Effects of agar-oligosaccharides and EGCG on the freshness and protein biochemical change of sea bass (Lateolabrax japonicus) fillets during ice storage

Sea bass (Lateolabrax japonicus) is an economically important marine species widely cultured in fisheries worldwide. During storage and transportation, L. japonicus are susceptible to be deteriorated due to the combined effects of microorganisms and endogenous enzymes, resulting in the change of freshness and protein biochemical, which affects its edible value. Therefore, it is particularly important to ensure its quality and safety by taking the appropriate regulation methods of fish quality changes during storage and transportation. In this study, the quality parameters including texture, color, and freshness parameters including thiobarbituric acid, K-value, total volatile based nitrogen, total viable count and sensory score, and the protein biochemical characteristics (myofibrillar protein content, secondary structure, sulfhydryl and carbonyl content) were measured periodically to analyze the effect of epigallocatechin gallate (EGCG) and agar oligosaccharide (AO) on the freshness of sea bass fillets during ice storage. The results showed that the total number of colonies and K values of the control group at the 16th day were as high as (6.78±0.15) lg (CFU/g) and 67.37%±1.75% respectively, and the sensory score was lower than 8, indicating the fillets were inedible at this time. While the samples of EGCG group (EG), the agar oligosaccharide group (AO) and the composite group can maintain value of the hardness, color, and other freshness related index. The TBA value of the control group were as high as (0.81±0.03) mg/kg after post mortem 20 days. However, the TBA values of the EG group, AO group, and composite group did not increase significantly during storage, and were always lower than the control group. In addition, the carbonyl content of the control group was significantly higher than that of other treatment groups from 12 days (P<0.05), and increased to (1.63±0.04) nmol/mg at the end period of storage, while the carbonyl content of the EG group, AO group and composite group was only (1.14±0.03), (1.28±0.08) and (0.99±0.09) nmol/mg respectively. The contents of protein, and sulfhydryl of the composite group samples were (70.58±2.02) mg/g and (39.39±0.99) μmol/g prot, respectively, which were higher than those in the control group and other treated groups at 20 days. The value of α-helix were15.38%±0.55%, 15.30%±1.85%, and 18.47%±0.74% in EG group, AO group, and composite group at 16 days, respectively, which were significantly higher than that of control with 13.26%±0.51%. The value of random coil was increased from the initial vale of 13.97%±0.91% to 24.36%±2.57% when storage for 16 days, while the value of composite group was only 16.75%±0.99%, indicating that the treatment of EG and AO suppressed the disordered changes of proteins.The results showed that EGCG and agar oligosaccharide had both good preserve effect, which could maintain the freshness of fish fillets, and significantly inhibit protein denaturation and oxidation of proteins. EGCG combined with agar oligosaccharide had better preservative effect than the single preservative, and extend the shelf life to more than 20 days. This research provides a theoretical basis for the preservation mechanism of EGCG and agar oligosaccharide and a reference for the development the novel efficient biological preservatives.