Chapter 12
Seafoods and Fishery Byproducts
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Natural and Unnatural Environments for Longer Chain Omega-3 Fatty Acids R. G . Ackman and H. Gunnlaugsdottir Canadian Institute of Fisheries Technology, Technology University of Nova Scotia, P.O. Box 1000, Halifax, Nova Scotia B3J 2X4, Canada
The highly unsaturated fatty acids characteristic of fish lipids include those with five (20:5n-3) and six (22:6n-3) ethylenic bonds. These are part of the total fatty acid intake in food systems in many parts of the world and there is really no other original food source except food fish and shellfish. In most fish the edible muscle is a friendly environment for highly unsaturated fatty acids, especially in frozen storage. However a variety of species-specific problems can be attributed to subdermal fats, or to trimethylamine oxide as a source of oxygen, or to enzyme activity even in frozen storage. Actual oxidation in minces is a man-made situation. One way to transfer these oxidation-susceptible fatty acids to our diet is to feed fish meal as a protein source to the broiler chicken. The oxidation status of the lipids in the original fish meal has been shown to have little effect on flavor when fish meal is fed to fowl at practical levels. The growing aquaculture industry also uses this protein source and salmonids benefit from the long-chain n-3 fatty acids, but fish oil and cheap fats are also involved. Fish meal and fish silage are both unnatural environments for these fatty acids, and oxidation does take place. Nutritional applications of longer-chain fatty acids from these sources require new and detailed studies. Fish (and shellfish) provide an almost unlimited variety of food resources which are, even in the North American region, an endless source of confusion in the popular and scientific nomenclature (1-3). For the purposes of this paper shellfish will be omitted and in the first section the discussion will focus on fish fillets, the most popular and widespread form of fish eaten in our western society. The form of the fish fillet is critical to the access of atmospheric oxygen but this is very important only in the case of fatty fish. The fish or fish product which is physically intact provides a "natural" environment for external and internal oxidation
0097-6156/92/0500-0208$06.75/0 © 1992 American Chemical Society St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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processes, and this environment can be degraded through filleting, skinning of fillets, mincing, and ultimately in the production of fish meal. Fish meal and fish silage, commercial products for non-human use, will be the subject of later sections of this paper.
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The Distribution of Fat in Fish: Skin Fat as a Quality Factor The extremes for fat distribution in fish can be typified by two quite different fish. One is the North Atlantic cod, Gadus morhua (4), where the muscle lipid is
