Oxidative and Hydrolytic Changes in Lipids of pelagic Fish during different Processing Steps
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Lipid oxidation and lipid hydrolysis are two major causes for deterioration in marine lipids, mainly due to the high content of polyunsaturated fatty acids. Differences in fatty acid composition, storage conditions and processes used on the material all influence the reaction rates and affect the end quality and shelf life of the product. The aim of this thesis was to determine in which degree storage factors influence lipid oxidation and lipid hydrolysis in muscles of mackerel and herring. Changes in lipid oxidation and lipid hydrolysis were analyzed and compared to storage factors in a) dark, white and mixed muscles of mackerel, stored at different storage temperature (4°C and -30°C) and storage periods (four and nine days, and one, seven and twelve months) and b) herring marinated in different salt concentrations stored at different storage temperatures (0°C and -4°C) and oxygen availability. Methods to determinate levels of conjugated dienes (CD) and thiobarbituric acid reactive substances (TBARS) in samples were performed to assess lipid oxidation while levels of free fatty acid (FFA) were obtained for evaluation of lipid hydrolysis. In mackerel, dark muscle had a significant higher (p<0.05) lipid content compared to white muscle as expected. No significant differences in primary oxidation products (CD) and significant differences in secondary oxidation products (TBARS) were observed in and between muscle types both at the different storage periods and temperatures. All muscles had significant hydrolysis activity at day nine compared to day four at 4°C storage. Storage at -30°C had a preserving effect on white and mixed muscles while dark muscle remained high in FFA content for one, seven and twelve months. It was found that marinating herring with reduced oxygen availability significantly hindered development of lipid oxidation (CD and TBARS) while had no effect on lipid hydrolysis. Storage temperature (0°C and -4°C) had no significant influence on lipid oxidation and lipid hydrolysis as well. Marinating had a significant enhancing effect on the development of lipid hydrolysis (FFA) compared to before and after salting samples. Thus, it was found that mackerel should be packed in closed packages and stored at -30°C to prevent degradation in fillets, while herring should be marinated with reduced oxygen availability to prevent oxidative rancidity.