Post-harvest changes in fish
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, odour, 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 muscle | fish should be allowed to pass through rigor at temperatures as close to 0�C as practically possible |
| autolytic enzymes involved in nucleotide breakdown | ATP ADP AMP IMP | loss of fresh fish flavour, gradual production of bitterness with Hx* (later stages) | same as above |
| cathepsins | proteins, peptides | softening of tissue making processing difficult or impossible | avoid 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 | softening | removal of calcium thus preventing activation |
| collagenases | Connective tissue | gaping of fillets | connective tissue degradation related to time and temperature of chilled storage |
| TMAO demethylase | TMAO | formaldehyde | store fish at temperatures less than or equal to -30�C |
| *: 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.