Effect of High Salt Oxidation System on Physicochemical Properties and Structure of Yak Myofibrillar Proteins

In order to elucidate the effect of protein oxidation on physicochemical properties and protein structure of yak longissimus dorsi muscle, muscle samples were treated in a Fenton oxidation system (H₂O₂ concentrations of 1, 10, 20, 40 and 60 mmol/L) containing 0.6 mol/L NaCl at pH 5.0 or 8.0 for 1 h. The results showed that with the increase of H₂O₂ concentration, the carbonyl content, surface hydrophobicity and dityrosine content of yak myofibrillar proteins increased, while the contents of total sulfhydryl and active sulfhydryl, solubility, and tryptophan fluorescence intensity decreased. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the numbers of myosin heavy chain and actin bands decreased after protein oxidation, and covalent cross-linking occurred between protein molecules via disulfide bonds, which was accompanied by an increase in the centrifugal and cooking loss of muscle. The transverse relaxation time (T₂) of muscle was measured by low-field nuclear magnetic resonance (NMR) spectroscopy and the microstructure of muscle was observed by scanning electron microscopy (SEM). It was found that the increase in the centrifugal loss and cooking loss of muscle was due to the destruction of muscle microstructure caused by protein oxidation as well as the decrease in unbound water content and the increase in free water content. Muscle centrifugal loss, cooking loss, free water content and the degree of damage of muscle microstructure at pH 8.0 were lower than those at pH 5.0. The results showed that under the same oxidation condition, yak muscle proteins are more prone to oxidative damage and lower water-holding capacity near the isoelectric point.