Mechanistic study on the effects of “blanching + additives” pretreatment on microwave freeze-drying of longan

For the problems that are prone to occur during the Microwave Freeze-Drying (MFD) process of longan, such as enzymatic Browning, loss of heat-sensitive nutrients and structural collapse. In this study, a response surface test was first designed, and a combined pretreatment process of “blanching + additives” was optimized, achieving both high efficiency and high quality drying of longans. Then, by means of the determination of enzyme activity and the content of heat-sensitive nutrients, combined with the microstructure observation of Micro-CT and environmental scanning electron microscopy (ESEM), the influence mechanism of the “blanching + additive” pretreatment process on the quality of longan MFD was revealed. The research results show that the optimal pretreatment process for longan before MFD is: boiling water blanching for 50 s +0.2% citric acid +0.5% sodium chloride +0.5% isoascorbic acid soaking for 30 min. By using this process for pretreatment, on the one hand, it can increase the content of Epicatechin (EC) and Vitamin C (Vc) by inhibiting the activities of Peroxidase (POD) and Laccase (LAC) (reducing them by 31.85% and 54.35% respectively), thereby increasing them by 23.77% and 155.56% respectively. On the other hand, it can also enhance the brittleness, color and drying efficiency of the product by increasing porosity and pore uniformity. More importantly, this study, in combination with biological analysis, revealed the mechanism by which “hot blanching and passivation” and “additive inhibition” synergistically regulate enzyme activity, component retention and structure formation, providing a theoretical basis and technological guidance for the high-quality processing of longan MFD.