Screening Of Bacteria Producing Soy Sauce Flavor And Study Of Characteristic Volatile Compounds

Soy sauce aroma style liquor is one of the four basic types in China. The unique brewing process results in a special microbial flora in Daqu and fermented grains. Microorganisms in high temperature Daqu affect the formation of soy sauce flavor. So it is helpful to reveal the formation mechanism of soy sauce flavor through analysis of microorganism in high tempera -ture Daqu. In this paper, strains producing soy sauce flavor were obtained from high tempera -ture Daqu and characteristic volatile compounds of their fermentation liquid were detected. The main results are as follows:1) Three strains of bacteria which can produce soy sauce flavor were screened from high temperature Daqu. These strains produced weak soy sauce flavor in wheat bran through solid fermentation for 15d; they produced very strong soy sauce flavor in wheat-extract medium by submerged fermentation for 6d. The culture conditions of three strains in submerged fermen -tation were optimized primarily. After three strains were inoculated in wheat-extract medium and cultured at 55℃for 6 d with an agitation speed of 150 r/min, liquid fermentation broth produced very strong sauce flavor. Through analysis of morphology, physiological and biochemical characteristics and 16S rDNA sequencing, three strains were identified as Bacillus licheniformis.2) After BL-J, BL-L1, BL-L2 were cultured in wheat-extract medium for 6 d, the broth was prepared by liquid-liquid extraction. The extraction phase of the fermentation product, which smelled sauce aroma, was used to analyze its volatile components by GC-MS. Acetoin, tetramethylpyrazine and furaneol which were three important compounds in soy sauce aroma liquor, were detected from the fermentation broth. The cultured broth with inoculation of BL-J, BL-L1, BL-L2 was compared with the cultured broth without inoculation by sensory evaluation. BL-L1 was the strain that produced the strongest soy sauce aroma. We speculated that acetoin, tetramethylpyrazine and furaneol were the characteristic compounds of the strain. During the BL-L1 fermentation period, the concentration of acetoin and furaneol decreased, and that of tetramethylpyrazine increased. Tetramethylpyrazine which was formed in the later fermentation time, was generated by conversion of acetoin.3) The high-temperature environment of 55℃was an important condition for BL-L1 to produce soy sauce aroma. Tetramethylpyrazine which was the characteristic compound of fermentation broth, was significantly higher at 55℃than at 37℃or 45℃.At the fermentation time of 4d, quantities of microorganisms had decreased significantly. Then the broth was dealt with sterilization by membrane filtration and inactivation of enzyme by high temperature separately. The treated fermentation broth continued to be cultured for another 2d. Compared with the fermentation products at 55℃for 2d after sterilization and inactivation of enzyme by high temperature at 4d, the fermentation products at 55℃for 2d after sterilization by membrane filtration at 4d smelled stronger sauce aroma. The bioenzyme secreted by the strain BL-L1 in the culture medium was certified to promote the formation of sauce aroma. The free amino acids of fermentation broth at 55℃for 2d after sterilization by membrane filtration and sterilization and inactivation of enzyme by high temperature at 4d, including glutamic acid, valine and leucine, were significantly lower than that of fermentation broth at the fermentation of 4d.