Changes and relationships of viscoelastic and physical properties of Chinese yam during a novel multiphase microwave drying process

Changes in viscoelastic properties and physical qualities including rehydration ratio (RR), volume ratio (Vr), porosity and texture of Chinese yams during multiphase microwave drying (MMD) were studied and the intrinsic relationship between them was analyzed. Results showed that RR, Vr and porosity rose and then fell with drying time during MMD, with the endpoint of sublimation drying being the turning point. Scheme III with the highest microwave power (0.2 W/g) in sublimation drying resulted in the lowest RR, Vr and porosity but the highest hardness. The Maxwell model was well fitted to describe the viscoelastic behaviors of yams (R2 > 0.99). Enhanced stress relaxation behavior during sublimation drying was observed. In contrast, a decrease in stress decay but an increase in residual elasticity were found when MMD switched from sublimation drying to evaporation drying. Pearson's correlation analysis showed that relaxation modulus was significantly negatively correlated with RR (r = −0.981, p < 0.01) and porosity (r = −0.961), while positively correlated with hardness (r = 0.94, p < 0.05) and the brittleness (r = 0.96, p < 0.01). Principal component regression models between the viscoelastic indexes and physical features with high R2 (>0.9) values could explain their relationships well. These findings have implications for the production of high-quality products through controlling drying processes.