Isolation and Screening of Caproic Acid-producing Bacteria from Shedian Laojiu Cellar and Acid Production Conditions
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Abstract:
Caproic acid-producing bacteria represent important acid-producing functional bacteria in pit mud. The generated hexanoic acid is the major component of ethyl caproate, which confers the characteristic aroma of Luzhou flavored liquor, and its application in liquor production may improve the aroma and quality. Here, caproic acid-producing bacterial strains were screened and purified from the pit mud and yellow water of different Shedian Laojiu cellars. Through copper sulfate color development and gas chromatographic detection, 96 caproic acid- and butyric acid-producing strains were screened from 118 strains, majority of which were facultative anaerobic bacteria. Moreover, using 16S rRNA sequencing, 10 microbial strains, including Lysinibacillus, Bacillus, Rummeliibacillus, Stenotrophomonas, and Corynebacterium, were identified. Antagonistic confrontation assay of these caproic acid-producing bacteria revealed no antagonistic effects among the 10 strains, indicating the feasibility of mixed culture. When sodium acetate concentration in the medium was increased from 0.5% to 1.0%, the total amount of butyric and caproic acid in the product was the highest. However, the total amount of caproic and butyric acid decreased when sodium acetate concentration exceeded 1.5%. Moreover, 74% of the strains produced more total acid under anaerobic conditions than under aerobic conditions, and 63% of the strains produced more total acid at 30℃ than at 37℃ under anaerobic conditions. During the 18-day fermentation process of five caproic acid-producing strains, bacterial cell concentration gradually increased with culture time. Butyric acid level peaked at the middle of fermentation, followed by a gradual decrease; however, caproic acid accumulated gradually at the later stages of fermentation, with the peak yield reaching 3 g/L.
