利用低场核磁共振分析烘烤过程烟叶水分迁移干燥特性

Analysis of moisture migration and drying characteristics of tobacco during flue-curing by low field NMR

为探究烘烤过程烟叶水分状态及迁移变化规律,以上部叶为试验材料,利用低场核磁共振技术分别对烘烤过程烟叶叶片和主脉进行弛豫特性及质子密度成像分析。结果表明:(1)叶片含有自由水、半结合水和结合水3种状态水分,主脉含有自由水、半结合水2种状态水分,烘烤过程烟叶主脉各对应状态水分流动性大于叶片;(2)42~48℃和54~60℃分别为叶片和主脉水分状态迁移变化的关键时期;烘烤过程烟叶叶片和主脉中自由水最先被干燥去除,而半结合水和结合水需要转化为自由水才能散失,其中少量结合水难以通过干燥脱除。(3)烘烤过程主脉水分由"主脉—侧脉—叶片"通道迁移效率较高,由主脉直接向叶片转移效率较低;另外,叶片厚度收缩率和主脉直径收缩率分别与其自由水散失规律密切相关。研究揭示了烘烤过程烟叶的干燥特性,可以为其烘烤工艺优化提供理论依据。

In order to study changes of moisture and migration characteristics of tobacco during curing process, tobacco upper leaves were used as test material in this study. Relaxation properties and proton density imaging of tobacco blade and tobacco midrib were analyzed by low field nuclear magnetic resonance technology during curing process. Results showed that tobacco blade contained 3 kinds of water phase, namely free water, immobilized water and bound water, and tobacco midrib contained 2 kinds, namely free water and immobilized water, and the mobility of each water phase in tobacco midrib was stronger than in tobacco blade during curing. The key period of water phase transition variations of tobacco blade and tobacco midrib was 42~48℃ and 54~60℃. Free water of tobacco blade and tobacco midrib was removed by drying at first stage of curing. Immobilized water and bound water were dehydrated by converting into free water and part of bound water was difficult to be removed by drying. Water migration channel of tobacco from tobacco midrib through lateral vein to tobacco blade had higher water migration efficiency, while water migration channel of tobacco from tobacco midrib to tobacco blade had lower water migration efficiency. The shrinkage rate of tobacco blade thickness and tobacco midrib diameter were consistent with the dehydration rule of their free water. The paper revealed the drying characteristics of tobacco during curing, and could provide a theoretical basis for optimizing curing technology.