the amount generally used Packing Consumer packs of fish are usually wrapped before they are frozen, but large blocks are sometimes frozen unwrapped The thicker and more elaborate the wrappings the longer the freezing time will be Once fish have been packed it is generally not practicable to use ice to cool them; it is therefore especially important to avoid delay at this stage The rate of packing should never exceed the rate of freezing so much that packed fish have to wait more than an hour before entering the freezer Interleaving of waxed paper or plastics film between fillets, especially in large blocks, enables the user of the frozen product to remove individual fillets without having to thaw the whole block Single fish, fillets or portions are also often frozen individually before packing in outer containers, in order to make them more convenient to handle; they are described as individually quick frozen, iqf Packaging, apart from having sales appeal and keeping the product clean, should protect the contents from deterioration due to drying and oxidation; the ideal material prevents moisture getting out and prevents air getting in The pack should fit the product tightly to prevent oxidation and drying within the pack Information on a pack of frozen fish should enable the producer to identify when and where it was made, and should give the purchaser advice on storage time, thawing time and cooking Freezing Three kinds of freezer are used for freezing fish in the UK, the air blast, the plate and the spray freezer In the first kind, heat is removed from the fish by blowing a stream of cold air over them; in the second, the fish lose heat by direct contact with hollow metal plates through which a cold fluid is passed; in the third, the product is in direct contact with a refrigerant The air blast freezer is most useful where the product is of irregular shape or where a number of products of different shape and size have to be handled in the same freezer The process can be either batch or continuous An air speed of about m/s is usually suitable, although a higher air speed may be justified for a continuous belt freezer Small quantities of fillets or small fish can be frozen in batches without wrappers in open metal trays with slightly tapered sides The frozen blocks can then be removed by spraying the base of the trays with water for a short time, a process that can be mechanized The blocks should be glazed when they are removed from the trays and can also be wrapped if required Fillets in large quantities are often frozen individually on a moving belt in a continuous blast freezer; a spiral construction is sometimes used to reduce floorspace Small flat consumer packs are not well suited for freezing in an air blast since the packaging material extends the freezing time FIG Loading an air blast freezer It is good practice to distribute the product uniformly across the air stream so that the air does not bypass the fish, as shown in Figure Frost should be removed from the cooling coils at regular intervals to enable the freezer to perform efficiently The design and performance of air blast freezers for fish are discussed in more detail in Advisory Note 35 Plate freezers can be of the vertical or horizontal kind Vertical plate freezers are used principally for freezing large blocks of whole fish, both at sea and on shore Whole white fish are loaded, usually without wrapping, between pairs of vertical cold plates to produce frozen blocks up to 10 cm thick and up to 50 kg in weight Small whole fish which are more susceptible to physical damage can be frozen in water in bags between vertical plates in the same way; the resultant casing of ice also helps to prevent rancidity in fatty fish such as herring or mackerel during subsequent cold storage It is possible to reduce the thickness of the block to cm when freezing small fish, thus reducing the freezing time required The most common mistake made with the vertical plate freezer is to remove the blocks before they are properly frozen More detailed advice on the operation of vertical plate freezers is given in Advisory Note 34 Horizontal plate freezers are used principally for freezing flat packs of fish up to about cm in thickness The plates are brought to bear on the packages under slight pressure, thus preventing distortion during freezing Unwrapped blocks of small fish can also be handled in a horizontal plate freezer provided good contact can be made The main causes of poor performance with this kind of freezer are removal of the product before freezing is complete and failure to maintain good contact between plates and product Poor contact can be caused by not filling the packs, so that an air-space remains between the top of the fish and the wrapping, or by failing to keep the plates free from knobs or ridges of ice The freezing of packs of different thickness in the same load can also result in the thinner packs making little or no contact with the upper freezing plate FIG Causes of poor performance in horizontal plate freezers Spray freezers are used mainly for freezing individual fish products up to cm thick, such as shellfish meats, thin fillets and portions; the products are moved through a tunnel where they are subjected to a spray of liquid refrigerant In one system the product is first plunged into a pool of liquid refrigerant to freeze the surface of each portion and so prevent them sticking together Freezing times are generally short, and continuous freezers of this kind can be fitted into mechanical handling schemes; small batch spray freezers are also available Accurate control of residence time and refrigerant flow is important; otherwise freezing can be inefficient, with excessive loss of refrigerant to atmosphere Some systems employ sensing elements which check the input load and adjust the flow of refrigerant to suit Since some refrigerant is lost to atmosphere, care must be taken to ensure adequate ventilation of the workspace by means of extraction fans Freezing times for fish products cannot be predicted accurately by calculation Freezing time is influenced by the shape and size of the product, the characteristics of any packaging material, the degree of contact between product and refrigerant, and the refrigerant temperature A calculated freezing time can be a useful guide but, whenever a new product is first frozen in a particular freezer, an accurate freezing time should be obtained by direct measurement A routine estimate of freezing time, based only on product thickness, can be inaccurate and misleading Advice on temperature measurement during freezing is given in Advisory Note 94, and detailed advice on the freezing times of fish products is given in Advisory Note 62 Most modern freezers are capable of freezing fish quickly, provided they are operated with care and common sense Between freezer and cold store Delays between freezer and cold store are all too common; a rise in temperature at this stage can result in considerable loss of quality It is not generally realized that frozen fish warms very quickly when left standing in a comparatively warm factory Individual consumer packs at -30°C will warm at a rate of about degree every minutes until they are about -10°C, and even a large carton of frozen fish in warm air can rise 5°C in half an hour Unfortunately fish at -10°C looks and feels as hard as fish at -30°C, and it is therefore almost impossible to tell by appearance whether frozen fish have become warm Not only is quality affected in this way, but extra heat is introduced into the cold store, thus raising store temperature and affecting products already stored there Fish that are to remain unwrapped in cold store should be glazed as soon as they are removed from the freezer, in order to give them a protective skin of ice; this reduces drying of the fish during subsequent cold storage The glaze is added to the fish by spraying with or dipping in cold water for a few seconds The product should then be transferred immediately to the cold store Care must be taken not to damage frozen fish in transit; although the product seems robust it is easily harmed by rough handling, and the damage may not show until it is thawed Freezing smoked fish Smoked fish that are likely to be kept for more than a few days should be frozen and cold stored as soon as they have cooled after removal from the kiln Stale smoked fish should never be frozen Smoked whole fish can be frozen in an air blast freezer, either on trays or in boxes Thick wooden boxes can increase the freezing time considerably Boxes should be left open during freezing to reduce the freezing time Smoked fillets can be frozen in consumer packs or in larger blocks; plate freezers can be used for both kinds of pack It is difficult to freeze kippers into a large cohesive block because the surface of the fish is dry after smoking, but it is possible to pack kippers singly in layers, with adjacent fish slightly overlapping, to make a frozen block that can be handled without it falling apart Cold storage The recommended temperature for cold storage of fish and fish products in the UK is -30°C Safe storage times for a variety of fish products, together with advice on correct cold storage practice, are contained in Advisory Note 28, Cold Storage of Frozen Fish INFLUENCE OF TEMPERATURE 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 10 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 During the first stage of cooling, the temperature falls fairly rapidly to just below 0°C, 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 Figure 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 11 unfrozen water The result is that, unlike pure water, the complete change to ice is not accomplished at a fixed temperature of 0°C, 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 The figure shows that by the time the fish temperature is reduced to -5°C about 70% of the water is frozen It also shows that even at temperatures as low at -30°C, a proportion of the water in the fish muscle still remains in the unfrozen state Figure 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 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 12 fluid when the fish was thawed The smaller ice crystals formed, when fish is frozen quickly, were thought to 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 -2°C 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 0°C 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 0°C to -5°C 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 -30°C With normal freezing practice in the UK, this latter requirement is defined by stating that the warmest part of the fish is reduced to -20°C 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 -35°C and the average temperature will then be near 13 30°C 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 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 -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 not represent what happens in practice Average freezing rates vary between 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 Table Freezing rates mm/h Slow bulk freezing in a blast room to 30mm/h Quick freezing in a tunnel air blast or plate freezer 50 to 100 mm/h Rapid freezing of small products 14 100 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 -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 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 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 Advice on handling fish before freezing at sea is given in Chapter 13 15 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 -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 16 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: Frozen in bulk and thawed after storing, to be used as newly caught, unfrozen fish 17 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 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 18 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 Table Practical storage life for fish From IIR Guide to Refrigerated Storage (Appendix 1) Storage life, months -18°C -25°C -30°C Fatty fish, sardines, salmon,ocean perch 12 Lean fish, cod, haddock 18 24 Flat fish, flounder, plaice, sole 18 24 Lobster, crabs 12 15 Shrimp 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:  There is, for every frozen product, a relationship between storage temperature and the time it takes at this temperature for the product to undergo a certain amount of quality change  Changes during storage and distribution at different temperatures are cumulative and irreversible over the entire storage period and sequence is without influence on the size of the accumulated total quality change 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 19 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 quality loss during storage and transport of cod fillets at three different temperatures has been calculated The distribution in this case includes 106 days at -30°C, 30 days at -25°C and 14 days at -18°C 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 -18°C, the corresponding quality loss would have been obtained in 60 days By keeping the product at -30°C 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 20