Abstract: In the present study, fresh sweet potato leaves were by four different drying methods of hot air drying, vacuum freeze drying, microwave vacuum drying and far infrared drying. Low field nuclear magnetic resonance and drying curves were used to evaluate changes in moisture content, state and distribution as well as mass and heat transfer in sweet potato leaves during drying. Besides, comprehensive quality assessment of the dried products was conducted with respect to micromorphology, color and rehydration performance. Results showed that the water in sweet potato leaves was classified into two types: bound water T21 (0.06–13.67 ms) and free water T22 (13.67–580.52 ms). Microwave vacuum drying showed the fastest drying speed, reducing the moisture content from 84.42% to 9.97% in 0.75 h. Both mass and heat transfer occurred from the inside to the outside of sweet potato leaves, and the porous structure in the surface of sweet potato leaves exhibited decreased degree of wrinkling after drying, with no significant change being observed in the microstructure. The microwave vacuum dried product had the highest levels of brightness and green color without obvious yellowing. In addition, its rehydration capacity (6.69 ± 0.29) was equivalent to that of the dried samples obtained by vacuum freezing drying and hot air drying, but significantly higher than that from far infrared drying (4.92 ± 0.73) (P < 0.05).

Key words: sweet potato leaves, low field nuclear magnetic resonance, drying curve, microstructure, rehydration capacity