Identification of Key Microorganisms and Metabolic Pathways Associated with the Formation of Off-Flavour Compounds in the Pit Mud of Strong-Flavour Baijiu

Off-flavours represent one of the most prevalent and severe causes of deteriorating pit mud quality in Strong-Flavour Baijiu production. However, the key compounds responsible for these off-flavours and their formation mechanisms remain poorly understood, thereby limiting quality control in the production process of Strong-Flavour Baijiu pit mud. To investigate the origin of off-flavours in pit mud, this study employed gas chromatography–mass spectrometry coupled with metagenomic methods to compare samples from normal and off-flavoured pit mud. The results show that the main cause of off-flavours in pit mud is the abnormal accumulation of acids such as heptanoic acid and hexanoic acid due to imbalanced nutrient ratios, along with insufficient synthesis of key esters including ethyl hexanoate, ethyl butyrate, and ethyl lactate; microorganisms such as Fermentimonas, Methanoculleus, Hortaea and Proteiniphilum, which are non-strictly anaerobic and acid-sensitive in the pit mud, are key microbes associated with these odour compounds. Furthermore, based on the annotation from the KEGG database, this study further inferred the possible microbial metabolic pathways that could lead to the formation of these off-flavour compounds, including starch and cellulose degradation pathways, biosynthesis pathways of valine, leucine, and isoleucine, pyruvate metabolism pathways, and butyric acid metabolism pathways. In summary, the study systematically analysed the key substances related to the odour of the pit mud of strong-flavour Chinese Baijiu and its microbial sources, providing a theoretical basis for the quality regulation of the pit mud during production and developing high-quality artificial pit mud.