Introduction
The purpose of cold storage
Recommended storage temperature
Cold storage life of frozen fish
Factors limiting storage life
Design of cold stores
Cold store operation
This note gives the reasons why fish must be kept at a low temperature during cold storage, and indicates for how long different products can be kept. Factors that limit cold storage life are discussed, and advice is given on how these can be kept to a minimum.
A few brief comments are given on cold store design and operation, mainly for the guidance of users rather than builders.
Preparation and quick freezing of the product prior to cold storage are described in Advisory Note No. 27, ‘Quick Freezing of Fish’.
The spoilage of fish flesh resulting from the action of enzymes and bacteria can be slowed down by lowering the temperature; when fresh raw material is properly frozen and then kept at a sufficiently low temperature, spoilage can be almost entirely stopped. Some deterioration of the frozen product takes place during cold storage, but the changes are so small under the right conditions that to the ordinary consumer the thawed product is indistinguishable from fresh after many months in store.
Generally speaking, the lower the holding temperature, the longer is the period of safe keeping; hence temperature of storage is the most important single factor affecting the storage life of frozen fish.
The recommended storage temperature for all frozen fishery products in Britain is minus 20°F. Bacterial action is completely arrested at this temperature, and undesirable changes in the flesh caused by enzymes go on only very slowly.
Some products can be kept safely even at higher temperatures provided storage is only for short periods, but since it is not always possible to ensure that a product stays in store no longer than originally intended, it is generally safer to use minus 20°F storage at all times. Some of the poor quality fish on sale to the public has all too often been stored at a high temperature for a long time.
Recommended temperature for long term storage of frozen fish
Even when fish are properly frozen within a few hours of catching and then stored at minus 20°F, they will not keep indefinitely. The product will deteriorate very slowly until eventually it becomes unattractive to look at and unpleasant to eat, although it may take years rather than months for this to occur.
Fatty fish, for example herring and salmon, do not keep so well as white fish such as cod, which have a very low fat content. Smoked fish do not keep so well as unsmoked fish.
Initial freshness influences storage life; stale fish spoils more rapidly during cold storage than fresh fish. The table shows the potential storage life, at various temperatures, of fish in good condition.
The figures given in the first part of the table are based upon the results of experiments carried out at Torry Research Station (TRS) over a number of years. All the samples were from very fresh fish, stored in ice for not more than 24 hours between catching and freezing. All but the smoked fish were well glazed, packed in wooden boxes lined with parchment paper and kept at temperatures within 1 Fahrenheit degree of those stated. Samples were tasted and compared with corresponding fresh fish at regular intervals.
Figures in the second part of the table are taken from the International Institute of Refrigeration (IIR) Code of Practice, ‘Recommended Conditions for Cold Storage of Perishable Foodstuffs’.
Figures in the columns headed ‘good’ give the period in which the stored product is for all purposes as good as fresh. The column marked ‘inedible’ indicates the time when the product becomes so distasteful to a consumer accustomed to fresh fish as to be uneatable.
The figures given cannot be more than approximations of the limiting periods.
COLD STORAGE LIFE OF FROZEN FISH
Storage Temperature
|
Type of Fish |
15°F |
0°F |
-5°F |
-10°F |
-13°F |
-20°F |
||||||
|
|
good |
inedible |
good |
inedible |
good |
inedible |
good |
inedible |
good |
inedible |
good |
inedible |
|
(i) (TRS) more than |
||||||||||||
|
white fish (gutted) |
1 mnth |
4 mnth |
|
|
4 mnth |
15 mnth |
|
|
|
|
8 mnth |
4 years |
|
smoked white fish |
1 mnth |
3 mnth |
|
|
3 |
10 mnth |
|
|
|
|
7 mnth |
1 year |
|
herring (gutted) |
1 mnth |
3 mnth |
|
|
3 mnth |
6 mnth |
|
|
|
|
6 mnth |
1 |
|
kippers |
3 wks |
2 mnth |
|
|
2 mnth |
5 mnth |
|
|
|
|
4 |
9 mnth |
|
kippers |
|
|
|
|
|
|
|
|
|
|
more than 1 year |
|
|
whole cooked lobster |
|
|
|
|
|
|
|
|
|
|
6 mnth |
|
|
cooked lobster meat |
|
|
|
|
|
|
|
|
|
|
3 mnth |
|
|
whole cooked crab |
|
|
|
|
|
|
4 mnth |
|
|
|
8 mnth |
|
|
cooked crab meat |
|
|
|
|
|
|
3 mnth |
|
|
|
6 mnth |
|
|
mussel meats |
|
|
|
|
|
|
3 mnth |
|
|
|
8 mnth |
|
|
(ii) (IIR) |
||||||||||||
|
cod, haddock, etc. |
|
|
3-5 mnth |
|
|
|
|
|
6-8 mnth |
|
8-10 mnth |
|
|
flatfish |
|
|
4-6 mnth |
|
|
|
|
|
7-10 mnth |
|
|
|
|
fatty fish |
|
|
2-3 mnth |
|
|
|
|
|
3-5 mnth |
|
6 mnth |
|
|
lobster and crab |
|
|
2 mnth |
|
|
|
|
|
|
|
|
|
|
shrimp |
|
|
6 mnth |
|
|
|
|
|
|
|
|
|
|
oysters |
|
|
2-4 mnth |
|
|
|
|
|
|
|
|
|
|
scallops |
|
|
3-4 mnth |
|
|
|
|
|
|
|
|
|
|
clams |
|
|
3-4 mnth |
|
|
|
|
|
|
|
|
|
Protein changes:
Fish proteins become permanently changed during freezing and cold storage. The speed at which this denaturation occurs depends very largely upon temperature. At temperatures not very far below freezing point, 28°F for example, serious changes occur rapidly; even at 15°F the changes are so rapid that an initially good quality product can be spoilt within a few weeks.
Badly stored fish are easily recognizable; the thawed product, instead of being glossy and translucent, is opaque, white and dull. The firmness and elasticity of the properly stored product are replaced by sponginess, and in very bad examples the flesh may break up. Juice can easily be squeezed out. Cooked samples, instead of having the succulent curdiness of cooked fresh fish, at first feel wet and sloppy in the mouth, and on further chewing become dry and fibrous.
When smoked, such inferior fish sag and gape and have an unattractive, matt surface. Smoked products made from properly stored frozen fish acquire an attractive glossy surface because the brine dissolves some of the protein and this solution subsequently dries on the cut surface of the fish; denatured protein is insoluble in brine, so that the surface of poorly stored fish remains dull after smoking.
Protein denaturation is more obvious in white fish than in fatty fish. Nevertheless, herring stored at 15°F for more than a month or so do not split well, and kippers made from them are dull.
Deterioration due to protein denaturation can therefore be checked by ensuring that storage is at as low a temperature as possible, preferably minus 20 F.
Fat changes:
The fat of fish may become unpleasantly altered during cold storage. Fish oils readily combine with oxygen, and some of the enzymes normally present in fish muscle, particularly those in the red strip of muscle just under the skin of fatty fish, assist this reaction. Badly affected fish have a most disagreeable odour and flavour; the fat becomes gummy, and the flesh develops a yellow, rusty appearance. Flavour may vary from that of a mild cod liver oil to an acrid burning or painty taste which is definitely objectionable.
These changes take place more rapidly at higher temperatures of storage, and are sometimes accelerated by the presence of small amounts of certain chemicals, such as salt, that probably increase enzyme activity. For this reason, fatty fish should never be brined before freezing.
Attempts to slow down or delay these fat changes by adding chemicals called antioxidants at some stage before cold storage have so far proved ineffective.
Frozen herrings can be protected to some extent against attack by atmospheric oxygen by glazing them well before putting them in store. In addition, herrings frozen in a compact block keep better than herrings frozen singly, and the larger the block the better the keeping quality. Fillets do not keep so well as whole herrings, possibly because the process of filleting opens up the fish and allows free access of oxygen.
Since drying and the addition of salt accelerate the development of rancidity, frozen smoked fatty fish have a much shorter storage life than the frozen raw fish. Since it is not possible to protect smoked fish by glazing them, rancidity in kippers can be of serious consequence. However, it has been shown that by packing them in suitable plastic bags, sealed under vacuum before freezing, it is possible to obtain very considerable extensions of storage life of frozen kippers, and of herrings too.
Vacuum packaging materials need to be impervious to moisture and atmospheric oxygen; a foil laminate and certain plastic laminates have proved most effective in these respects. Frozen kippers, properly packed in one of these materials, have been stored in very good condition for over a year. It may be possible to extend the process from small consumer packs to large blocks of frozen fatty fish with equal success. The wrapping materials suitable for vacuum packing of fish are discussed in more detail in a later note in this series.
Frozen fish drying in cold store
Dehydration changes:
Frozen fish may dry slowly in cold store even under good operating conditions. This is undesirable for reasons other than the most obvious one that the product will lose weight. Most important, drying accelerates alteration of the protein and oxidation of the fat; both effects are described above. Texture and appearance are adversely altered.
Frozen fish that have suffered severe drying in cold store have a white, toughened, dry and wrinkled appearance on the surface that is characteristic of the condition known as freezer burn. The skin of the thawed fish may have a similarly dry, wrinkled look, and if drying has been exceptionally severe, the flesh beneath can become spongy and as light as balsa wood.
The frozen product is protected against drying either by glazing or by an impervious wrapping. All frozen fish not wrapped in a moisture-proof material must be glazed before storing. Glazing means covering the exposed surfaces of the frozen fish with a thin skin of ice by either spraying or brushing it with water, or by dipping it in water for a few seconds. The skin of ice so formed will evaporate slowly while the fish is in cold storage, so that regular inspection of the fish is necessary to ensure that the glaze is renewed when required.
Other precautions that can be taken to reduce drying of the stored product are mentioned in the following sections on design and operation of cold stores.
The following very brief notes are intended to serve as a rough guide for owners and users of cold stores for frozen fish.
It is worth repeating here that any store intended for long term storage of frozen fish should be capable of maintaining the contents of the store at the recommended temperature of minus 20°F at all times. The temperature of the store must be kept as steady and uniform as possible, since any fluctuation increases the damage to the product and also encourages dehydration.
The temperature difference between the product and the cooling unit in the store must be kept as small as possible, since the bigger the temperature difference, the faster will water vapour leave the product to appear as frost on the cooler; thus freezer burn will be excessive when the temperature difference is large. In practice it is therefore desirable to use a cooler with as large a surface area as possible in still air, rather than a small cooler with fans, since movement of air can also accelerate drying. The cooling grids should be spread over as much of the internal surface of the store as possible, walls as well as roof. This also helps to prevent heat that leaks through the walls from reaching the product.
Drying of the product can also be greatly reduced by restricting the amount of heat entering the store. In very small stores, the biggest single source of heat is often caused by opening the door, but in general most of the heat comes through the insulation. For a given thickness of a particular type of insulation, size and shape of store are important; a small cold store will have a bigger heat leak in proportion to the amount of product it will hold than will a big one of the same shape; a store whose shape is approximately that of a cube will have a far smaller heat leak than a long, low, narrow store having the same capacity. Here are two examples to illustrate these points: a store 45 feet by 30 feet by 15 feet will hold more than three times as much produce as one measuring 30 feet by 20 feet by 10 feet, but has not much more than twice the surface area; a store 200 feet by 50 feet by 10 feet has nearly twice the surface area of one measuring 60 feet by 60 feet by 28 feet for the same capacity. Goods can be stacked up to 20 feet or more high with fork lift trucks.
Pillars and supports that pass through the insulation must themselves be well insulated; large amounts of heat can enter the store through uninsulated structures.
When a cold store door is opened, cold air pours out and warm air rushes in to take its place; some well designed large stores have only one door, but if there is more than one it should not be possible to open more than one at a time.
Large refrigerated air locks are not common; the modern tendency is to provide a small non-refrigerated air lock having inner and outer doors that cannot be open at the same time; the exchange of air when a door is opened is then very small. Ideally an alarm should ring when a door remains open too long. It is sometimes possible in very large cold stores to dispense with an air lock altogether.
A curtain of air blown downwards from above the doorway can reduce exchange of warm and cold air, but the non-recirculating type in common use is often difficult to adjust so that it works properly; it can be a useful aid when the door is opened at intervals, but its use can be much abused. It must not be depended upon to screen a door left open continuously.
Hatches can be used to feed the product into or out of the stores; larger doors need then to be opened far less frequently. Hatch openings should be as high up in the store wall as possible, to prevent excessive loss of cold air.
Rigid insulation materials should be used wherever possible for low temperature insulation. Fibrous or loose granulated materials are not quite so suitable because strong convection currents of air can be set up inside the insulation. In addition, loose materials tend to settle down after a while, leaving an air space, for example at the top of a wall. With all types of insulation, effective fitting, sealing and overlapping of slabs is necessary to prevent air movement within the insulation. Avoid the use of flammable insulation material to reduce the risk of fire.
Moisture will always tend to pass from the warm air outside a cold store to the colder air inside, where it will condense on the cold pipes and frozen produce. But some of the water that passes through an unprotected wall will freeze at the point in the insulation where the temperature is at 32°F; the insulation will thus gradually become less efficient as ice builds up within it and may eventually be completely destroyed. There is therefore a need for a vapour barrier on the warm face of the insulation. There should be no barrier on the inside face so that any moisture present in the insulation when erected, together with any that gets in during operation, can eventually pass right through with little damage to the insulation.
Large low temperature stores built directly on the ground often need ventilation under the floor insulation, or underfloor heating, in order to prevent gradual build up of ice which may distort the floor or eventually wreck the building; this phenomenon is known as frost heave.
A cold store must always be fitted with an accurate thermometer that can be readily observed by the operator from outside the store; the temperature of the air should also be recorded continuously on a chart. Advice on measuring temperature in cold stores is given in Advisory Note No. 94, ‘Temperature Measurement and Fish’.
Never attempt to freeze fish in a cold store and never put frozen fish that is warmer than the store directly into the store. Cold storage plant rarely has sufficient reserve capacity to cope with the extra heat load; warm produce brought into the store will not only take a very long time to cool, but can also warm other products already in store, with consequent loss of quality.
Cold store plant is not powerful enough to freeze fish
Do not stack goods directly on the cold store floor, or close to the walls; leave an air space below and around the goods so that cold air can pass between the goods and the store structure. Pallets or dunnage can be used on the floor, and vertical battens can be fixed to the walls. The National Association of Frozen Food Producers recommends that products should be at least 4 inches off the floor, 8 inches away from walls and 18 inches from roof cooling units, and that 1-inch dunnage should be used between every second tier of packages.
Never have more than one door open at a time, and do not leave it open longer than necessary.
Produce stacked close to the doorway should be screened to protect it from warm air coming in.
Packages of any one product should be removed from the store in the same order in which they were put in; all packages should be clearly date-stamped so that stock rotation, on the principle of first in, first out, can be strictly observed.
All products in store should be protected either by a wrapping that is impervious to water vapour, or by adequate glazing; glazing should be renewed at intervals when required.
Samples of products in store should be inspected at regular intervals and their temperature checked in the manner described in Note No. 20.
Store air temperature should be continuously recorded, and checked at regular intervals; any movement outside the tolerated limits should be immediately reported so that action can be taken to put things right.
All cooling pipes in the store should be defrosted at regular intervals; efficiency of the cooler will be considerably reduced if frost is allowed to accumulate.
Products being removed from store to store should be loaded
into transport in accordance with the advice given in Advisory Note No. 8,
‘Road Transport of Frozen Fish’.
mnth