Mechanized Qu-making may contribute to lactic acid bacteria overproliferation in Hongqu rice wine brewing: Insights into microbial dysbiosis and flavor quality deterioration.

Hongqu (HQ) serves as the core saccharification and fermentation starter (commonly referred to as "Qu") for Hongqu rice wine (HQW), a traditional Chinese fermented alcoholic beverage celebrated for its unique sensory characteristics and potential health-promoting properties. Driven by industrialization, mechanized Qu production has been progressively implemented to enhance process reproducibility, scalability, and operational efficiency. However, the implications of this technological transition on microbial community assembly, metabolic function and flavor formation during HQW brewing remain insufficiently characterized. To address this gap, this study employed an integrated metagenomic and metabolomic approach to comparatively analyze the taxonomic composition and functional metabolic profiles of mechanized Hongqu (MHQ) and traditional Hongqu (THQ), and further monitored their dynamic succession throughout HQW brewing process. Results demonstrated that MHQ exhibited significantly higher saccharification capacity and markedly enriched abundance of Saccharomyces cerevisiae, yet displayed a substantial reduction in Aspergillus niger compared with THQ (1.06% versus 43.41%). Paradoxically, despite these favorable starter attributes, HQW fermentation inoculated with MHQ induced an uncontrollable proliferation of lactic acid bacteria (LAB), predominantly represented by Pediococcus acidilactici, Lactiplantibacillus plantarum and Weissella paramesenteroides. This LAB-dominant consortium proliferated markedly during HQW fermentation, resulting in a community compositional shift toward bacterial dominance, evidenced by a fungi-to-bacteria ratio of 1.00:5.88 in MHQW, whereas THQW retained fungal dominance, with a corresponding ratio of 1.00:0.26. This structural shift coincided with significant declines in the relative abundances of functional fungi, including Saccharomyces cerevisiae and Monascus purpureus, possibly involving changes in niche occupation and acidification. These microbial community changes were associated with a metabolic shift characterized by excessive accumulation of organic acids, dysregulated biogenic amine profiles, depletion of free amino acids, and diminished synthesis of key volatile flavor compounds. Quantitatively, MHQW exhibited significantly higher final titratable acidity (12.67 g/L vs. 5.76 g/L), lower ethanol yield (17.29% v/v vs. 20.39% v/v), elevated total organic acid content (16.62 g/L vs. 6.28 g/L), and reduced total free amino acid concentration (3366.23 mg/L vs. 4303.93 mg/L) relative to THQW. Collectively, these findings indicate that mechanized Qu-making may disrupt the delicate "fungi-bacteria" ecological equilibrium essential for robust and balanced HQW fermentation, potentially favoring LAB proliferation. This study highlights the potential value of rational microbiome design to control LAB proliferation while maintaining functional fungi, which is important for optimizing mechanized Qu-making processes and improving flavor quality and fermentation robustness in HQW production. Collectively, our work provides a mechanism-informed framework for advancing Huangjiu modernization through rational microbiome engineering.