Metagenomic and metatranscriptomic analyses of microbial genera and volatiles produced during the fermentation of doubanjiang meju

Fermented broad beans (Vicia faba L.), commonly known as meju, serve as a crucial raw material for producing Pixian Doubanjiang (DBJ), a traditional condiment in Chinese cuisine. However, there is limited information on the dynamics of fungal populations and the activity shifts of key enzymes during DBJ meju fermentation. This study aimed to elucidate the microbial composition, active genera, expressed genes and pathways during the DBJ meju fermentation. The general chemical components, free amino acids, enzymes and volatile compounds were also investigated; the correlations between active genera and physicochemical factors were analyzed, at different fermentation stages. The results demonstrated that protease was the predominant enzyme during meju fermentation. A total of 32 major volatile compounds were identified, with most alcohols and aldehydes showing a sharp increase from the early to the middle stages, followed by stabilization until the end of fermentation. Significant shifts in metatranscriptomic composition at the genus level were observed, with Aspergillus, Staphylococcus, and Tulasnella emerging as the core active genera in the process. Notably, cellulase activity was positively correlated with the presence of Tulasnella. Additionally, Aspergillus and Tulasnella were found to play a crucial role in developing the unique aroma of DBJ meju. Our findings on the succession of active genera and their correlation with physicochemical factors are expected to provide substantial evidence for potential quality control and enhancement of this renowned Chinese condiment.