Pretreatment-driven shrinkage regulation in Chinese yam during multiphase microwave drying

Multiphase microwave drying (MMD) is an innovative drying technology that significantly reduces drying time compared to freeze drying (FD) while maintaining similar product quality. However, shrinkage during the evaporation stage remained a major limitation hindering MMD from replacing FD. In this study the effects of three pretreatment methods—ultrasound (US), freeze-thaw cycles (FT), and film coating (FC)—on the shrinkage characteristics of Chinese yam during MMD were investigated.

To elucidate the mechanisms governing shrinkage behavior, we integrated low-field nuclear magnetic resonance (LF-NMR) analysis, which enables rapid detection of the effects of pretreatments on water status, with 2D/3D microstructural characterization using scanning electron microscopy and X-ray micro-computed tomography. Product quality was evaluated based on rehydration capacity and textural properties. The results showed that US pretreatment significantly reduced shrinkage (P < 0.05), with the 400 W treatment achieving the lowest ratio (3.2 %), representing a substantial improvement over the control. This reduction was attributed to the US-induced transformation of bound and immobile water into free water, and the formation of large, uniformly distributed micro-channels. This expanded pore area and optimized pore density distribution, thereby effectively suppressing shrinkage. Additionally, US-400 significantly enhanced rehydration capacity and improved textural properties (P < 0.05), resulting in superior product quality. One freeze-thaw cycle (FT-1) reduced shrinkage by 6 % compared to the control group, but additional FT led to severe shrinkage and quality deterioration. The cassava starch coating with 2 % glycerol (FC-2) reduced shrinkage by 15.8 % compared to the control, but increased drying time, hardness, and crispness, ultimately lowering product quality. In conclusion, US pretreatment at 400 W effectively minimized shrinkage during MMD, offering theoretical and technical support for producing high-quality dried Chinese yam.