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| | | Post-harvest changes in fish |
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|  In human nutrition, fatty acids such as linoleic and linolenic acid - important for preventing skin diseases - are considered essential as they cannot be synthesised by the organism. In marine fish, these fatty acids constitute only around two percent of the total lipids - a small percentage compared with many vegetable oils. However, fish oils contain other "essential" polyunsaturated fatty acids which act in the same way as linoleic and arachidonic acids. As members of the linolenic acid family (first double bond in the third position, w-3 counted from the terminal methyl group), they also have neurological benefits in growing children. One of these fatty acids, eicosapentaenoic (C20: 5 w 3), has attracted considerable attention since Danish scientists found a significant presence of it in the diet of a group of Greenland Eskimos who proved virtually free from arteriosclerosis. Studies in the United Kingdom and elsewhere have documented that eicosapentaenoic acid in the blood is an extremely potent anti-thrombotic factor.Immediately after capture, several chemical and biological changes take place in dead fish which can ultimately lead to rejection for human consumption because of spoilage. Fish post-harvest losses are significant, especially in developing countries. Estimated at 10 to 12 million tonnes, they account for around 10 percent of global capture and cultured fish. Therefore, understanding the post-harvest changes that occur in fish is very important in developing appropriate measures to reduce losses and preserve the quality and safety of the finished products. | | | | The most obvious changes fish undergo after capture are sensory, the foremost being the onset of rigor mortis due to a loss of the limp elastic texture of the muscle which contracts before becoming hard and stiff. This condition usually lasts for a day or more in iced fish, then rigor resolves. Other changes relate to the appearance, odor, texture and taste. | | | | Sensory changes of fish are due to the enzymatic breakdown of major fish molecules. These reactions are catalysed either by autolytic or bacterial enzymes, as summarized in the table below. | | | | Summary of Autolytic Changes in Chilled or Frozen Fish Enzyme(s) Substrate Changes Encountered Prevention glycolytic enzymes Glycogen production of lactic acid, pH of tissue drops, loss of water-holding capacity in musclehigh temperature rigor may result in gaping fish should be allowed to pass through rigor at temperatures as close to 0°C as practically possiblepre-rigor stress must be avoided autolytic enzymes involved in nucleotide breakdown ATPADPAMPIMP loss of fresh fish flavour, gradual production of bitterness with Hx* (later stages) same as aboverough handling or crushing accelerates breakdown cathepsins proteins, peptides softening of tissue making processing difficult or impossible rough handling during storage and discharge chymotrypsin, trypsin, carboxy-peptidases proteins, peptides autolysis of visceral cavity in pelagics (belly-bursting) problem increased with freezing/thawing or long- term chill storage calpain Myofibrillar proteins softeningmolt-induced softening, in crustaceans removal of calcium thus preventing activation collagenases Connective tissue gaping of filletssoftening of muscle tissue connective tissue degradation related to time and temperature of chilled storage TMAO demethylase TMAO formaldehydeinduced toughening of frozen gadoid fish store fish at temperatures less than or equal to -30°Cphysical abuse and freezing/thawing accelerate formaldehyde-induced toughening *: Hx: Hypoxanthine. TMAO: Trimethylamine oxide | | | | Microbially induced changes result from bacteria found on all the outer surfaces (skin and gills) and in the intestines of live and newly-caught fish. These bacteria invade the muscle and cause gradual degradation of several of its constituents (carbohydrates, nucleotides, amino acids and other NPN molecules), producing undesirable volatile compounds such as trimethylamine, volatile sulphur compounds, aldehydes, ketones, esters and hypoxanthine, as well as other low molecular weight compounds. | | | | The last cause of fish spoilage is lipid oxidation and hydrolysis that leads to the development of rancidity, even with storage at subzero temperatures. This is due to the large amount of polyunsaturated fatty acid moieties found in fish lipids. In fact, this is a major cause of spoilage of frozen fish. | | | | |
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