In an effort to investigate the effect of addition of different levels (0%, 0.2%, 0.5% and 1.0%) of vacuum freezedried (VFD) garlic powder to myofibrillar protein (MP) extracted from yak meat on its emulsification properties in the Fenton oxidation system, MP was evaluated for active sulfhydryl content, protein solubility, and surface hydrophobicity, percentage of interface adsorbed protein, emulsion viscosity, emulsification activity, emulsion stability (clarification index, integrated light transmittance, sedimentation rate) and microstructure after oxidation for 12 h at 4 ℃ in the oxidation system. The results showed that addition of high levels of VFD garlic powder (0.5% and 1.0%) significantly enhanced the emulsification activity, emulsion stability and oil phase distribution uniformity of MP in the Fenton oxidation system (P < 0.05). Adding 0.5% of VFD garlic powder provided good protection for yak meat MP in the Fenton system, resulting in a 2.55-fold increase in active sulfhydryl content, a 19% increased in protein solubility and a 25.41% reduction in surface hydrophobicity. Adding 1.0% of VFD garlic powder significantly improved the emulsification characteristics of yak meat MP in the Fenton oxidation system, increasing the viscosity of yak meat MP emulsion by 2.02 times, and the emulsification activity by 49.80%, and reducing the clarification index, integrated light transmittance and sedimentation speed by 17.06%, 37.44% and 50.79%, respectively. These results suggested that VFD garlic powder significantly improves the emulsification characteristics of yak meat MP in the Fenton oxidation system, and its effect depends on the addition amount of VFD garlic powder. Addition of 1.0% of VFD garlic powder is more effective.

Physicochemical properties, total bacterial counts and volatile flavor compounds of fermented yak meat sausages with 0.0%, 1.5%, 3.0% and 4.5% NaCl were evaluated in this experiment. The results showed that the pH and lightness value of the sausage with 4.5% NaCl were significantly higher than those of the other three groups (P < 0.05). The hardness, adhesiveness and chewiness decreased with decreasing amount of added NaCl, while the storage modulus and total bacterial count increased. Additionally, the water-holding capacity and resilience of the sausage with 3.0% NaCl were significantly higher than those of the other groups (P < 0.05), and the yellowness value was the lowest among the four groups. As NaCl content decreased from 4.5% to 1.5%, the thiobarbituric acid reactive substances (TBARS) value and sulfhydryl content showed an upward trend, and the protein carbonyl content showed a downward trend, while the redness value, elasticity, and cohesiveness did not significantly differ. Seven categories of volatile flavor compounds including aldehydes, alcohols, and acids were detected in fermented yak meat sausages. Three principal components were identified by principal component analysis (PCA), and 12 key volatile flavor compounds with relative odor activity values (ROAV) ≥1) such as hexanal, nonanal, and isovaleraldehyde were obtained. The relative contents of volatile flavor compounds and the key flavor substances of fermented yak meat sausage varied with NaCl level. With decreasing NaCl content from 4.5% to 1.5%, the relative content of aldehydes derived from lipid oxidation increased. Sensory evaluation showed that with the decrease in NaCl content from 3.0% to 0.0%, the appearance, texture, taste, and overall acceptability showed a downward trend, while there was no significant difference in sensory scores between the sausages with 3.0% and 4.5% NaCl.

To explore the effect of reducing the sodium content on the lipid oxidation and flavor quality of naturally fermented yak meat sausages, low-sodium and control sausages were prepared by adding 2% and 4% sodium salt and natural ripening or constant temperature ripening, respectively. Physicochemical indicators were measured during the ripening process. The results showed that regardless of the ripening method used, the water content, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and hydroxyl radical scavenging capacity of the low-sodium group were significantly lower than those of the control group (P < 0.05), and the pH was significantly higher than that of the control group (P < 0.05). The degrees of lipid hydrolysis and oxidation in the four treatment groups increased with ripening time, being significantly higher in the low-sodium group than in the control group for each ripening method (P < 0.05). There was no significant difference in lipoxygenase activity between the two groups of natural maturation (P > 0.05), while at the late stage of constant-temperature ripening, the lipoxygenase activity in the low-sodium group was significantly higher than that in the control group (P < 0.05). In addition, a total of 69 volatile flavor compounds were detected from the sausages, and the cumulative contribution rate of the first three principal components was as high as 96.206%. Principal component analysis (PCA) showed that on the 9th and 17th (end) day of maturation, the relative contents of alcohols, aldehydes and other substances generated by lipid oxidation in the low-sodium group were higher than those in the control group. Different sodium contents had a significant effect on the hydrolysis and oxidation of lipids in sausage. Directly reducing the sodium content to 2% may promote the oxidation of lipids.

The effects of the addition of different amounts (0.01%, 0.05% and 0.10%) of banana flower extract (BFE) on the physicochemical quality of fermented yak meat sausage were investigated using butylated hydroxy toluene (BHT) as a positive control, and the changes in the pH, moisture content, color, water retention, texture, rheological properties, total bacterial count, volatile flavor substances and sensory quality of the sausage during fermentation and storage were evaluated. The results showed that the sausage with 0.10% BFE had stronger water retention and bacterial inhibition ability, while increasing BFE addition reduced the textural characteristics and storage modulus of the fermented sausage. A total of seven classes of volatile flavor substances, including alcohols, aldehydes, acids and esters, were detected in all samples during fermentation, and three principal components were obtained by principal component analysis, with a cumulative contribution of 75.014%. In conclusion, BFE has strong antioxidant activity and significantly improves the physicochemical properties of fermented yak meat sausage, and the most pronounced effect is obtained with 0.01% BFE addition.

In this study, the effect of Mg²⁺ on the physiochemical, structural and gel properties of mutton myofibrillar protein was studied. The results indicated that with an increase in Mg²⁺ concentration (0, 0.01, 0.02, 0.03, 0.04 and 0.05 mol/L), the physicochemical properties of mutton myofibrillar protein (MP) including solubility, emulsification activity, emulsion stability and particle size increased significantly (P < 0.05), the absolute value of the zeta potential decreased, the proportion of β-sheet increased significantly (P < 0.05), and surface hydrophobicity increased significantly (P < 0.05). The endogenous fluorescence intensity of tryptophan was largest at a Mg²⁺ concentration of 0.04 mol/L. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the band intensity of myosin and relevant light chain proteins increased. Rheological analysis showed that the storage modulus (G’) first increased and then decreased. Compared with the control group, MP gel strength increased by 70.49% at a 0.03 mol/L Mg²⁺ concentration. Gel cooking loss and water-holding capacity (WHC) were negatively correlated with Mg²⁺ concentration (P < 0.05). Low-field nuclear magnetic resonance (NMR) analysis showed that with increasing Mg²⁺ concentration, immobilized water in the MP gel was transformed into free water. Microstructure observed by scanning electron microscopy (SEM) showed that the higher the concentration of Mg²⁺, the more porous the gel. Correlation analysis showed that Mg²⁺ concentration was highly correlated with the structure and gel characteristics of MP. Therefore, Mg²⁺ can significantly improve the gel properties of mutton MP.

To explore the effect of different levels of lipid oxidation on the structural and functional properties of myofibrillar protein (MP) in yak meat, the oxidation of side-chain amino acids, structure, rheological characteristics, and emulsion stability of MP incubated in the presence of different concentrations (0, 0.1, 1, 5, 10 and 20 mmol/L) of malondialdehyde (MDA) were measured. The results showed that with an increase in MDA concentration, the contents of carbonyl group, MDA-MP adducts and dimeric tyrosine and the relative content of β-turn in MP overall increased, while total sulfhydryl content, surface hydrophobicity, endogenous fluorescence intensity, and the relative contents of α-helix and β-sheet decreased. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) results indicated gradual loss of myosin heavy and light chains, implying that interaction with MDA could induce MP oxidation and cross-linked aggregation and result in structural transition from an ordered to a disordered state. In addition, moderate oxidation (with MAD concentration < 10 mmol/L) increased the storage and loss moduli and improved the gel quality of MP; however, excessive oxidation (with MAD concentration ≥ 10 mmol/L) led to serious aggregation of MP and a decrease in the storage and loss moduli as well as a decrease in the emulsion stability and color deterioration. In conclusion, MDA oxidation can promote the oxidation of MP, and alter the structural and functional properties.

The effects of adding different levels (0.01%, 0.05% and 0.10%) of banana flower extract (BFE) versus dibutylated hydroxy toluene (BHT) on protein oxidation in naturally fermented yak meat sausages were investigated. lys The carbonyl content, Schiff base content, total sulfhydryl content, active sulfhydryl content, surface hydrophobicity, dimeric tyrosine content, endogenous fluorescence intensity and solubility of myofibrillar proteins (MPs) from fermented yak meat sausage were measured at different fermentation times, and the secondary structure and composition of MPs were characterized by Fourier transform infrared (FTIR) spectroscopy, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The results showed that compared with the blank control, the addition of BFE decreased the carbonyl group content, Schiff base content, surface hydrophobicity, dimeric tyrosine content and endogenous tryptophan fluorescence intensity of MPs, and increased the total sulfhydryl group content, active sulfhydryl group content and solubility. FTIR spectroscopy showed that the addition of BFE improved the reveal the more stability of the secondary structure of MPs. SDS-PAGE showed that BFE reduced the diffusion of myosin heavy chain and had a protective effect on myosin. This study showed that BFE inhibited protein oxidation in naturally fermented yak sausage, and this effect was positively correlated with BFE concentration, being most pronounced at 0.10% BFE concentration.

Sarcoplasmic proteins (SP) derived from yak meat were oxidized by malondialdehyde (MDA) at different concentrations. The effects of lipid oxidation on physicochemical properties and color stability of SP were investigated by evaluating side-chain amino acid oxidation, protein structure and color of SP and the oxidation status of myoglobin (Mb). The results showed that after MDA oxidation, the a* value, b* value, C* value, and deoxymyoglobin and oxymyoglobin contents of SP significantly decreased (P < 0.05), and the L* value, metmyoglobin content, and ferrylmyoglobin concentration significantly increased (P < 0.05), indicating that MDA oxidation lowered color stability. The contents of carbonyl groups and dimeric tyrosine, the fluorescence intensity of SP-MDA adducts, and the relative contents of β-helix and β-turn significantly increased (P < 0.05), and total sulfhydryl content, surface hydrophobicity, intrinsic fluorescence intensity, and the relative contents of α-helix and random coil significantly declined (P < 0.05). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed blurred expansion of small molecule bands and the formation of macromolecular aggregates, suggesting that MDA promoted the oxidation and aggregation of SP. Pearson correlation analysis revealed significant correlations between MDA oxidation and physicochemical properties and color stability of SP (P < 0.05). This study reveals that MDA alters the structure of SP by directly oxidizing it or mediating Mb oxidation, causing cross-linked aggregation of SP and reducing its color stability.

In order to develop sustained-release films with excellent performance, this study used polyvinyl alcohol (PVA) and chitosan (CS) as a film-forming substrate, β-cyclodextrin (β-CD) as a sustained-release agent, and oregano oil (OEO) as an antibacterial and antioxidant to prepare β-CD/CS/PVA composite films with different amounts of added OEO. The effects of addition of different amounts of OEO on the basic properties, physicochemical structure, antibacterial and antioxidant properties, and slow-release performance of the composite films were investigated. Fourier transform infrared (FTIR) spectroscopy indicated that none of the components altered the structure of the composite film. Increasing the content of OEO improved the ductility, hydrophobic properties, and light barrier properties, and also significantly increase the antibacterial and antioxidant capacities (P < 0.05). The diameters of inhibition zones of the composite film with 2% OEO against Staphylococcus aureus and Escherichia coli was 8.42 and 9.24 mm, respectively, and its DPPH radical scavenging rate was 80.84%. The composite film had excellent sustained-release performance in 95% ethanol, and the active substance in the composite films with 0.5% and 1.5% OEO could be released continuously for 540 min in this medium. It was concluded that the composite film with 2% OEOs had good overall performance, showing great potential for development.

Fresh milk with added hyaluronic acid (HA) was fermented by inoculating Streptococcus thermophilus and Lactobacillus bulgaricus. By measuring the pH, water holding capacity, viable bacteria count, rheological properties, volatile flavor substances, stability of yoghurt, the effects of HA with various molecular masses and various amounts of HA on the quality of yoghurt were evaluated. The results showed that various molecular masses of HA had different effects on the quality of yoghurt, but all could significantly improve its water holding capacity, smoothness and fineness (P < 0.05). The addition of high molecular mass (1280 kDa) HA improved the elastic modulus (G’) and viscosity modulus (G”) of yoghurt, enhanced its stability index, promoted the growth and reproduction of L. bulgaricus and S. thermophilus, but caused obvious whey syneresis at the end of fermentation. Medium molecular mass (350 kDa) HA inhibited the growth and reproduction of L. bulgaricus, increased the pH, reduced the titratable acidity, and optimized the sensory quality of yoghurt. Low molecular mass (180 kDa) HA reduced the stability of yoghurt. To sum up, medium molecular mass HA can improve the quality of fermented milk.