Dissecting the microbial, physicochemical, and flavor dynamics of core and peel layers in Houhuo Daqu: Insights into quality regulation

Houhuo Daqu (HHD) exhibits significant heterogeneity between its core and peel layers, yet their differences remain underexplored. This study integrates metagenomic sequencing and electronic sensory technologies to compare the physicochemical properties, microbial communities, and flavor profiles of HHD's core and peel. Results reveal distinct microbial communities and diversity between the layers. Both are dominated by bacteria (>90 % relative abundance). The core shows significantly higher relative abundance of Bacillus licheniformis, Bacillus haynesii, and Bacillus paralicheniformis, while the peel has elevated levels of Streptomyces sp. NHF165, Pantoea agglomerans, and Lactiplantibacillus plantarum. Bacillus licheniformis is linked to acetic acid biosynthesis. Flavor analysis indicates both layers are rich in pyrazines, contributing to HHD's distinctive aroma. Enzyme activities differed markedly between the core and peel. Structural equation modeling, regression, and ENVFIT analyses show that amino acid nitrogen directly influences enzymatic activity and indirectly affects it by shaping microbial community and diversity. Additionally, amino acid nitrogen significantly impacts HHD's taste and aroma, modulated by starch and ash content. These findings highlight amino acid nitrogen as a key factor for controlling HHD quality in future production.