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The enzymes that cause spoilage of fish are active at low temperatures and fish oils become oxidised at fairly low temperature, fish catches subjected to temperatures above freezing deteriorate fast. The entire fish catch is usually aboard the fishing vessel is packed with crushed ice or by mechanical refrigeration. The use of antibiotics and other preservatives in the crushed ice retards bacterial action to some extent.
The most common chilling methods are wet ice, mixtures of ice and sea water or ice-cold water or salt-water icing. The shelf life of cold water fish chilled to 0oC immediately after post-mortem is 1-2 weeks and fish from tropical waters will have longer shelf-life. Ice necessary to cool 1 kg of lean fish from 25oC to 0oC is 0.25 kg. Ice may be used in the form of blocks, plates, tubes shells, softs and flakes. Ice flakes are more popular due to cooling efficiency and economics. The ideal temperature of chilled fish is 0oC to -2oC.
The quality of frozen stored fish is influenced by the species, composition, size, how and where caught, post harvest handling before freezing, the state of rigour and quality when frozen and the freezing process. Oxidative deterioration, dehydration, toughening, loss of juiciness and excessive drip are some of the major problems in frozen stored fish.
Post-harvest handling should be carried out carefully to minimise deterioration of micro-organisms and by autolysis prior to freezing. Freezing should be carried out as quickly as possible. Quality fishes should be gutted and intestines removed on board vessel itself and kept in ice boxes. Prefrozen fish should be handled at zero degree centigrade. Toughness and loss of juiciness can be minimised by storing the products below -18oC.
According to Central Institute of Fisheries Technology at Kochi, oxidative changes of lipids in fish can be reduced by eliminating oxygen, avoiding contamination with oxidative calalysts, adding antioxidants, avoiding irradiation and using very low storage temperatures.
Fish are glazed with layers of ice by dipping in cold water. Dipping and freezing may be repeated several times to build up thick glaze to protect the frozen fish from surface oxidation and from freezer burn (drying out) during frozen storage. Glazing and protective packing can prevent dehydration.
Thaw drip can be reduced by controlling glycolysis prior to freezing, using certain chemicals and following good practices for freezing, storage and thawing.
The rate of freezing can exert some influence on the final quality of thawed fish. The attainment of fish temperature to -18oC lower, in a period of two hours, produces good results. All freezing methods can be used for freezing fish. Plate freezing is used for retail packages of fish. Air-blast freezing is used for individually packaged fish and shell fish product or when unpackaged and individually frozen. Excessive dehydration can occur during air-blast freezing of unpackaged products. Liquid immersion freezing is used for freezing fish and shrimp at sea. It is not advisable to freeze fatty fish in sodium chloride solutions because it accelerates oxidation of lipids during frozen storage. Liquid nitrogen freezing can be used for freezing fish and shell fish.
There is a critical zone in freezing depending on the temperature and time. ‘Critical zone’ can be overcome when the temperature of the product has been reduced over the shortest period of time. The initial critical low temperature expected for seafood is about -40oC, which is the temperature at which bacterial and enzyme activity on fish comes to a standstill. The sharp dip in temperature is crucial, because a longer freezing time means larger ice crystals being formed within the fish causing damage to edible tissue. Once the peak freeze has been achieved, and then the storage temperature can be about -23oC. If the frozen state at -21o to -23oC may retain its quality for as long as 2 years.
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