Chilling and Icing of Fish
Ø Protection against quick melting of ice
Ø Preservation of ice and fish in community ice box
Ø Ice storage life of adequately iced fish
Ø Factors influencing the quality of fish during chilling/icing
Ø Changes in fish during the chilling/icing process
Chilling and icing of fish:
Protection against quick melting of ice
1. Ice will melt slowly if the ice box is kept in the shade or under a shed, not directly under the sun;
2. Keeping the door of fish hold or ice box closed will reduce the rate of ice melting;
3. Adequate insulation of ice box will reduce the rate of ice melting. Insulation can be done by placing different insulating materials in between the two layered side walls, for example-styrofoam, polyestyrine, cork, glass fibre, plastic, polythene, saw dust, tea, etc. Most of these are very expensive. Ice box can be insulated by cheaper materials too. These include old clothes, used nets, dried plant leaves, etc. A kind of plant leaves, Hogla can be used as an effective insulator.
4. Small and thin pieces of ice absorb heat from fish more quickly than the large ice crystals and maintain an equilibrium temperature state inside the box. Within 30 to 60 minutes of icing, melting of ice stops in the ice box. Ice crystals solidify again around the fish making a large solid mass of ice. Under adequate insulation, small ice crystals can rapidly and effectively lower the temperature. Therefore, rate of ice melting is also reduced by this method.
Preservation of ice and fish in community ice box
Excess or unsold fish of the day can be preserved in community ice box kept in fish market. Ice block can also be preserved in community ice box in remote areas along the coastal belt or islands, where ice plant does not exist.
Community ice box is a large ice box, preferably 4 x 8 x 3.5 feet deep in size, primarily designed(Nowsad, 2005b) to preserve fish for next day’s sale. This is so named by the fishers community of Cox’s Bazar because of its operation through community based approach. Small enterprise groups (5-6 members) of the ECFC project in the coastal fishing villages have jointly constructed these large ice box and kept in common places, preferably in the fish markets. The boxes have been jointly used by the business group members on space sharing basis.
Individually, one fisherman can not afford the cost of this large ice box, but collectively, they can draw substantial benefit from it. The space inside is partitioned into several portions and allocated for different members. Ice blocks can also be preserved in such ice box for couple of days or weeks. To avoid solidification, small pieces of wood or bamboo may be placed in between the blocks while keeping ice blocks for several days. In coastal areas or islands, ice blocks can be transported from the ice plant by the empty fish hold of the fishing boats. Insulated rickshaw van, power trailer, etc. can be used in other areas.
Ice storage life of adequately iced fish
During ice storage, generally-
v Non-fatty fish can be kept longer than fatty fish;
v White fleshed fish can be kept longer than drak fleshed fish;
v Freshwater fish can be kept longer than marine fish;
v Tropical fish can be kept longer than temperate fish;
v Tilapia can be kept for 28 days, but Indian major carp, rohu for 35 days;
v Maitta/Surma (mackerel) can be kept within the range of 9 and 15 days;
v Hilsha (river shad) can be kept for a range between 15 and 17 days.
Fish kept at 0°C in crushed ice remain fit for human consumption for about 18 days;
Fish kept at 5°C remain fit for about 6 days;
Fish kept at 10°C remain fit for only 2 days.
Every 5°C reduction of body temperature towards 0°C can increase the keeping quality of fish by 3 folds.
Factors influencing the quality of fish during chilling/icing
Following important factors should be considered to maintain the quality of fish during icing/chilling.
I. Care in handling: Transfer of fish in and out of board and during transportation might cause physical damage which can provide access through cuts and wounds for spoilage bacteria. Careful handling during chilling/icing results in considerable improvement of the quality of fish;
II. Delay in icing: Delay in icing even in winter causes considerable adverse effect on the keeping quality of fish when the fish lie on deck after catch. Fish packed into ice boxes straight from the net and then left undisturbed are noticeably superior to fish taken from bulk lots and packed inshore boxes at landing.
III. Evisceration and removal of gills: According to the practice and also supported by official regulations in many countries, it is advisable to remove gills and guts from the larger species of fish in cold condition. There are two reasons for gutting: first, the large number of bacteria in the intestine spoil the fish; second, the continuous action of the digestive juice of the fish cause greater incidence of autolytic spoilage.
IV. Cleanliness: Cleanliness is important for high keeping quality of fish in two ways:
a) Natural source of bacteria can largely be removed by washing out of the gut cavity and off the slime from the body surface.
b) Fish is handled in a hygienic manner during icing so that the chances of contamination are kept to a minimum.
Changes in fish during the chilling/icing process
i. Protein and weight loss: The fish chilled with ice shows gradual weight loss in the lower layers. Losses which occur in iced fish fillets and steaks are largely or entirely due to formation of free liquid drip. Melting water carries with it a considerable percentage of soluble proteins, salts, other flavouring and nutritive substances of the fish.
ii. Discolouration: Excessive pressure of ice on fish during chilling results bruising and damage and consequent discolouration of flesh during icing. If the fish is not gutted soon after being caught, the powerful digestive enzymes attack the viscera and belly walls called belly burn or disruption in course of few days at ice temperature which also cause discolouration. It is well known that pelagic fishes with full of stomach may develop torn bellies long before the sign of spoilage set in.
iii. Rancity: In case of fatty fishes in low temperature at even 0 to -2°C, rancidity may develop and the rancid flavor becomes a limiting factor of keeping quality during long time storage in ice.
iv. Shrinkage: Shrinkage is a common phenomenon in fish packed with ice, particularly in the upper layers. The shrinkage with ice, particularly in the upper layers. The shrinkage in the lean fish is higher than that of fatty fishes. Subcutaneous layer of fat serves to reduce the evaporation of tissue moisture.
v. Weight gain by fish in RSW: Fish in RSW gain weight and salt uptake is probably the most important factor that limits the application of RSW system. Fish intended for normal processing and marketing can acquire salt which would make them unacceptable for this purpose. Salt uptake in industrial fish is also critical since it is concentrated during processing and the upper limit is usually equivalent to a concentration of about 0.5% in the raw fish.