Multi-platform metabolomics reveals the effects of different staged curing processes on tobacco quality and metabolic mechanisms

This study takes B2F grade and B3F grade tobacco leaves of Yunyan 87 as research materials. On the basis of the conventional curing process, three treatments, namely SG (yellowing stage + color-fixing stage), SS (color-fixing stage) and SW (stem drying stage), are designed by adjusting process parameters at different curing stages. Metabolomics integrated with LC-MS and GC-TOF-MS is employed to explore the mechanism of curing process on tobacco quality and the causes of quality differences between tobacco leaf grades. The results show that tobacco leaves treated with the SG process present the lowest starch content, the highest contents of sugars and polyphenols, as well as the optimal sensory evaluation score and average price. The SG process promotes the accumulation of aroma precursors in the phenylpropanoid metabolism pathway and downstream flavonoids. Although the SS process also reduces starch content and elevates the contents of sugars and polyphenols, its overall effect is inferior to that of the SG process.The SW process leads to the accumulation of indole derivatives from tryptophan metabolism and excessive organic acids in the tyrosine pathway. The disparities in quality observed between B2F and B3F tobacco leaves are attributed to the fact that B2F leaves accumulate more abundant terpenoids, flavonoids and other functional substances, while containing lower levels of irritant compounds. This study clarifies the impacts of parameter adjustment at each stage of tobacco curing on leaf quality, thereby offering a theoretical framework to support the establishment of a standardized technology system for tobacco flue-curing.