Analysis of shrinkage deformation behavior in microwave vacuum drying of Chinese yam based on evolution of physical properties

Shrinkage of fruits and vegetables during drying directly affects the sensory quality of products, and understanding its behavior and mechanisms is crucial for quality improvement. This study utilized image processing techniques to analyze the shrinkage behavior of Chinese yam during microwave vacuum drying (MVD). The drying characteristics and physicochemical properties (textural properties, viscoelastic properties, and moisture state) of Chinese yam were investigated under microwave power densities (MPDs) ranging from 2 to 10 W/g to elucidate the mechanisms of shrinkage. Additionally, the color and bioactive compounds (total phenolics and flavonoids) of the dried products were also evaluated. Results showed that the shrinkage behavior during MVD exhibited a two-phase pattern: a rapid shrinkage phase followed by a slower shrinkage phase, with a distinct inflection point (moisture content range of 0.82–1.19 g/g dry basis) dependent on moisture dynamics. Volumetric shrinkage resulted from combined radial and longitudinal contraction. Significant longitudinal shrinkage manifested as central collapse was observed at low MPDs (2 and 4 W/g). The radial shrinkage (top-view projected area) ranged from 39.76 % to 41.81 % across conditions, showing no significant differences. The rapid loss of free water was the primary driver of shrinkage, while the increase in sample hardness (textural property) and residual modulus (viscoelastic property) reflected the enhancement of mechanical strength, effectively resisting shrinkage induced by drying. Additionally, high MPDs not only improved volume retention but also demonstrated favorable effects on color preservation and retention of bioactive compounds.