Mechanisms for reducing cigar tobacco leaf irritancy during acidic fermentation.

Abstract

This study explored the effects of an acidic fermentation environment on the quality of cigar tobacco leaves using air-cured “Dexue 1” tobacco leaves as the raw material. Malic acid was used as a pH regulator and three concentrations were screened to identify an optimal level for co-fermentation with 1.5% glucose. The potential mechanisms by which acidic fermentation reduces tobacco leaf irritancy were also analyzed. Fermentation under acidic conditions led to a more balanced chemical composition and to reduced irritancy. In the T2 (0.8% malic acid) and T3 (0.8% malic acid + 1.5% glucose) treatment groups, free nicotine proportion decreased by 37.52% and 46.98%, respectively, compared to the unfermented treatment (CK). Enzymatic activities of carbon- and nitrogen-metabolizing enzymes (alpha-amylase, neutral protease), a browning-related enzyme (polyphenol oxidase), and a polyphenol synthesis enzyme (phenylalanine ammonia lyase) were significantly higher in fermented tobacco leaves than in unfermented ones. Microbial abundance and diversity were also substantially elevated post-fermentation. Dominant bacterial genera included Staphylococcus, Pantoea, Ralstonia, Pseudomonas, and Bacillus. Staphylococcus exhibited a significantly positive correlation with protein and nicotine content, as well as a significant or highly significant negative correlation with total sugar, amino acids, and NH3 content. Metabolomic analysis revealed that the primary enriched metabolic pathways were the citrate cycle and phenylalanine metabolism. Staphylococcus was positively correlated with acetyl-CoA, isocitrate, and trans-2-hydroxycinnamic acid levels, whereas Bacillus was significantly positively correlated with cis-aconitate and citrate. Under acidic conditions, N exists as NH4+, which is more stable and less likely to volatilize during combustion, thus reducing NH3 release. Moreover, the proportion of free nicotine decreased as pH decreased. Collectively, these factors reduced the irritancy of cigar tobacco leaves fermented under acidic conditions.