Protein oxidation in Atlantic mackerel (Scomber scombrus) during chilled and frozen storage
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There are numerous health benefits associated with the consumption of fatty fishes such as mackerel. They provide important nutrients linked to their lipid, proteins, vitamins and minerals. As well as being a rich source of omega-3 fatty acids, fatty fish is an excellent protein source. Fish provides proteins of high biological value which contains essential amino acids with high digestibility. There are many steps during pre-handling, handling, storage and processing which may affect lipids, proteins and vitamins of fatty fishes leading to the reducing nutritional values. To maximize the health benefits of fatty fish, it is therefore important to know how these steps will affect the different components of fish and how these compounds can be retained during different processing steps. The main aim of this work has been to study the oxidation of sarcoplasmic and myofibrillar proteins of white, black and whole mackerel muscle during chilled (+4̊ C) and frozen storage (-30̊ C). The samples were either chilled stored for 4 and 9 days or frozen stored for 1, 7 and 12 months. The contents of protein carbonyl and total thiol were used to measure the extent of protein oxidation during fish storage. Protein carbonyl was measured by the use of a conventional DNPH-based and an ELISA method. The second aim was to evaluate the effects of protein oxidation on water holding capacity and protein solubility. The lipid oxidation during chilled and frozen storage of the same samples as used in the present work, have been evaluated in a master project and the results was used to consider the relationship between lipid and protein oxidation.The conventional DNPH-based method was found not to be a useful method for measuring the content of protein carbonyls in fatty fish like mackerel due to the incomplete resolubilization of the protein pellet in guanidine hydrochloride. Carbonyl contents of sarcoplasmic and myofibrillar protein of all muscle types were relatively stable during chilled and 1 month frozen storage of samples. This could implicate that protein carbonylation was not developed in samples during refrigerated and 1 month frozen storage. However, the total thiol group generally decreased in myofibrillar proteins during chilled and 1 month frozen storage, which could be an implication of protein thiol group oxidation in those samples. WHC was relatively stable during chilled and 1 month frozen storage. Sarcoplasmic protein solubility was also stable during this period of storage while the myofibrillar protein solubility generally decreased. Frozen storage for 7 and 12 months significantly influenced the oxidative stability of sarcoplasmic and myofibrillar proteins of all mackerel muscle types. Sarcoplasmic and myofibrillar protein carbonyl contents increased significantly during frozen storage at -30̊ C for 7 and 12 months compared to the chilled and 1month frozen storage. At the same time, the total thiol group contents decreased significantly. These results show that duration of frozen storage has a strong impact on protein oxidation in mackerel. Increased protein carbonyl contents and decreased thiol group contents in 7 and 12 months frozen samples occurred concomitantly with a loss of WHC and protein solubility. It could be concluded that protein oxidation had a significant effects on the loss of WHC and protein solubility. Myofibrillar proteins had significantly higher carbonyl content, lower total thiol group content and lower protein solubility compared to the sarcoplasmic proteins in both chilled and frozen stored samples. This may implicate the higher stability of sarcoplasmic proteins with regard to protein oxidation and freeze denaturation compared to myofibrillar proteins. While the black and mixed muscle had a significantly higher sarcoplasmic and myofibrillar carbonyl contents compared to the white muscle, no clear relationship between muscle types and loss of thiol groups, WHC and protein solubility was observed. The results from the lipid oxidation study on the same mackerel samples show decreased level of TBARs after 7 months frozen storage. The increased carbonyl contents and decreased thiol group contents at the same time may indicate a correlation between protein and lipid oxidation progress in mackerel.