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 m2 s−1 at 50 °C and 5.7777E−09 m2 s−1 at 60 °C, respectively. The moisture mobility of the samples at different drying stages was monitored by the NMR analyzer. T2 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 T22 peak from about 80 ms to about 30 ms, especially for the ones dried at 60 °C. The reduced relaxation time of T22 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.
Highlights
In tobacco primary processing, fresh tobacco curing after harvesting, an important drying operation unit, could convert tobacco leaves from agricultural products into tobacco industry materials
thermogravimetric analysis (TG) can be employed at isothermal, lowtemperature condition, which simulates the process of fresh tobacco curing to investigate the drying mechanisms
We present an application of the analytical method combining the TG and 1H nuclear magnetic resonance analysis (NMR) to study the drying characteristics of fresh tobacco leaves (FTL)
Summary
Fresh tobacco curing after harvesting, an important drying operation unit, could convert tobacco leaves from agricultural products into tobacco industry materials. Academy of Agricultural Sciences, Zhengzhou, China 4 China Tobacco Henan Industrial Co., Ltd, Zhengzhou, China suitable curing technology according to their drying characteristics. On the other hand, when fresh tobacco leaves (FTL) were cured from the initial moisture content of 80–85% to the final moisture content of 16–18%, the curing conditions of FTL during curing processing need to be adjusted according to the drying characteristics [4] and thermal properties [5] of materials at different drying stages. Some drying kinetics parameters, such as the drying rate constant, the moisture diffusion coefficient and the activation energy, can be obtained from TG drying curves [7, 8]. The change in internal moisture mobility and distribution in green tobacco leaves during drying is still unknown by the present TG method
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