Application Of Streptococcus Thermophilus With High-yield Acetaldehyde In Fermented Milk

Two high-yield acetaldehyde Streptococcus thermophilus (IMAU80809and IMAU10636) and one typical strain Lactobacillus bulgaricus IMAU40078were studied for the fermentation characteristics and production ability of flavor substance during fermentation and storage. The experiment was carried out by comparing the behaviors and metabolites of fermentation by two Streptococcus thermophilus strains with single or mixed with different proportions Lactobacillus bulgaricus (10:1,100:1). The main conclusions were as follows:During fermentation, the titratable acidity(TA) of single or mixed fermentation samples increased slowly in the period of0-2hours, but since then the TA values increased rapidly till the end of fermentation. Mixed fermentation samples were more faster than single fermentation to reach the end of fermentation (less than pH4.6). With the fermentation, the number of viable cells increased significantly, and the number of viable cells of two S.thermophilus single fermentation and mixed fermentation had reached over108cfu/mL at the end of fermentation.During storage, the concentrations of diacetyl and acetaldehyde and lactic acid increased significantly at first and then stabilized along with the time. The production of diacetyl of single Streptococcus thermophilus strains fermentation were the largest. On the fourteenth day of the storage, the highest diacetyl contents of Streptococcus thermophilus IMAU80809was3.58μg/mL, and Streptococcus thermophilus IMAU10636was3.26μg/mL on the seventh day of the storage. The acetaldehyde and lactate contents of at a ratio of10:1mixed fermented milk samples were the highest. On the seventh day of the storage, acetaldehyde contents of two mixed fermented milk samples reached the maximum, were18.09μg/mL and18.97μg/mL, respectively. On the fourteenth day and the seventh day of the storage, lactic acid contents reached the maximum, were16.22g/L and17.87g/L, respectively.The quality of different fermented milks were evaluated by sensory evaluation. The yogurt at a ratio of10:1fermentation is better than other yogurts. Compared with other fermented samples, the samples at a ratio of10:1fermentation had appropriate acidity and better flavor, so the sense evaluation had the highest score, were8.9points and8.8points, respectively. Base on these results, the starter cultures at a ratio of10:1has the potential to be used as a commercial starter.