 |  | Post-Harvest Losses of Fish in the Tropics (NRI) | ||
 |  | Nutritional losses | ||
|  | (introduction...) | ||
| | Where nutritional losses occur | ||
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Nutritional loss can occur in fresh fish and some loss of nutrients is inevitable in all forms of food processing Fish processing is no exception, and in traditional methods the losses may be serious. Procedures in which the fish is heated to fairly high temperatures, such as smoking, can result in damage to the nutritional value of fish protein, with losses in availability of Iysine and other essential amino acids. These losses may not be significant if the people eating the processed fish have an adequate diet in other respects. In some parts of the world people eat relatively small quantities consumption from Gujarat to Delhi. This was about 10% of the actual wholesale income of the market and probably represents a typical figure. It was reported that 10-25% of fish arriving in Delhi was of poor quality, and attracted only about 50% of its high-quality price and that a further 5% loss was incurred by net physical drip loss. Since greater care is taken in handling the higher value species compared with the lower value ones, extent of economic loss can be related to species value.
One factor which must be borne in mind in considering economic losses is the availability of food for lower income groups. Sadly there are many people in developing countries who can only afford fish which has deteriorated and lower in value. The economic loss to the fishermen, processor or trader makes food available for these people. It can often happen that improvement in quality of a product which is to be consumed domestically could increase the price beyond the means of some indigenous purchasers.
Fresh fish
Fresh fish, as has already been emphasised, is extremely perishable, and is subject to bacterial spoilage. As the fish spoils, its nutritional value decreases, as the bacteria causing the spoilage degrade the protein which is intended for human consumption. However bacterial action produces nitrogenous compounds with noxious odours, and the affected fish will become highly unattractive before there is too much nutritional damage.
Bacterial spoilage in fresh fish can produce toxins which cause food poisoning; histamine contamination is prevalent among pelagic fish such as mackerel and sardine. Pathogenic bacterial contamination of fresh fish caused by poor handling, and washing the fish in polluted water can also cause food poisoning.
Cured fish
Traditional processing of cured fish frequently involves high temperatures, particularly when the fish is smoked. It used to he thought that salting and drying effectively preserved the nutritional value of the original fresh fish, and much of the older literature (for example Cutting, 1962) seemed to confirm this. More recent work, particularly that carried out at the Natural Resources Institute (NRI), has indicated that this is not the case. Measurements used to he based on the gross composition of the product hut now more sensitive analytical methods are available. It has become clear that a more appropriate measure of nutritional loss should he based on the biological as availability of the muscle constituents of the fish flesh. Recent work has shown that traditional processing methods can cause a loss of nutrient availability when fish muscle constituents, although still physically present, are not able to he utilized by the human hot y. (Carpenter anti Booth (1973) showed that fairly high temperatures, of about 150° as are encountered in smoking' affect the availability of Iysine, one of the amino acids fount in fish protein. More recent work carried out at NRI has indicated that loss of availability of Iysine and other essential amino acids Could' also occur at much lower temperatures, such as 0°. This raises the possibility that nutritional losses can occur when fish is sun dried . Research is being carries out on the extent to which this damage occurs in practical fish drying operations.
Other nutrients present in fish muscle which can he affected by the heat used in traditional curing methods include methionine and other sulphur amino acids, and vitamins and K.
Rancidity developement in cured fish can also lead to nutritional loss Rancidity is caused by the oxidation of fish lipids, and so oily fish such as sardine anti mackerel is
particularly prone to it. Oxidation continues during storage of cured fish, leading to the development of a bright yellow/orange colour and distinctive and unpleasant 'painty' odours. The product will become most unattractive to consumers, and may be completely rejected.
The development of rancidity is undesirable from a nutritional point of view for two reasons. Firstly, because as fish oils oxidize they react with and damage the protein, and their nutritional value decreases. Secondly, because there is evidence suggesting that rancidity are toxic when ingested.
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