Formation of phenolic compounds from roasted sesame hull lignin: Impact of lignin structure

The phenolic compounds generated from sesame hull lignin roasting contribute to sesame oil quality, yet their formation mechanism remains unclear. In this study, a thermal desorption unit with gas chromatography coupled to mass spectrometry (TDU-GC-MS) was used to simulate the roasting of five lignins isolated from sesame hull: two types of milled wood lignin (MWL-1 and MWL-2), acetic acid lignin (AAL), alkali lignin (AL), and ethanol lignin (EL). The effect of lignin structure on phenolic product formation during roasting was investigated. UV-vis, FT-IR, and NMR showed that all five lignins were GSHC-type lignins. However, the morphologies, molecular weight, and thermal properties of these five lignins differed according to SEM, GPC, and TG. The 2D-HSQC NMR analyses showed that the two MWLs and AAL contained a higher content of G units (35.0 %-42.9 %) and β-O-4 linkages (5.6-8.3/100 Ar). The phenolic content of the roasting degradation products was also high (21.94 %-25.73 %), and the phenolics were dominated by G-type phenolics (19.66 %-22.87 %). AL and EL had high H unit contents of 45.3 % and 47.4 %, respectively, and the roasting phenolic degradation products were dominated by H-type phenolics of 9.32 % and 8.98 %, respectively. Results reveal that the types of phenolic products formed through the thermal degradation of lignin during sesame hull roasting depend on the structural unit composition of the lignin in sesame hulls. High β-O-4 linkage content favors phenolic formation. This study contributes to the understanding of the formation pathway of phenolics resulting from the roasting degradation of lignin in sesame hulls during sesame oil processing.