1Final processing of fish Heating Canning The canning process is a sterilization technique that kills microorganisms already present on the fish, prevents further microbial contamination, and inactivates degradative enzymes. In this process fish are hermetically sealed in containers and then heated to high temperatures for a given amount of time. Canned fish can be stored for several years.

2However, sterilization does not kill all microorganisms, and bacterial growth and gas production may occur if the products are stored at very high temperatures. Because the severe thermal conditions of canning cause the disintegration and discoloration of the flesh of many species of fish, only a few types of fish are available as canned products. The most common types are tuna, salmon, herring, sardines, and shrimp. The thermal processing does not have a detrimental effect on the high-quality protein of the fish

3In addition, these species are often canned with their bones left intact. The bones become soft and edible, significantly increasing the level of calcium present in the fish product. Tuna is an exception; because of special handling considerations, the bones of tuna are removed prior to canning. Tuna is normally caught far offshore and must be frozen and held for some period of time prior to canning.

4During this freezing and holding period unsaturated fatty acids are oxidized, causing the tuna to become rancid. The rancidity is removed by precooking, and the bones are removed at this time in order to facilitate the cutting and preparation of the meat for canning. Freezing Of the many processing methods used to preserve fish, only freezing can maintain the flavour and quality of fresh fish.

5Freezing greatly reduces or halts the biochemical reactions in fish flesh. For instance, in the absence of free water, enzymes cannot react to soften and degrade the flesh. The three steps for freezing fish include immediate cooling and holding, rapid freezing, and cold storage. If fish is frozen improperly, structural integrity may be compromised because of enzymatic degradation, texture changes, and dehydration.