This study aimed to explore the effect of intramuscular connective tissue changes during freeze-thaw cycles on the quality of bovine rumen smooth muscle. The changes in the shear force, pH, thawing loss rate, collagen solubility, collagen-degrading enzyme activity, cross-linked pyridine content and microstructure of bovine rumen smooth muscle subjected to different freeze-thaw cycles were analyzed. The results showed that the activity of collagen-degrading enzymes including β-galactosidase (β-gal) and β-glucuronidase (β-GD), the contents of cross-linked lysine pyridinoline (LP) and hydroxylysine pyridinoline (HP), collagen solubility and thawing loss rate significantly increased with increasing freeze-thaw cycles (P < 0.05). Similarly, the degree of rupture of endomysium and perimysium as well as the gap between muscle fibers increased, so that the arrangement of muscle fibers became looser. The shear force and pH significantly decreased when the number of freeze-thaw cycles was greater than 1 (P < 0.05). The thawing loss had a significantly negative correlation with pH (P < 0.05), but a significantly positive correlation with the contents of cross-linked HP and LP (P < 0.01). The shear force a had significantly negative correlation with β-gal activity, β-GD activity, and collagen solubility (P < 0.01). In conclusion, the decrease in pH and the formation of crosslink can lead to an increase in thawing loss, and the decrease in shear force mainly depends on the damage of ice crystal to muscle and protein degradation. The rate of shear loss decreases slowly and the rate of thawing loss increases after 3 freeze-thaws. Therefore, controlling freeze-thaw cycles within three cycles can reduce the loss of enterprises while ensuring the quality of smooth muscle.

The aim of this study to develop a stable anthocyanin indicator film to monitor the freshness of chilled beef. Composite anthocyanin microcapsules (CAM) were prepared and used to prepare indicator films using carrageenan (Car) and sodium carboxymethyl cellulose (CMC) as film-forming matrix with different amounts (5%, 10%, 15% and 20%, V/V) of added CAM, and the structural and physical properties of the resulting films were characterized, and their application to monitor beef freshness was investigated. The results showed that the addition of CMC significantly improved the mechanical properties, barrier properties and thermal stability of Car films. The film with 15% CAM had the highest tensile strength (17.76 MPa) and lowest water vapor transmission rate (1.35 × 10^-11 g/(m·s·Pa)), and showed a tightly and orderly arranged cross-section. The Fourier transform infrared spectra showed that the components of the indicator film were structurally similar and compatible. When the indicator film was exposed to ammonia, its color changed from pink to blue-green and eventually to yellow. The indicator film was applied to monitor the freshness of beef during storage at 4 ℃. The results showed that beef reached secondary freshness and spoilage on day 4 and 7, respectively, and that the color of the indicator film changed from pink to purple and green correspondingly. In conclusion, the film with 15% CAM is the most sensitive to changes in the freshness of beef and thus can be used to detect it.

This study aimed to investigate the effects of nitric oxide (NO)-mediated hypoxia-inducible factor-1α (HIF-1α) on the glycolysis and tenderness of postmortem yak muscle and its regulatory mechanism. In this study, Yak Longissimus dorsi was injected with 0.9% saline (control), 200 μmol/L S-nitrosoglutathione (GSNO), 200 μmol/L GSNO + 100 μmol/L YC-1 (HIF-1α inhibitor) or 200 μmol/L GSNO + 50 μmol/L PD98059 and aged, and NO level, HIF-1α, mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway activation, glycolysis-related enzyme activities, and shear force of muscle were analyzed at different aging time points (3, 6, 9, 12, 24 and 72 h). The results revealed that NO levels in the GSNO group were significantly higher than those in the control group during 3–72 h (P < 0.05), reaching a peak at 6 h. ERK1/2 and p-ERK1/2 expression levels were higher in the GSNO group than the control and GSNO + PD98059 groups during 3–72 h (P < 0.05), reaching a peak at 24 h. HIF-1α expression level in the GSNO group was significantly higher than that in the control and GSNO + YC-1 groups (P < 0.05), peaking at 12 h. Hexokinase (HK), pyruvate kinase (PK) and lactate dehydrogenase (LDH) in the GSNO group reached their maximum values at 9, 12 and 12 h, respectively (P < 0.05) and were significantly higher than those in the control group; at 3, 12 and 72 h, the glycolytic potential in the GSNO group was significantly higher than that in the two other groups (P < 0.05); at 6, 9 and 72 h, the pH in the GSNO group was significantly lower than that in the other groups (P < 0.05). In addition, shear force and hematoxylin and eosin (HE) staining were used to assess the tenderness of yak meat. Shear force, myofiber cross-sectional area and diameter were lower and the space between myofibers was higher in the GSNO group than in the two other groups. In conclusion, during the early period of postmortem aging, NO mediates the MAPK/ERK signaling pathway, thereby activating HIF-1α expression and upregulating HK, PK and LDH activity, accelerating the rate of glycolysis of and reducing pH, and ultimately improving the tenderness of yak meat. The results of this study may serve as a potential regulatory mechanism for regulating the tenderness of yak meat and provide a theoretical basis for improving the theoretical system of glycolysis and tenderness during postmortem aging.