Abstract
Hexanoic acid (HX) is a crucial flavor compound and precursor of ethyl caproate (EA), which determines the quality of Chinese Luzhou-flavor liquor (CLFL). The isolation, purification, identification, and optimization of fermentation conditions of HX-producing bacteria are essential for industrial CLFL production. In this study, one strain of HX-producing bacterium was isolated from six candidate bacterial strains and identified as Clostridium sartagoneforme. Then, the growth characteristics and HX production of C. sartagoneforme were investigated. Sodium acetate medium was identified as the optimal fermentation medium from four candidate media. C. sartagoneforme yielded 800.85 ± 12.87 mg/100mL HX in sodium acetate medium. Then, to further optimize the formula of the fermentation medium, the carbon and nitrogen sources and inorganic salt component of the fermentation medium were investigated using HX yields as an optimization index. Optimization was performed with a single-factor experiment and the Taguchi design method. The single-factor experiment showed that the highest HX outputs were obtained when the sodium acetate medium contained 2.5 g/L yeast extract, 1.8 g/L KCl, 20 g/L sodium acetate, 15 mL/L ethanol, and 1.5 g/L glucose. In the orthogonal experiment designed using the Taguchi design method, HX yields reached 2018.29 ± 46.37 mg/100mL in sodium acetate medium that contained 3.5 g/L yeast extract, 1.8 g/L KCl, 25 g/L sodium acetate, and 15 mL/L ethanol.
Highlights
Hexanoic acid (HX) is the precursor of ethyl caproate (EA), the main flavor component in Chinese Luzhou-flavor liquor (CLFL) according to the National Standard GB/T 10781.1-2006
The results indicated that the HX yields of clone 5 in 15-day fermented broth was significantly higher than those of clones 1, 2, 3, 4, and 6 (Sigtow-tailed = 0, 0, 0, 0 < 0.01)
An efficient HX-producing strain was isolated from 280-year-old pit mud
Summary
Hexanoic acid (HX) is the precursor of ethyl caproate (EA), the main flavor component in Chinese Luzhou-flavor liquor (CLFL) according to the National Standard GB/T 10781.1-2006. One of the simplest and most widely adopted methods for improving CLFL quality is prolonged fermentation time. The relative HX amount, EA content, and CLFL quality increase as fermentation time increases Another widely adopted method for improving CLFL production is multiple fermentation rounds (double fermentation). In this method, lees are retained in the bottom of the fermentation pit and are mixed with new fermentation starter powders (e.g., daqu in Chinese) for another one or two fermentation periods. After two or three fermentation periods, the amounts of HX and other organic acids increase and the microbial biochemical reaction of spirits with HX or other organic acids is enhanced, increasing total acid and ester contents and improving liquor flavor. The traditional methods to improve CLFL quality, decrease production efficiency and increase production cost [4]-[9]
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