Abstract: In order to gain an understanding of variations in moisture content, distribution and states during the vacuum microwave drying of flavored yam chips, inverted spectra of transverse relaxation time T2 of flavored yam chips dried at different microwave powers were measured using low-field nuclear magnetic resonance (LF-NMR). The results showed that the higher microwave power, the shorter the time it took to remove free water and immobilized water; a greater effect was noted on free water. But excessively high microwave power caused coking of the materials. A linear relationship was seen between the moisture content of flavored yam chips and the total peak area of NMR, which could predict the drying time. NMR images showed that the moisture increased or decreased from the exterior to the interior during the drying process. At the end of drying, the remaining moisture was mainly present in the inner layer of flavored yam chips. Changes in moisture content had a significant effect on the quality of flavored yam chips after drying. LF-NMR imaging can provide visual information of moisture changes during the drying process of flavored yam chips, and this study can provide a basis for designing and optimizing a vacuum microwave drying process for flavored yam chips and for improving the product quality.

Key words: flavored yam chips, vacuum microwave drying, low-field nuclear magnetic resonance, transverse relaxation time T2, peak area