Effect of oxidation modification induced by peroxyl radicals on the physicochemical and gel characteristics of grass carp myofibrillar protein

Protein oxidation of fish products during storage and processing is of interest recently. In this study, the effect of peroxyl radicals generated from 2,2 '-azobis (2-amidinopropane) dihydrochloride (AAPH) on the physicochemical and gel properties of myofibrillar protein (MP) from grass carp was investigated. After incubation with AAPH (0.04, 0.2, 1, 5, 25 mM), the carbonyl and dityrosine content, and surface hydrophobicity index of MP were higher than that of the non-oxidation control. Conversely, this treatment decreased the free ammonia, total and free sulfhydryl content of MP progressively. The transformation from alpha-helix to beta-sheet and beta-turn in MP treated with AAPH was found. The fluorescence quenching of tryptophan occurred in the 5 and 25 mM AAPH groups. When the AAPH concentration reached to 25 mM, the protein solubility decreased to 58%, compared to the control group. Meanwhile, this AAPH treatment damaged the elastic feature of MP and also decreased the water holding capacity, gel strength and texture of gel, which were closely depended on the oxidation degree. The poor and rough microstructure provided solid and visible evidence for the loss of gel properties in oxidation treatment groups. In short, these results gave a deeper understanding for the changes of physicochemical and gel properties of fish MP subjected to oxidation environment.