Analysis of Rewetting Characteristics and Mechanical Properties in Leaf Blades and Main Veins of Cured Tobacco Leaves

Abstract

The moisture content and mechanical properties are crucial factors influencing the processing quality of cured tobacco leaves. Understanding the rewetting characteristics and mechanical properties of leaf blades and main veins is essential to for optimizing tobacco processing. This study measured the rewetting behavior and mechanical properties of leaf blades and main veins under varying temperature (20–40 °C) and relative humidity (RH, 50%–90%) conditions. Rewetting curves were obtained and dynamic models were constructed. Low-field nuclear magnetic resonance (LF-NMR) analysis was utilized to obtain information of relaxation time (T2), and correlation models between moisture state and mechanical properties were established. The results indicated that the rewetting rate of leaf blades and main veins decreased gradually with increasing moisture content, while higher temperature and RH enhanced the rewetting process. The rewetting time of leaf blades at 30 °C with 90% RH was reduced by 71.43% compared to that at 30 °C with 70% RH. For main veins, the rewetting time at 30 °C with 90% RH was shortened by 50.00% compared to 30 °C with 80% RH. The Peleg model was determined as the optimal dynamic model for describing the rewetting characters for leaf blades and main veins. Two moisture states were observed: strongly bound moisture and weakly bound moisture. The peak area of strongly bound moisture increased gradually, while the peak area of weakly bound moisture initially increased and then decreased. At the moisture content of 0.15 g/g, the strongly bound moisture content of leaf blades decreased by 7.75% compared to main veins, whereas the weakly bound moisture content of leaf blades increased by 3.04 times. However, the proportion of weakly bound moisture decreased despite the increasing proportion of strongly bound moisture. Furthermore, the relaxation time of leaf blades and main veins increased during rewetting. The mechanical properties analysis revealed a correlation between breaking force, strain, and elastic modulus with the content of strongly bound moisture. This study provides a theoretical basis for the development of improved rewetting processes in tobacco production.