Effects of Different Freezing Treatments on Properties and Structure of Konjac Glucomannan Gel

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.