This note describes how to handle fatty fish, particularly herring, sprats, pilchards and mackerel, from the time they are caught until they are processed at the cannery. These species predominate in the British fish canning industry.
Advice is given on chilling, freezing, storing and thawing the catch; both shipboard and shore practice are described. The advantages of proper chilling or freezing are emphasized by pointing out not only the improvement in quality but also the added safeguard against the multiplication of harmful bacteria that may survive the canning process.
Canning in the context of this note means cooking the fish in the can at a high enough temperature and for long enough to halt the activity of all bacteria and enzymes that are present. In practice, processing times and temperatures are normally more than adequate to sterilize the pack but, if there are unusually large numbers of bacteria present in the fish, standard heat processing treatments may fail.
Many types of bacteria are capable of forming spores, which are thick-walled and highly resistant to heat. Some spores can remain undamaged by the canning process and later, under appropriate conditions, may germinate to produce actively growing bacteria again. One spore-forming type of bacteria, Clostridium botulinum, is particularly important because of its ability to produce a lethal poison in foods; processing times and temperatures sufficient to destroy it are generally accepted as the minimum standard of processing for canned fish products. Other types of spore-forming bacteria, however, are even more resistant to heat, since these cause spoilage of the canned fish product; processing times and temperatures are chosen to eliminate the most resistant of these. How easily and completely the spores can be killed by any particular heat-processing treatment depends very much on the number of spore-bearing bacteria present in the raw material at the start. Since none of them, including Clostridium botulinum, will grow to any significant extent below 4°C, chilling or freezing of the catch prior to processing makes canning much easier and safer. Good hygiene during processing will then keep the risk of contamination to a minimum.
Chilling or freezing of the fish immediately after capture also helps to ensure a better quality canned product by reducing spoilage during the time the fish are awaiting processing.
Fish that are to be canned within a few days of capture may be satisfactorily preserved by chilling. At chill temperatures, 0 to 2°C, not only is bacterial action slowed but also the action of enzymes in the gut is reduced.
Small fatty fish like herring and sprats are seldom gutted and washed at sea, because of their great number and small size. The gut enzymes, particularly in fish that have been feeding heavily, are very active and can quickly digest the belly wall and subsequently soften the flesh; burst bellies are very common in feedy summer herring. Pilchards with soft flesh are impossible to descale without damaging their appearance. Thus it is extremely important to chill the fish quickly immediately after catching.
Careful handling of small fish is also very important; broken, damaged or squashed fish are of little use for canning.
The fish can be cooled in three ways, by mixing them with ice, by immersing them in chilled fresh water or in chilled sea water.
Chilling with ice:
The fish should be boxed at sea to reduce damage and to avoid unnecessary handling. Enough ice should be mixed with the fish in the box to last either until the fish are processed or until they can be re-iced. A sprinkling of ice on top of the fish is not good enough; there should be a layer of ice in the bottom of the box, a sprinkling of ice among the fish and another layer of ice on top. The layer of fish should be not more than 150 mm deep. The kind of ice used is not very important, provided the ice is ice and not a slushy mixture of ice and water. Flake ice, because of the large surface area of the pieces, will melt rapidly and therefore cool the fish rapidly; it must be remembered, though, that flake ice takes up more space than crushed block ice, and therefore there must be plenty of room in the box for adequate ice as well as the required weight offish.Chilling in refrigerated fresh water:Sprats, properly iced in boxes immediately after catching, will keep in good condition for canning for up to 3 days. Herring treated in the same way will keep for 4-6 days, depending on their initial quality.
Boxed, iced fish should be put into a chillroom as soon as they arrive at the canning factory, re-iced if necessary, and kept there until they are processed. The chillroom should be kept at a temperature between 1 and 2°C; the ice is then free to melt, thereby cooling the fish with cold melt water, but the surrounding air is not so warm that ice is wasted. Fish without ice should never be kept in the chillroom.
Water at 1-2°C is a suitable alternative for keeping fish cool at the factory while awaiting processing. The water can be cooled by adding ice or by using mechanical refrigeration.Chilling in refrigerated sea water (RSW):Chilled water has some advantages over ice; the fish are less likely to be squashed, and the mass offish and water can be handled mechanically in the factory. Using this technique, cooling is fairly uniform throught the mass of fish.
Sea water has a lower freezing point than fresh water; therefore fish in refrigerated sea water (RSW) can be kept at a slightly lower temperature than 0°C, usually between 0 and -2°C. Sea water is usually more convenient for use on board ship, but cold brine containing up to 8 per cent salt can be used for storage at the factory.Herring in RSW can be kept for 5-6 days and still be used to make a good quality canned product, even when the fish are feedy or rich in fat. The stowage rate should be no higher than four parts of herring by weight to one part of water. The RSW should be cooled by mechanical refrigeration plant, using a heat exchanger.
As with fresh water, storage in RSW makes handling much less laborious, particularly for small species like sprats. Ice and boxes can be dispensed with, pump discharge and bulk storage at the factory are quite feasible, and the quality of the raw material can be as good as, or better than, iced fish of the same age.
Some salt is absorbed by the flesh of the fish when stored in RSW, and the surface slime is removed. Some control of salt uptake is desirable, because too much salt may affect the quality of the product. The more fish there are in a given amount of RSW, the less salt the individual fish will absorb; therefore it is desirable to keep the ratio of fish to water as high as possible, up to the limit given above. It should be possible to control the stowage rate in tanks at the factory to give the required salt content for canning, thus dispensing with the brining stage during processing.
For long-term storage before canning, fatty fish like herring and sprats can be quick frozen and kept in cold storage. Good quality raw material should be frozen in a proper manner, as described in Advisory Note 27. The freezing can be done at sea, or on shore, provided the interval between catching and freezing is short; where the waiting time is likely to be more than a few hours, the fish should be kept in ice or chilled water as described earlier; chilled herring and sprats for freezing should not be kept longer than 36 hours after catching. Fish in poor condition when frozen will be in even worse condition when thawed for canning.
Care should be taken not to damage small fish, particularly sprats, during freezing; for example, sprats are sometimes damaged when overfilled trays are loaded into a horizontal plate freezer, because the pressure exerted by expansion during freezing may crush the fish within the block. Less damage is caused to small herrings and sprats by freezing them in a vertical plate freezer, preferably with 10 per cent water added to fill the spaces between the fish. Blocks should be no more than 50 mm thick.
Blocks of frozen fatty fish should be well glazed and, if possible, suitably wrapped before storing them.
Herring, sprats, pilchards and mackerel should be stored at minus 30°C. At this temperature they will keep in good condition for 7 months. The main limiting factor for fatty fish in cold store is rancidity; when the temperature is too high, storage time too long, or the glazing inadequate, the fat in the fish goes rancid. In addition, texture of the flesh becomes progressively tougher in cold store.
When the fish are removed from cold store for canning, they should be thawed as quickly as possible to reduce spoilage.
Herring are usually nobbed before canning; that is the head
and long gut are removed. Thus frozen whole herring have to be thawed completely
before they can be nobbed. It is possible to avoid a separate thawing operation
by nobbing the herring before freezing them, and then thawing them sufficiently
for processing during the brining stage, by immersing them in saturated brine at
about 10°C. The partially thawed herring can then be packed into cans and
heat-processed in the normal way. Herring frozen singly require about 10 minutes
in the brine to thaw them sufficiently for packing; herring frozen in trays in a
single layer require 20 minutes. Brining time has to be long enough to toughen
the skin; otherwise the fish may stick to the lid of the can after
packing.
