Changes in Physicochemical Properties, Protein Oxidation and Free Amino Acids of Trachinotus ovatus during Curing and Air-Drying

This study investigated protein oxidation and the changes of free amino acids during the dry salting and ripening of Trachinotus ovatus. For this purpose, the moisture content, salt content and textural characteristics of dry-salted fish, and the carbonyl content, sulfhydryl content, proteolysis index, free amino acids (FFA), secondary structure and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profile of myofibrillar protein from dry-salted fish were analyzed. The results showed that the salt content of cured fish samples increased, and the water content decreased; the hardness increased, and the springiness increased at first and then decreased during curing and air-drying. Protein oxidation occurred during the processing of dry-cured fish. The carbonyl content of myofibrillar protein significantly increased, while the sulfhydryl content decreased. Meanwhile, the α-helix content decreased, while the random coil content increased. The results of SDS-PAGE showed that the myosin heavy chain (MHC) and actin significantly changed and the myosin heavy chain band became lighter and even disappeared after air-drying, indicating that myofibrillar protein degradation occurred. The proteolysis index increased during the whole process. The non-protein nitrogen content decreased after curing but increased during air-drying and the content of total free amino acids reached the highest after one day of air-drying. The contents of umami amino acids and sweet amino acids increased during the whole process. This study can provide a theoretical basis for quality control in the processing of salted aquatic products.