Characterization of moisture mobility and diffusion in fresh tobacco leaves during drying by the TG–NMR analysis

Abstract

Curing is an essential procedure in tobacco primary processing. The moisture mobility and diffusion characteristics during drying are the key factors that affect the curing condition optimization. In the present study, the thermogravimetric analysis (TG) was integrated to the online nuclear magnetic resonance analysis (NMR) to investigate the drying characteristics of fresh tobacco leaves (FTL) during hot air drying at 50 and 60 °C. The results of TG were used to analyze the drying kinetics and obtain the effective diffusion coefficient of moisture of FTL during drying. The moisture diffusion coefficient of FTL was 4.1782E−09 m 2  s −1 at 50 °C and 5.7777E−09 m 2  s −1 at 60 °C, respectively. The moisture mobility of the samples at different drying stages was monitored by the NMR analyzer. T 2 spectra showed that the initial ratios of semi-bound water and bound water in FTL were 93.2 and 6.8%, respectively. The drying rate showed a decreasing trend with the reduction in relaxation time of T 22 peak from about 80 ms to about 30 ms, especially for the ones dried at 60 °C. The reduced relaxation time of T 22 peak indicated that water activity in FTL was weakened during drying. Moisture distribution obtained by magnetic resonance imaging in FTL at different drying stages can reveal the moisture transfer path.