In this study, the effects of protein oxidation and NaCl concentration on the structure of yak myofibril protein (MP) were investigated. To this end, MP was oxidized for 1 h in a Fenton oxidation system and treated for 1 h with different NaCl concentrations under three pH values (5.0, 6.0 and 8.0), respectively. The total sulfhydryl content, surface hydrophobicity, solubility, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) pattern, and microstructure of treated MP were determined. The results showed that at a fixed pH, the total sulfhydryl content and solubility decreased with increasing H₂O₂ concentration, and the surface hydrophobicity increased. In addition, the degree of expansion declined horizontally. At a fixed H₂O₂ concentration, the total sulfhydryl content and solubility increased with increasing pH, and the surface hydrophobicity decreased, accompanied by lateral expansion, narrowing of the A-band and destruction of the Z-line. At a fixed pH, the solubility increased with increasing NaCl concentration, and the surface hydrophobicity decreased, accompanied by lateral expansion and destruction of the transverse bridge. At a fixed NaCl concentration, the solubility increased with increasing pH, the surface hydrophobicity decreased, and lateral expansion occurred. At pH 8.0 and 1 mol/L NaCl concentration, both the M-line and Z-line were obviously destroyed, the structure of MP became blurred, the myotome was broken, the transverse bridge was destroyed, and coarse filaments were separated from fine ones. In conclusion, both protein oxidation and NaCl had adverse effects on yak MP under different pH conditions, and protein oxidation had the lowest negative impact on MP structure at pH 6.0 and a NaCl concentration of 0.4 mol/L.

In an effort to elucidate the effect of protein oxidation on its water-holding capacity (WHC), yak muscle was treated in a Fenton oxidation system containing 0.1 mol/L NaCl at pH 5.0 or 8.0. The results showed that the contents of malondialdehyde (MDA) and carbonyl groups overall increased with increasing concentration of H₂O₂ in the Fenton system, and the total sulfhydryl content decreased, which reflects an increase in the degree of oxidation. The contents of MDA and otal sulfhydryl groups were higher at pH 5.0 than that at pH 8.0. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) displayed that the oxidation induced backbone breakage and covalent cross-linking by disulfide or non-disulphide bonds of myofibrillar proteins. As a result, the space between muscle structures was reduced and the WHC decreased, which was manifested by an increase in centrifugation loss and cooking loss. Transverse relaxation times (T₂) from low-field nuclear magnetic resonance (NMR) and microstructure analysis by scanning electron microscopy (SEM) demonstrated that as the degree of oxidation increased, the content of immobilized water decreased, the content of free water increased, and the microstructure was damaged, causing water loss from the muscle. Under the same oxidation conditions, the centrifugation loss, cooking loss and free water content were lower at pH 8.0 than at pH 5.0, which illustrated that yak muscle had higher WHC at pH 8.0.

In this study, essential oil was extracted from the fruit of Litsea cubeba by hydrodistillation, and its major components were analyzed by gas chromatography-mass spectrometry (GC-MS). The antimicrobial activity of the essential oil against Cladosporium was determined and its mechanism of action was explored by examining the antibacterial rate and mycelium inhibition rate of Cladosporium, the liposome leakage assay, and the determination of leakage of intracellular ions and macromolecular contents and ATPase activity. The results indicated that the major components of L. cubeba essential oil were terpenoids such as citral and monoterpene oxides. Its minimum inhibitory concentration against Cladosporium was 0.64% (V/V). The essential oil was effective in inhibiting the growth of Cladosporium mycelium and altering its molecular structure and functional groups. The initial target for the antimicrobial activity of the essential oil was the cell membrane, where it increased the permeability of the phospholipid bilayer in a concentration-dependent manner, leading to the leakage of fluorescein encapsulated in liposomes without collapsing the bilayer. It also caused the leakage of intracellular K⁺, Ca²⁺, and macromolecules, thereby inhibiting the growth and reproduction of Cladosporium. Scanning electron microscope observation further confirmed that L. cubeba essential oil disrupted the morphology of Cladosporium mycelium, resulting in severe loss of intracellular contents.

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.