In this study, braised pork belly was frozen at different temperatures (−18, −38, and −80 ℃), and the resulting quality changes were evaluated in terms of water-holding capacity, texture, color, moisture distribution and microstructure. The results showed that freezing led to a significant reduction in the moisture content of braised pork belly. After thawing, the water-holding capacity of the −80 ℃ frozen group was 86.49%, which was significantly higher than that of the −18 ℃ and −38 ℃ groups (P < 0.05), and was close to that of the fresh sample (89.30%). The results of low-field nuclear magnetic resonance (LF-NMR) confirmed that freezing at low temperatures delayed the migration of immobilized water (T₂₂) to free water (T₂₃), thereby maintaining the water-holding capacity of the product. In terms of texture, the hardness, springiness and chewiness of the lean part were significantly lower than those of the control group (P < 0.05), while those of the fat part showed an increasing trend. The texture quality of the −80 ℃ frozen group was significantly better than that of the other groups (P < 0.05). Microstructural observations revealed that freezing at −80 ℃ effectively inhibited the growth of ice crystals and reduced damage to muscle fiber structure (P < 0.05). The porosity between muscle fiber bundles was only 6.47%, significantly lower than that of the −18 ℃ (11.67%) and −38 ℃ (9.25%) groups. After thawing, the fat droplets were more uniformly distributed in the −80 ℃ group, with a smaller relative area of 30.53% compared with the −18 ℃ (39.74%) and −38 ℃ (32.67%) groups. The b* value of the lean layer and the L* value of the fat layer in the −18 ℃ freezing group were significantly higher than those in the other treatment groups (P < 0.05). Different freezing temperatures had no significant effect on the a* value of braised pork belly or the b* value of the fat layer (P > 0.05). In conclusion, freezing at −80 ℃ was more effective in preserving the water-holding capacity and textural properties of braised pork belly, thereby maintaining its overall quality.

In order to explore the effect of alternating electric field (AEF) assisted freezing on energy metabolism in beef during thawing and aging, bovine Longissimus dorsi muscle samples were assigned to three groups: direct aging at 4 ℃ (control), thawing and then aging at 4 ℃ after 30 days of freezing at −18 ℃ (FA), and thawing and then aging at 4 ℃ after 30 days of AEF assisted freezing −18 ℃ (EA). The changes of muscle pH, adenosine triphosphate (ATP) content, glycogen content, lactic acid content, lactate dehydrogenase activity, hexokinase activity and Ca²⁺-ATPase activity were analyzed during the aging process. The results showed that as aging time increased, the pH of the OA group initially increased, then decreased and finally increased again, reaching the ultimate pH at the third day of aging. The pH of the FA and EA treatment groups remained relatively constant during aging. For all groups, there was a decreasing trend in ATP content. The ATP content of the OA group decreased from 7245.43 to 752.38 μmol/g after seven days of aging. The ATP content of the EA group was significantly lower than that of the OA group on day 0 of aging (P < 0.05). The muscle glycogen contents in the FA and EA groups were significantly lower than that in the control group during the entire aging process (P < 0.05), and the lactic acid content, and hexokinase and lactate dehydrogenase activities decreased significantly with aging time (P < 0.05). On day 5 of aging, the glycogen and lactate contents, and lactate dehydrogenase activity in the EA group were significantly lower than those in the FA group (P < 0.05), and the activity of Ca²⁺-ATPase was significantly higher than that in FA group (P < 0.05), indicating that alternating electric field can accelerate the process of energy metabolism. In conclusion, aging after freezing and thawing can prolong the stiffening time of beef, and AEF assisted freezing effectively can promote energy metabolism, thus improving muscle quality. These findings may provide a basis for the development of new storage and preservation technologies for meat products.

The effects of immersion freezing and air freezing at different temperatures (-18 and -38 ℃) on the quality of braised beef with potato were studied by physicochemical measurements, microstructural observation and sensory evaluation. The results showed that freezing at -38 ℃ maintained better quality of braised beef with potato compared with freezing at -18 ℃, and immersion freezing was significantly superior to air freezing. The freezing time of beef and potato in the dish under -38 ℃ immersion freezing was shortened by 96.02% and 97.34%, respectively, compared with air freezing at -18 ℃, and the time required to pass through the maximum ice crystal formation zone for beef and potato was 9.67 and 10.92 min, respectively, suggesting improved freezing rate of the dish. Under this condition, the L^*, a^* and b^* values of beef and potato were closer to those of the unfrozen dish; the hardness and elasticity of beef were significantly higher than those of the other frozen treatment groups (P < 0.05), and the shear force was 25.35 N, which was closer to that of the unfrozen sample (26.70 N), indicating good toughness of beef; the hardness of potato was 341.95 g, which was significantly higher than that of (P < 0.05) air-frozen samples, indicating good texture. Scanning electron microscopy (SEM) showed that beef muscle fibers were not broken after immersion freezing at -38 ℃, the gap between muscle fiber bundles was the smallest, arranged in an orderly manner; the cell contour of potato was clear and compact. The overall sensory score of the immersion-frozen sample was higher than that of the air-frozen one, and the sensory scores of beef and potato in the dish after -38 ℃ immersion freezing were 88.80 and 86.80 points, respectively, indicating higher acceptability. These findings provide a theoretical basis for the development of freezing preservation technologies for industrialized Chinese dishes.