Fish and sea food may be spoiled by autolysis, oxidation, or bacterial activity or most commonly by combination of these. Most fish flesh, however is considered more perishable than meat because of more rapid autolysis by the fish enzymes and because of the less acid reaction of fish flesh that favors microbial growth.
Factors influencing kind and rate of spoilage
- The kind of fish
The various kinds of fish differ considerably in their perishability. Thus some flat fish spoil more readily than round fish because they pass through rigor mortis more rapidly but a flat fish like the halibut keeps longer because of the low pH ( 5.5 ) of its flesh. Certain fatty fish deteriorate rapidly because of oxidation of the unsaturated fats of their oils. Fishes high in trimethylamine oxide soon yield appreciable amounts of the ‘ stale – fishy” trimethylamine.
- The condition of the fish when caught
Fish that are exhausted as the result of struggling, lack of oxygen, and excessive handling spoil more rapidly than those brought in with less ado, probably because of the exhausation of glucogen and hence smaller drop in pH of the flesh. “ Feedy” fish i.e. those full of food when caught are more perishable than those with an emplty intestinal tract.
- The kind and extent of contamination of the fish flesh with bacteria
These may come from mud, water, handlers and the exterior slime and intestinal content of the fish and are supposed to enter the gills of the fish, from which they pass through the vascular system and thus invade the flesh, or to penetrate the intestinal tract and thus enter the body cavity.
Chilling the fish is the most commonly used method for preventing or delaying bacterial growth and hence spoilage until the fish is used for is otherwise processed. The cooling should be as rapid as possible 0 to – 1C, and this low temperature should be maintained. The warmer the temperature, the shorter the storage life of the fish.
Bacteria causing Spoilage
The bacteria most often involved in the spoilage of fish are part of the natural flora of the external slime of fishes and their intestinal contents. The kinds of bacteria causing spoilage vary with the temperatures at which the fish are held. At lower temperatures species like Pseudomonas, acinetobacter, Moraxella and Flavobacterium species are most common. At higher temperature bacteria of genera Micrococcus and Bacillus are predominant. Discoloration of fish may occur during spoilage, yellow to greenish – yellow colors caused by Pseudomonas fluorescens, yellow micrococci and other, red or pink colors from growth of Sarcina, Micrococcus or Bacillus specie or by molds or yeasts and a chocolate brown color by an asporogenous yeast. Pathogens parasitizing the fish may produce discoloration or lesions.
Spoilage of special kinds of Fish and Seafoods
- Slat fish are spoiled by salt tolerant or halophilic bacteria of the genera Serratia, Micrococcus, Bacillus, Alcaligenes, Pseudomonas, and others which often cause discoloration, a red color being common. Molds are the chief spoilage organisms on smoked fish.
- Marinated fish should present no spoilage problems unless the acid content is low enough to permit growth of lactic acid bacteria or the entrance of air content is low enough to permit growth of lactic acid bacteria or the entrance of air permits mold growth.
- Crabmeat is deteriorated by Pseudomonas, Acinetobacter and Moraxella at chilling temperatures and mainly by Proteus at higher temperatures. Species of Pseudomonas, Alcaligenes, Flavobacterium and Bacillus have been incriminated in the spoilage of raw material Lobesters. Crabs and oysters may contain species of Vibrio, including V.parahaemolyticus. The levels in these products fluctuate with seasonal temperature changes.
- Oysters remain good condition as long as they are kept alive in the shell at chilling temperature, but they decompose rapidly when they are dead. Oysters are not only high in protein but also contain sugars, which result from hydrolysis of glycogen. At temperature near freezing,Pseudomonas, Acinetobacter and Moraxella species are the most important spoilage bacteria but Flavobacterium and Microcuccus species also may grow. At higher temperature the souring may be the result of the fermentation of the sugars by coliform bacteria, streptococci, lactobacilli and yeast to produce acids and a sour odor. Early growt of Serratia, Pseudomonas, Proteus and Closstridium may take place. An uncommon type of spoilage by an asporogenous yeast causes pink oysters.