Effect of Ultrasonic-Freeze-Thaw Pretreatment on the Drying Characteristics of Purple Cabbage Crisps

Vacuum freeze-dried purple-cabbage crisps suffer from long drying time and poor quality. In response to these problems, this study investigated the effects of ultrasonic, freeze-thaw and combined ultrasonic and freeze-thaw pretreatment on the drying characteristics and quality of freeze-dried purple cabbage crisps. The results demonstrated that all pretreatments effectively shortened the drying time, reduced the energy consumption, and improved the physicochemical quality and flavor of freeze-dried purple cabbage crisps, the effect of the combined treatment being the most pronounced. Compared with the control sample, the combined treatment shortened the drying time by 20.04%, reduced the energy consumption by 31.45%, decreased the thioside content by 22.42%, increased the VC content by 57.06%, enhanced the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and total antioxidant capacity by 57.17% and 51.33%, respectively, and reduced the hardness and chewiness by 39.22% and 77.98%, respectively, and increased the brittleness by 66.13%, and it was found to be the most effective in retaining active ingredients and preserving the color. Microstructural observation and low-field nuclear magnetic resonance (LF-NMR) analysis showed that damage caused by pretreatment to the cellular and tissue structure contributed mainly to the reduction in drying time and energy consumption and the quality improvement of purple cabbage crisps, and the change in water distribution was the secondary reason for the reduction in drying time and energy consumption. Volatile component analysis showed that the combined treatment facilitated the removal of pungent compounds such as isothiocyanate from purple cabbage crisps, and vacuum freeze-drying removed the strong mustard-like odor, the spicy compound allyl isothiocyanate, and methyl thiocyanate, responsible for onion odor, thus improving the flavor of samples subjected to ultrasonic-freeze-thaw treatment. This study provides theoretical and technological references for the development of recreational products and high-value utilization of purple cabbage.