Lipid Composition and Deterioration in Light and Dark Muscle of Rainbow Trout (Oncorhynchus mykiss)
Nutritionally valuable lipids, including lipid classes and their fatty acids in light and dark muscle, have been determined and analyzed in farmed rainbow trout. Dark muscle was found to have a higher content of total lipids than light muscle, 7.3 wt% versus 3.0 wt%, which was mostly accounted for by neutral lipids, especially triacylglycerols. The proportion of phospholipids in dark muscle was less than in the light muscle, 32.2 wt% versus 56.9 wt% of total muscle lipids. The differences in lipid classes are also indicated in the differences of fatty acid composition; the high content of phospholipid in light muscle causes a higher polyunsaturated fatty acid content. Docosahexaenoic acid (C22:6n-3) was the major fatty acid present in light muscle. Palmitic acid (C16:0) together with oleic acid (C18:1n-9) were the major fatty acids in the dark muscle. Rainbow trout were frozen as fillets and stored at -15oC for up to 34 weeks. Results from analyses of lipid hydrolysis, and subsequent lipid oxidation during frozen storage of light and dark muscle revealed that the major deterioration of lipids is due to hydrolysis and to a minor extent by oxidation. The content of free fatty acids increased after 34 weeks of frozen storage from originally 0.1 - 0.5 wt% to 7.3 wt% in dark muscle, and to 9.3 wt% of total lipids in light muscle. The following free fatty acids dominated after 34 weeks of storage: C16:0, C16:1n-7, C18:1n-9, C20:5n-3 and C22:6n-3. The phosphatidylcholine content of the muscle decreased by 25 - 50% after 34 weeks of frozen storage. Losses of polyunsaturated fatty acids from both neutral and polar lipids occurred during frozen storage. An increased content of polyunsaturated fatty acids, especially eicosapentaenoic acid (C20:5n-3) and C22:6n-3, was found in fish from low temperature water, 5oC, however there was also decreased lipid stability during frozen storage. Carotenoid supplementation to the diet of farmed rainbow trout did not improve the lipid stability during frozen storage, although the content of astaxanthin and a-tocopherol decreased during frozen storage.