Baijiu, a distilled spirit, is recognized as one of the six major spirits in the world, alongside brandy, whisky, vodka, gin, and rum (Liu and Sun, 2018). It can be categorized into twelve aroma types, each distinguished by its flavor profile. These types include strong-aroma, jiang-aroma, light-aroma, rice-aroma, zhima-aroma, chi-aroma, mixed-aroma, feng-aroma, laobaigan-aroma, te-aroma, fuyu-aroma, and drug-aroma type of flavor. Strong-aroma Baijiu dominates the market, accounting for over 70% of sales market and serving as a cornerstone of the Baijiu industry (Liu et al., 2017). The most notable characteristic of the production process for strong-aroma Baijiu is the use of cellar mud fermentation under anaerobic solid-state with multiple microbes called as “multi-microbe co-fermentation”. This process yields a product with a distinct cellar aroma, sweetness, and freshness, consisting of over 2,000 flavor substances, including alcohols, aldehydes, acids, esters, nitrogen compounds, and sulfur compounds (Liu et al., 2017). Among these substances, acids play a crucial role in determining the flavor of the product (Hong et al., 2023).

So far, researchers have identified a total of 147 types of organic acids in Baijiu (Miao et al., 2022). These acids can be divided into three categories: short- and medium-chain fatty acids (S/MCFA), long-chain fatty acids, and other acids. The S/MCFA, such as acetic acid, butyric acid, and caproic acid, play a significant role in reducing the pungency and bitterness of Baijiu and enhancing its crispness, thereby greatly influencing its taste (Niu et al., 2020, Wu et al., 2022). By adding appropriate amounts of organic acids, Baijiu can become more harmonious and leave a long-lasting aftertaste. Furthermore, the addition of organic acids can also promote the aging process of fresh Baijiu product (Miao et al., 2022). Different categories of Baijiu have varying proportions of acids, which greatly impact the overall sensation of the drink. In particular, strong-aroma Baijiu contains the highest levels of caproic acid. The acid not only contributes to the acidic flavor of the Baijiu but also acts as a precursor for the synthesis of ethyl caproate, a key aroma compound in strong-aroma Baijiu (Miao et al., 2022). Furthermore, studies have demonstrated that S/MCFA have several health benefits. They can reduce alcohol intoxication and alleviate acute liver injury (Wu et al., 2022, Xu et al., 2020). Additionally, these acids are vital for maintaining metabolic functions, acting as energy resources for intestinal epithelial cells, and modulating intestinal inflammation (Yao et al., 2022).

The quality and flavor of strong-aroma Baijiu are closely linked to the cellar mud’s quality, which is primarily influenced by the metabolic production of acids by microorganisms (Hu et al., 2016, Lin et al., 2023). Studies have shown that microorganisms in the cellar mud can produce various metabolites, particularly Clostridium sp., an anaerobic species that can metabolize S/MCFA like butyric acid and caproic acid using glucose, acetic acid, lactic acid, and other substrates in the Baijiu fermentation system (Fu et al., 2021). One specific bacterium, Clostridium kluyveri, is known for producing caproic acid by converting acetic acid and ethanol through the reverse β-oxidation pathway (Seedorf et al., 2008). However, due to the strictly anaerobic nature of the cellar mud’s prokaryotic microorganisms and the undefined culture conditions, high-throughput sequencing has revealed the presence of numerous uncultured microorganisms in the cellar mud. Recent findings include the discovery that Ruminococcaceae bacterium CPB6 can produce caproic acid using lactic acid as a substrate, as well as the identification of new species such as Caproicibacterium amylolyticum LBM18003T and Caproicibacterium lactatifermentans LBM19010T, which has expanded our understanding of caproic acid-producing bacteria in cellar mud for strong-aroma Baijiu production (Zhu et al., 2017, Gu et al., 2021, Wang et al., 2022). In recent years, new genera, and species of microorganisms have been identified from the cellar mud (Gu et al., 2021, Wei et al., 2021, Xu et al., 2022b, Fan et al., 2023, Wu et al., 2023). However, study about cellar mud microorganisms is still in its early stages, and there is a limited availability of isolated microbial resources when comparing with the annotated results based on high-throughput sequencing. Therefore, it remains important to isolate and identify key caproic acid-producing microorganisms in the cellar mud.

In this study, a bacterial strain was isolated from the cellar mud of strong-aroma Baijiu that was able to ferment glucose to produce caproic acid. The 16 S rRNA gene sequence of this strain showed similarity below the threshold of 98.70% for microbial species classification to that of the reference species Caproicibacter fermentans DSM 107079 T (Flaiz et al., 2020). This suggested that the strain might be a potential new species of the genus Caproicibacter. Additionally, the metabolic properties of the strain was investigated. Through whole genome sequencing and annotation, the metabolic pathway of caproic acid production was revealed, and the key thiolase for caproic acid synthesis of the strain was identified. This study offered novel caproic acid producing bacteria from Baijiu sources.