Selection Of Streptococcus Thermophilus With High Production Of Diacetyl And Expression Of The Related Functional Genes

In this paper, we treated thirty strains as the object of study, which were twenty-nine strains of Streptococcus thermophilus elected from our LAB Collection Center and commercial strain of L822provided by Chr-hansen. When finishing fermentation and under different storage time, the fermentation characteristics (such as pH value, titratable acid, viable count) and the diacetyl yield of thirty strains were checked to screen a high-diacetyl production of lactic acid bacteria. In order to determine the key genes of S. thermophilus producing diacetyl, the related functional gene expression of S. thermophilus producing diacetyl was also evaluated during48-hour refrigerated storage. In the meantime, we analyzed the correlation between diacetyl content in fermented milk and various functional genes. The main results were as follows:1. Among30strains of S. thermophilus, the fermentation time of IMAU20765, IMAU10636, IMAU80285, IMAU80287, IMAU80846, IMAU20728and IMAU20246were shorter than S. thermophilus L822. The result showed that the ability of producing acid of7strains of S. thermophilus was higher than the others. During storage, the pH value and titratable acid of fermented milk were not changed significantly. The viable count in fermented milk was maintained over than107cfu/g after48-hour refrigerated storage, except for the strains of IMAU20418, IMAU20141, IMAU20728and IMAU20588.2. The diacetyl content in fermented milk was measured during storage period. According to the clustering analysis by using SAS,30strains of S. thermophilus could be divided into3groups. There were9experimental strains in high content group of diacetyl,13strains of S. thermophilus in medium content group of diacetyl and8experimental strains in low content group. Among these strains, the diacetyl yield of4experimental strains(IMAU20713, IMAU20246, IMAU20738and IMAU20796) was maintained over than9mg/L during the storage. Especially the IMAU20246fermented milk had higher diacetyl content than L822, and the strain of IMAU20246with good fermentation, so it was suitable for commercial fermentation strain.3. The finding of this study showed that the related functional gene expression of S. thermophilus producing diacetyl was different depending on strains. During storage, the expression of pfl, adr, acoA, acoB, acoC and acoL had apparent differnces in9strains of S. thermophilus, but the change of Idh, als, aldB and nox wasn’t marked.Among them, the genes encoding for the pyruvate dehydrogenase complex had similar tendency in each of the strains.4. Accroding to the relevance analysis of diacetyl content and related functional gene expression, there was a positive correlation between diacetyl of these samples and the functional gene of als. On the contrary, there were negative correlations between diacetyl and the other related functional genes.