Fish Microbiology and Quality Assurance

Unit 8 - Spoilage of fresh and processed fish and fishery products

8.1. Spoilage of fish and shellfish
Owing to their high nutritive value the spoilage of fish and fishery products proceeds at a faster rate. Spoilage of fish and shellfish is the result activities of autolytic enzymes, oxidation and associated microorganisms.

Enzymatic spoilage
Enzymatic spoilage is caused by the autolytic fish enzymes. Fishes are highly perishable than meat because of more rapid autolysis by fish enzymes, and favorable conditions for microbial growth due to less acid reactions. The autolytic spoilage can be prevented by reducing the activity of enzymes by lowering the temperature.
Oxidation or non-enzymatic spoilage
Oxidation or non-enzymatic spoilage is caused by the oxidation of fish fat. The oxidative deterioration of many unsaturated fish oils leads to spoilage of fish. Thus, the fatty fishes spoil much faster than lean fishes.
Bacterial spoilage
Bacterial spoilage is caused by the activities of microorganism associated with the fish. Bacterial spoilage of fish begins only after the completion of rigor mortis, which results in the release of products of protein denaturation due to decrease in pH, which is utilizable by bacteria. Thus, prolonging rigor mortis helps to delay spoilage and thereby keeps fish fresh.

Rigor mortis is hastened by struggling of the fish, lack of oxygen and warm temperature. However, rigor mortis can be delayed by reducing enzyme activities by lowering pH and adequate cooling of fish. The pH of the fish has important influence on perishability because of its influence on growth of bacteria. Lower the pH of fish, slower will be bacterial decomposition of fish. Lowering of pH occurs during rigor mortis when muscle glycogen is converted to lactic acid.

Spoilage of both marine and fresh water fish occurs in the same manner. Fish contain high levels of protein and non- protein nitrogenous constituents (16~20 %), lack carbohydrate, and have varying amounts of fat depending on the species of fish. The non-protein nitrogenous compounds in fish include free aminoacids, volatile nitrogen bases- ammonia and trimethyl amine (TMA), creatine, taurine, betaines, uric acid, anserine, carnosine and histamine. Spoilage of fish begins from the surface, gill and intestine because of high bacterial load. From gills, intestine and surface microorganisms ¬gradually migrate to adjacent tissue and cause spoilage. Spoilage organism first utilizes simpler compounds and later fish protein releasing various off-odour compounds.
converted to lactic acid.