Identification and quantification of the caproic acid-producing bacterium Clostridium kluyveri in the fermentation of pit mud used for Chinese strong-aroma type liquor production

Chinese strong-aroma type liquor (CSAL) is a popular distilled alcoholic beverage in China. It is produced by a complex fermentation process that is conducted in pits in the ground. Ethyl caproate is a key flavor compound in CSAL and is thought to originate from caproic acid produced by Clostridia inhabiting the fermentation pit mud. However, the particular species of Clostridium associated with this production are poorly understood and problematic to quantify by culturing. In this study, a total of 28 closest relatives including 15 Clostridia and 8 Bacilli species in pit muds from three CSAL distilleries, were detected by culture-dependent and -independent methods. Among them, Clostridium kluyveri was identified as the main producer of caproic acid. One representative strain C. kluyveri N6 could produce caproic, butyric and octanoic acids and their corresponding ethyl esters, contributing significantly to CSAL flavor. A real time quantitative PCR assay of C kluyveri in pit muds developed showed that a concentration of 1.79 x 10(7) 16S rRNA gene copies/g pit mud in LZ-old pit was approximately six times higher than that in HLM and YH pits and sixty times higher than that in 12-new pit respectively. This method can be used to improve the management of pit mud microbiology and its impact on CSAL quality. (C) 2015 Elsevier B.V. All rights reserved.