Analysis of hygroscopicity and pyrolysis behaviour of propanediol esters synthesized by the enzymatic method

In order to develop food or tobacco usable flavour precursors, compounds 2-hydroxypropyl benzoate (3a), propane-1,2-diyl dibenzoate (4a), 2-hydroxypropyl cinnamate (3b) and propane-1,2-diyl (2E,2′ E)-bis(3-hydroxypropyl cinnamate) (4b) were synthesized by transesterification propanediol with vinyl benzoate and vinyl cinnamate under the biological enzyme Novozym435. The structures of the propanediol esters were characterized using nuclear magnetic resonance (¹H NMR, ¹³C NMR), infrared spectroscopy (IR) and high-resolution mass spectrometry (HRMS). The hygroscopicity and moisture retention capacity of reconstituted tobacco shreds with the synthesized propanediol were explored by 100 h hygroscopic desorption assay, model validation and low-field nuclear magnetic resonance imaging (LF-NMR). The results showed that the bound water content of 3a and 3b was lower than that of propanediol and higher than that of the control under both low (RH = 32%) and high (RH = 84%) humidity conditions. In addition, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technique was applied to investigate the thermal behaviour of 3a and 3b. Four and five pyrolysis products were formed by pyrolysis of 3a and 3b, respectively. The main pyrolysis products benzoic acid and cinnamic acid could be used as flavouring agents in food and tobacco. 3a and 3b had good moisture and humidity retention effects as well as uniform and sustained flavour release. The results of the study not only effectively improve the water absorbency of reconstituted tobacco and enhance the flavour, which is suitable for the storage and smoking process, but also provide a reference for the further development of new flavour moisturizers in the tobacco industry.