Research On Major Aroma Constituents In Tobacco Leaves During The Bulk-curing Process

Tobacco leaves after curing of four loading modes were used to explore the availability of evaluating the aroma quality by electric nose. The middle leaves of NC55, which were cured by intellectual curing-barn,were used to reveal the law of generating aroma during the bulk-curing. Standard bulk curing-barns were used to study physiological and biochemical changes and aroma generation character loading leaf with pole and clamp respectively. The key points of the two loading modes of aroma generation during curing were elucidated. The main results are follows:(1)Research on the availability of evaluating aroma quality by electric nose.It is found that electric nose can not be used to evaluate aroma quality, but can detect the differences among the materials through analyzing the whole information. Aroma quality must still be evaluated by analyzing the content of every key kind of aroma components.(2)Changes of aroma precursors and physiological index during tobacco leaves curing.Pigments and polyphenols were influenced by their key corresponding hydrolytic or oxidizing enzyme. Degradation of chlorophyll was more quickly and thoroughly than that of carotenoids. Most of chlorophylls and carotenoids were degraded during the stage of 40-42℃ and 42-47℃ respectively. Degradation of chlorophyll of leaves loading with pole was more quickly than that of leaves loading with clamp. Degradation of carotenoids of leaves loading with pole was more quickly in the early stage but turned more slowly in later stage during curing than that of leaves loading with clamp.. Polyphenols content reached the highest level at the stage of 45℃-47℃ but some polyphenols then decreased to the initial level. Water loss rose constantly during curing and more than 90% of the water was lost when curing process ended. The generation of total sugar and reducing sugar have finished before 40℃, so does the degradation of starch.The degradation of pigments and loss of water in pole mode was earlier than those in clamp mode during bulk-curing.(3)Changes of aroma components and definition of ‘five-aroma’ stages in bulk-curing.Macromolecule substances such as the plastid pigment and starch began to been degraded in the stage of 38℃-40℃, and this stage was defined as aroma preparation. When the stage of 42℃ ended, the macromolecular substances have been degraded thoroughly, and contents of the degradation products of caroteniods such as neophytadiene began to increase, and this stage was defined as aroma generation. In the stage of 45℃-47℃, contents of neophytadiene and the degradation products of phenylalanine reached to high level. Degradation products of caroteniods, Maillard reaction products and cembranenoids were generated largely. This stage was defined as aroma promotion. In the stage of 50-54℃, maillard reaction products and cembranenoids rose contantly, and this stage was defined as aroma increment. In the stage of 68℃ which was defined as aroma fixation, the curing process must be ended timely in order to avoid volatilizing aroma substance. The generation of aroma in pole mode was earlier than that in clamp mode.(4)Regulation on key temperature point during the bulk-curing.The aroma quality could be promoted through extending periods of 40℃、47℃、50℃、54℃ and ending the curing process timely at the stage of 68℃. For tobacco curing with pole loading mode, increasing the heating rate could avoid the browning reaction. For clamp loading mode it is effective to promote aroma quality through decreasing the wet bulb temperature at the stage of 45℃ and increasing the curing time at stage of 45℃-47℃.