Materials and methods General experimental proceduresDiscarded tobacco leaves (growth origin, Qujing, Yunnan province; year, 2014) were chosen. D-glucose (≥ 99.5%) and D-fructose (≥ 99.5%) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Furfural, 5-HMF (5-hydroxymethylfurfural), and 2-phenylethyl propionate were obtained from Sigma-Aldrich (Alexandria, America). Agilent 1290 HPLC was used for the HPLC-ELSD analysis (Agilent, Santa Clara, USA). Headspace GC-MS and GC-MS analyses were carried out by Agilent 6890GC/5973MS GC-MS (Agilent, Santa Clara, USA), which was equipped with a flame ionization detector. Tobacco leaves were cut using an SY-III tobacco cutting machine (Fubuang, Zhengzhou, China). The cut tobaccos were reacted in an HPX-9082MBE air oven (Boxun, Shanghai, China). Heating procedureThe tobacco leaves were cut. Then the cut tobaccos (20.0 g) were blended with glucose and water, which were heated in the air oven at the pre-set temperatures. The roasted tobaccos were taken out immediately at the presupposed time. Afterwards, the