Fish can be preserved by canning, chilling, freezing and curing.
Unlike most foods, the enzymes of fish operate at about 5oC, the temperature of the water from which they came. In order to keep fish for any length of time, they must be frozen, canned or cured.
Canning: In canning, fish is dressed and washed. It is cut into pieces and filled in cans in saline. The cans are double steamed under vacuum. The sealed cans are sterilised at steam pressure at 121.1oC for 90 minutes.
The sterilised cans are cooled in running water, wiped and kept in a cool place 10-15.5oC. Fishes like salmon, tuna, sardines, mackerel, herring, lobster, crab, clams, shrimp and mussels are canned. Of these, salmon and tuna are the most popular.
Canned shellfish may become dark or discoloured during storage. This discolouration is thought to be caused by ferrous sulphide formed by the hydrogen sulphide released from the fish and the iron in the can. The use of a special enamel can containing zinc is effective in preventing this discolouration. Shellfish is sometimes dipped in acetic, citric or tartaric acid before it is packed. This increases the acidity of the fish and reduces the possibility of ferrous sulphide formation. However, the colour changes in canned tuna are the result of a lack of vitamins in the flesh. Ascorbic acid and niacin are effective in returning the flesh to its normal pink colour.
Smoking: Smoking is a method of preserving surplus stock of fish. Smoked fish have a stronger flavour than fresh fish. The fish is either salted or not salted before it is smoked. It is then hung on rods in an oven or kiln. The smoke is blown over the fish for varying lengths of time. Smoked fish is available in the Indian markets.
Salting and Drying: Salting and drying of fish are the most important methods of fish preservation. The process of salting and drying includes the following steps.
Cleaning and dressing: Fish is cleaned and dressed. Dressing is done by splitting the fish open on the ventral side along with the cerebral column with the dorsal side as hinge. Gills and entrails are removed. In the case of large fish, fillets are prepared. Salting is done with the addition of potassium sorbate, sodium benzoate and acid sodium phosphate (0.5 percent). Salting is carried out at room temperature for 48 hours. Salted fish is washed over with 5 percent salt solution to remove excess of salt sticking to the surface of the fish.
Drying and packing: The salted fish is dried in the sun or cabinet driers with inlet air temperature at 82.2oC and outlet air temperature of 43.3oC to a moisture content of about 25 percent. The dried fish is packed in polythene bags and wrapped in wax paper and packed in wooden boxes.The sea food units which process the raw fish and turn it into value added delicacies-some of them are packed ready to eat curry, fish pickle, mussel pickle, oyster papad require giant cryogenic equipment apart from insulated vans to carry the raw material and finished product.