Impact of spatial heterogeneity on fermentation characteristics of cereal vinegar: A comparison of manual and mechanical methods

Owing to limited mass transfer, spatial analysis can more accurately reveal the solid-state fermentation (SSF) process of cereal vinegar. The purpose of this study was to evaluate and compare the fermentation characteristics of cereal vinegar using traditional manual operation (TO) and modern mechanical operation (MO) at different depths, to clarify the reasons for the differences in bacterial composition and flavor. High-throughput sequencing was used to investigate the composition and dynamic changes in bacteria at different depths during TO and MO, revealing that the spatial heterogeneity of SSF results in variation in bacterial diversity. The diversity of bacteria increased with an increasing depth with TO and MO. Kroppenstedtia, Lactiplantibacillus, Acetobacter, Lactobacillus, Sphingomonas, Limosilactobacillus, Ligilactobacillus, Aquabacterium, and Lentilactobacillus were significantly associated with volatile flavor compounds (VFCs). In particular, Acetobacter was significantly positively correlated with 9 and 14 VFCs with TO and MO, respectively. Moreover, the physicochemical and flavor properties changed markedly at different depths for TO and MO. High total titratable acid and acetic acid contents were observed with TO, whereas high lactic acid, amino acid, and ester contents were observed with MO. In conclusion, these findings provide new insights into understanding the SSF process of cereal vinegar and improving vinegar quality. •The characteristics of heterogeneity of cereal vinegar fermentation was revealed.•The spatial heterogeneity results in the variation of bacterial diversity and flavor.•Acetic acid accounts for the variation in bacterial diversity with depth.