基于低温冻融核磁共振技术的烟叶纳米孔隙结构评价

Evaluation of Nanopore Structure of Tobacco Leaves Based on Cryogenic Freeze-Thaw Nuclear Magnetic Resonance Technology

采用低温冻融核磁共振技术,结合Gibbs-Thomson效应确定液体凝固点温度与液体所处孔隙孔径的关系,提出了一种无损且非侵入式的烟叶纳米孔隙结构分析方法。以烘丝前后云南烟叶样品的B2F上部烟、C2F中部烟、X2F下部烟为对象,分析研究了烟叶部位、烘丝方式与烟叶纳米孔隙分布间的关系。结果表明:核磁共振信号量与样品中水分含量高度线性相关,拟合线性相关系数高于0.99,样品的核磁共振信号量直接可以反映烟叶样品中水分含量与体积的大小。烟叶纳米孔隙的孔径相对较小,且呈非均匀分布,约90%的孔隙孔径在[1.9,20.6]nm内,上部和中部烟(4.6,8.6]nm孔隙占比显著低于下部烟,而(10.6,20.6]nm孔隙占比显著高于下部烟,相较于下部烟而言孔隙结构更为疏松,孔隙结构的差异可能主要受烟叶部位、光照、营养、水分等因素的影响。烟叶经烘丝处理后纳米孔隙结构发生显著变化,气流式烘丝与滚筒式烘丝均使得较小孔径的孔隙占比提高,较大孔径的孔隙占比下降。

A non-destructive and non-invasive method for analyzing the nanopore structure of tobacco leaves was proposed using cryogenic freeze-thaw nuclear magnetic resonance technology,combined with the Gibbs-Thomson effect to determine the relationship between the liquid freezing point temperature and the pore size of the liquid.Taking B2F upper part,C2F middle part,and X2F lower part of Yunnan tobacco leaves before and after drying as the objects,the relationship between tobacco leaf parts,drying methods and the distribution of nanopores in tobacco leaves was analyzed and studied.As shown by the results,the nuclear magnetic resonance signal intensity was highly linearly correlated with the moisture content in the sample,with fitting linear coefficients higher than 0.99.The nuclear magnetic resonance signal intensity of the sample could directly reflect the moisture content and volume in tobacco samples.The pore size of nanopores in tobacco leaves was relatively small and unevenly distributed,with about 90%of the pore size within[1.9,20.6]nm.The proportion of pores(4.6,8.6]nm in the upper and middle parts of tobacco leaves was significantly lower than that in the lower part of tobacco leaves,while the proportion of pores in the(10.6,20.6]nm was significantly higher.Compared with the lower part of tobacco leaves,the pore structure was looser,and the differences in pore structure may be mainly influenced by factors such as tobacco leaf location,light,nutrition,and moisture.After drying,the nanopore structure of tobacco leaves underwent significant changes,increasing the proportion of smaller pore sizes and decreasing the proportion of larger pore sizes by both heated stream drying and drum drying.