Abstract:Low field-nuclear magnetic resonance (LF-NMR) was employed in this work to evaluate water mobility and distribution in taro samples during the processing of microwave vacuum drying (MVD) under different microwave intensities of 1.5, 2.0 and 2.5 W/g. Results showed that there was no constant drying rate period, and the entire microwave vacuum drying process occurred in the falling rate period. The average drying rates under microwave intensities of 1.5, 2.0 and 2.5 W/g were 0.149, 0.185 and 0.224 g/(g·min), respectively. The effective moisture diffusivity (Deff) values for MVD taro slices at different microwave intensities were 7.30×10-9、9.46×10-9、1.14×10-8 m2/s, respectively. Among the transverse relaxation time (T2), T21 (2.31~9.32 ms), T22 (10.72~49.77 ms) and T23 (57.22~705.48 ms) were corresponding to bound water, immobilized water and free water in fresh taro samples, respectively. The dramatically decreased content of free water for MVD taro was accompanied (p<0.05) by the increase in the proportion of bound water, although the content of bound water did not changed significantly (p>0.05). The proportion of immobilized water in taro samples increased, and then decreased with the drying time. The MRI results directly showed that the water-desorbing rate of outer layer in taro is equivalent to inner layer during microwave vacuum drying processing. The higher the microwave intensity, the faster relaxation signals disappear. This work obtaineded the laws of moisture diffusion in taro samples during microwave vacuum drying processing that the water diffusion and the conversion between the different components of moisture in MVD taro samples increased by the microwave intensity . It can provide a theoretical basis for industrial production of taro.
