The purpose of freezing
What happens during freezing
How to make a good product
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 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 do 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.
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 1. 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 1 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. 1. How temperature changes during freezing.
Different sizes of ice particle do 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 0 to -5°C should be not more than a few hours; most freezers nowadays are capable of achieving a freezing time of 2 or 3 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.
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 2 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 6 days, and at 16°C they last only 3 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 days 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 3 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 2 days in ice before they are frozen, and hake are sometimes too soft to stand up to the effects of freezing after only 1 day in ice.
FIG. 2. 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 7 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 7 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. 3. 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 3 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 1 kg of fish is the amount generally used.
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.
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 5 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. 4. 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 4. 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 5 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 5 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. 5. Causes of poor performance in horizontal plate freezers.
Spray freezers are used mainly for freezing individual fish products up to 3 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 1 degree every 2 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.
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.