Screening And Aroma Characteristics Of Lactic Acid Bacteria With Flavor-enhancement Abilities In Tibetan Mushrooms

In this study, Lactic acid bacteria, which possessed flavor-enhancement abilities, were isolated and identified. Meanwhile, the volatile aromatic compounds in yogurts which fermented by different strains were analyzed. Besides, in order to explore the mechanism of aroma production of different strains for yogurt, the real-time quantitative PCR method was adopted to determine the amount of gene-expression of different metabolic pathways. Aimed at enhancing the aromatic characteristics endogenous control.The main conclusions were as follows:(1) Lactic acid bacteria, which possessed better capacity of aroma production and existed in Tibetan mushrooms, were isolated and identified. The results showed that there were four Lactic acid bacteria strains identified from Tibetan mushrooms, including 1-33,1-34, 2-31, and 3-29, and all of them had much higher capacity of producing acetaldehyde and diacetyl. Moreover, the results also indicated that among those four strains, strain 1-33,1-34, and 2-31 were proved to be Lactobacillus plantarum, while strain 3-29 was considered to be Lactococcus lactis. Four strains could grow better at the temperature range of 15 to 450C,and the temperature of 37 ℃ was selected as the optimum growth temperature of the four strains. Four strains entered into the logarithmic growth phase about 3 hours later, and when they were used for preparation of yogurt, the acidity of yogurt increased significantly during the fermentation.(2) The aroma compounds of different yogurt were analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that the total number and contents of aroma compounds which detected from strain 1-33 and 1-34, were much higher than those from the control (commercial Lactic acid bacteria) yogurt. Meanwhile, the concentrations of acetaldehyde, diacetyl, acetoin, and other compounds in the strains of 1-33 and 1-34 were higher than those in the blank strains.(3 ) The odor-active compounds in different yogurts which fermented by four screened strains, were distinguished by odor activity value (OAV). The results showed that there were 18 aroma components with OAV>1 were detected in three yogurts which fermented by three Lactobacillus plantarum (strain 1-33, 1-34, and 2-31). Among those, there were 10 carbonyl compounds (such as acetaldehyde, diacetyl, 2-pentanone, 2,3-pentanedione, and acetone), 4 volatile acid compounds (including butyric acid, caproic acid, octanoic acid, and capric acid)were identified in three yogurts. Besides, 3-methyl-2-butanol, dimethyl sulfide, and D-limonene were also detected.(4) Seven sensory attributes (milk note, typical yogurt odor, creamy, vinegarish off-flavor,and overall acceptance) of three different yogurts were evaluated by sensory analysis. The results showed that the yogurt which fermented by strain 1-34, contributed milk note, typical yogurt odor, creamy, and overall acceptance significantly (p<0.05) when it compared with the control yogurt. However, there was no significant difference (p<0.05) between fermentation yogurt (fermented by strain 1-33) and the control. (5) The results showed that the changes of characteristic aroma compounds were different during fermentation and storage of yoghurts fermented by strain 1-33 and 1-34. And at the same time, in order to study the mechanism of aroma production of different strains for yogurt, the changes of aroma compounds were connected with the expression of different key enzymes. The results showed that the amounts of gene-expression of β-galactosidase (lacA and lacL), 6-phosphofructokinase (pfk) and pyruvate were gradually increased during the fermentation process. It also could be seenthat the total content of characteristic volatile compounds, acetaldehyde, diacetyl, and acetoin were also increased gradually. The amounts of gene-expression of lacA, lacL, and pfk were decreased gradually during the storage, while the total contents of characteristic aroma compounds increased. The amounts of gene-expression of pdhD and the content of acetaldehyde were both decreased, similarly, the amounts of gene-expression of a-als and the content of diacetyl were also decreased gradually. The amounts of gene-expression of a-aldB were decreased gradually, but the content of acetoin was gradually increased.