Konjac glucomannan (KGM) gel was frozen by air freezing, immersion freezing and ultrasonic-assisted freezing, separately and analyzed for changes in its properties and structure by a texture analyzer, an X-ray diffractometer (XRD), low field nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), and cold field scanning electron microscopy (SEM). The results showed that compared with air freezing and immersion freezing, ultrasonic-assisted freezing increased the cooling rate of the gel, reduced the syneresis rate, and increased the gel strength without changing the crystallinity of KGM. When the ultrasonic power was 300 W, the syneresis rate and strength of the gel reached their best level. Compared with air freezing, ultrasonic-assisted freezing reduced the syneresis rate by 23.56% and increased the gel strength by 61.23%. The DSC heat flow curves showed that ultrasonic-assisted freezing increased the eutectic point of the gel and lowered its thawing temperature. The microscopic images showed that the network structure of the gel frozen at an ultrasonic power of 300 W was the densest, with the smallest pores. In conclusion, 300 W ultrasonic-assisted freezing can effectively improve the gel characteristics of KGM, which provides a theoretical foundation for the production of new KGM gel foods.

In an attempt to investigate the effects of different freezing pretreatments on the emulsification and gelation properties of myofibrillar protein (MP), this study aimed to analyze changes in the emulsifying activity index (EAI), emulsifying stability index (ESI), emulsion particle size, rheological properties, gel strength, cooking loss, water distribution and morphology of MP extracted from porcine Longissimus dorsi muscle pretreated by air freezing (AF), immersion freezing (IF) or ultrasonic-assisted immersion freezing (UIF). The results showed that AF and IF remarkably lowered the emulsifying activity and emulsion stability of MP compared with the control group (held at 4 ℃ for 6 h) and resulted in significantly larger and non-uniform sizes of emulsion droplets. UIF markedly increased the emulsifying activity but did not change the emulsion stability or droplet size. UIF improved the viscoelasticity and gel strength of heat-induced MP gels and reduced the cooking loss compared with the other treatment groups, and water distribution in the gel network was more uniform. high-resolution field-emission scanning electron microscopic (FE-SEM) observation revealed that the network structure of UIF-treated MP gels was more compact than that of the other treatment groups, closer to that of fresh MP gels. In summary, UIF-treated MP had better emulsification and gelation properties, indicating that UIF could maintain the processing quality of frozen meat better.

In order to investigate the effect and underlying mechanism of deacetylated konjac glucomannan (DKGM) on the structure and gel properties of pork myofibrillar protein (MP), the changes in the gel strength, water retention, water distribution, microstructure, secondary and tertiary structures and gel molecular forces of MP were analyzed after the addition of DKGM at different levels (0%, 0.125%, 0.25%, 0.5% and 1%). The results showed that the gel strength of MP increased with the increase of DKGM addition, reaching the maximum at an addition level of 0.25%, which was 1.55 times higher than that of the control group. The addition of DKGM could retard the flow of water, thus improving the gel water-holding capacity. Cold-field scanning electron microscopy revealed that the addition of DKGM could promote the formation of more uniform and dense network structures in MP gels. The structure and force analysis showed that the addition of an appropriate amount of DKGM could promote the unfolding of MP molecules and the exposure of hydrophobic groups, increase the content of active sulfhydryl groups, induce α-helix-to-β-fold transition, enhance the hydrophobic interactions and disulfide bonds of MP gels, and thus improve the gel strength and water-holding capacity.

In this study, the effects of ultrasonic-assisted immersion freezing (UIF) with different ultrasonic powers on beef quality were measured. The changes in freezing rate, cooking loss, thawing loss, tenderness, color difference, moisture distribution and microstructure of beef were analyzed after traditional air cooling or ultrasonic-assisted freezing at different ultrasonic powers (0, 200, 400 and 600 W). Fresh beef was used as control. The results showed that UIF could significantly increase the freezing rate of beef, affect the color and water state, and reduce the structural destruction of muscle tissue and quality deterioration during the freezing process. At an ultrasonic power of 400 W, the fastest freezing rate was obtained, as well as the lowest cooking loss and thawing loss of 32.44% and 1.66%, respectively, and shear force closest to that of the fresh meat. In addition, the minimum freezable water of 51.11% was observed. Low-field nuclear magnetic resonance (NMR) analysis showed that the amplitude of T₂₁ was the highest, and the amplitude of T₂₂ was relatively small, indicating a more uniform water distribution. Microscopic observation showed that an appropriate ultrasonic power reduced the structural damage caused by freezing to muscle fibers, thereby improving the quality of frozen meat.

To investigate the effect of different freezing methods on the cooking quality of pork, porcine Longissimus dorsi muscle frozen by three different methods: air freezing (AF), immersion freezing (IF) and ultrasonic-assisted immersion freezing (UIF) were stewed after being thawed at 4 ℃ , and their moisture content, cooking loss, shear force, color, volatile flavor components and taste were analyzed. Fresh (unfrozen) meat was used as control. The results showed that the moisture content of cooked UIF meat (57.81%) was significantly higher than those of cooked AF and IF meat, which was the closest to that of the control group, and the cooking loss was less than that of any other treatment group. The shear force of the UIF group was the smallest and slightly lower than that of the control group, indicating that ultrasound had a positive effect on the tenderness of cooked pork. There was no significant different in brightness between the control and UIF groups. Principal component analysis (PCA) of the electronic nose sensor responses for volatile flavor compounds showed that the first and second principal components accounted for 98.346 0% and 1.317 4% of the total variance, respectively. Linear discriminant analysis (LDA) of the electronic tongue data showed that the UIF group was not significantly differentiated from the control group and they were similar in taste, indicating that ultrasound slowed down the quality and flavor deterioration of frozen pork and brought its quality closer to that of fresh meat.