Abstract: This study was undertaken in order to explore the water diffusion characteristics of Chinese yam tubers during microwave vacuum freeze-drying. Low field nuclear magnetic resonance (LF-NMR) was used to analyze water distribution change during the drying process. In addition, changes in water effective diffusion coefficient, water content and drying rate were measured. The results showed that moisture migration from high degree of freedom to low degree of freedom occurred during the drying process. Water effective diffusion coefficient increased from 1.129 × 10-9 to 5.439 × 10-9 m2/s as the microwave power increased from 1.5 to 4.4 W. Similarly, moisture diffusion coefficient, water migration rate and the amount of unbound water converted to bound water increased with increasing microwave power. The Page and Newton models were fitted to the experimental data. Our comparative evaluation showed that the Page model could better describe the drying process with excellent goodness of fit and R2 of greater than 0.99. This study can provide a theoretical basis for real-time monitoring of moisture in the drying process of Chinese yam tubers.

Key words: microwave vacuum freeze-drying, low-field nuclear magnetic resonance, moisture diffusion characteristics, drying model