Quality changes of whitespotted conger (Conger myriaster) based physicochemical changes and label-free proteomics analysis during frozen storage

Abstract

Whitespotted conger (Conger myriaster) muscle proteins were susceptible to oxidative denaturation during frozen storage. The objective of this study was to investigate the alterations in quality through physicochemical analysis and proteomics after whitespotted conger stored at temperatures of −18 °C and −60 °C. The microstructural observation revealed the noticeable variations such as increased interstitial space and fractured muscle fibre with extension of frozen storage time, and the muscle fibre of whitespotted conger stored at −60 °C were more intact than those stored at −18 °C. The raised TVB-N value indicated that the freshness of whitespotted conger decreased during 120-day frozen storage period. Analysis of myofibrillar protein content and SDS-PAGE demonstrated that compared to −18 °C, lower storage temperature (−60 °C) could better maintain the structure of whitespotted conger muscle by inhibiting protein degradation and oxidation. To reveal the mechanism of protein degradation, label-free quantitative proteomic analysis was performed through LC-MS/MS. The structural proteins including domain-associated proteins and actin-related proteins were up-regulated during frozen storage, but the phosphoglycerate kinase, phosphoglycerate mutase, and fructose-bisphosphate aldolase were down-regulated. Storage at −18 °C accelerated the up- or down-regulation of those differentially abundant proteins. According to KEGG analysis, up- or down-regulated pathways such as glycolysis/gluconeogenesis, carbon metabolism, biosynthesis of amino acids, and calcium signalling pathway mainly accounted for the protein degradation and quality reduction of whitespotted conger at low temperature. These results provided a theoretical basis for improving the quality stability of whitespotted conger during frozen storage.