Changes in protein fractions of rainbow trout (Oncorhynchus mykiss) gravads during production and storage

Gravad fish products belong to a group of low-processed products obtained from fresh fish by sprinkling fillets with a mixture of sugar and salt and then placing them in cold storage. Little is known about changes in the tissue during the production and storage of gravads. The purpose of the present study was to investigate the type and scope of changes in rainbow trout (Oncorhynchus mykiss) gravad proteins during processing and vacuum storage at 3 °C and −30 °C. The results showed that the solubility of protein was barely affected by the gravading process, but did change during the cold storage and freezing of the gravad. Catheptic activity decreased with gravading and further storage. A SDS-PAGE analysis revealed that the intensity of the M-protein and troponin bands was reduced, while the bands of α-tropomyosin and the myosin heavy chain increased when the trout muscle was subjected to the gravading process. Two bands with molecular weights of 255 and 135 kDa disappeared in the gravad, and new bands with molecular weights of 163 and 117 kDa appeared. An HPLC analysis revealed that gravading had a limited effect on the relative areas of peaks corresponding to low molecular weight substances of protein origin, while more significant changes were observed during gravad storage. Moreover, the gravading process caused a lowering of the actin and myosin denaturation temperature, which was proved using the DSC method. In conclusion, changes in rainbow trout muscle protein appeared to have a crucial effect on product quality.