Isolation, identification and fermentation conditions of highly acetoin-producing acetic acid bacterium from Liangzhou fumigated vinegar in China

Liangzhou fumigated vinegar is famous in China for its distinct flavor. How to enhance flavor in traditional fermented foods, such as vinegar, has attracted a large interest in recent years. Acetoin (ACT) is a precursor of tetramethylpyrazine (TTMP), a popular food flavor found in different traditional fermented foods. We obtained 200 acid-producing isolates from the solid fermentative substrate of Liangzhou fumigated vinegar by the method of forming transparent zones on GYC agar, and 28 bacteria producing ACT by Voges–Proskauer colorimetric assay and gas chromatography analysis, including 15 strains of Acetobacter, eight strains of Lactobacillus and five strains of Staphylococcus. Based on ACT level, the isolate C92 with the highest yield was selected for further analysis. It was identified as Acetobacter pasteurianus by physiological and biochemical characteristics, as well as 16S rRNA gene sequence similarities. We examined the ACT level of strain C92 under different conditions, and found that a temperature of 31.31 °C, pH of 6.67, shaking speed of 209 rpm, and inoculation volume of 8.27 % were the best conditions for strain C92 to produce a yield of 19.04 g/L at 40 h post-culture. In a time–ACT curve, ACT reached the maximum value at 40 h and then decreased afterward, suggesting that ACT produced by C92 is a growth-associated primary metabolite from sugar. Therefore, the C92 isolate is regarded as a strain with promising ACT-production ability in vinegar. More studies are needed before C92 is applied in the industrial production of vinegar.