Screening Of Streptococcus Thermophilus For Yoghurt Starter Cultures

Starter culture of lactic acid bacteria plays a crucial role in the quality of yoghurt.It makes good sense of screening out superior strain culture to promote the quality of yoghurt. Two hundred and eighty-nine strains of S. thermophilus, formerly isolated from traditional fermented milk, were screened for favorable fermentation characteristics such as fermentation speed and sensory evaluation. Twelve S. thermophilus strains were screened out by comparing with the fermentation characteristics of S. thermophilus Yc, which was obtained from a commercial starter.Milk were fermented by S. thermophilus and L. bulgaricus ND02with1:1ratio (1×107cfu/g for each) at42℃.The fermented milk sample were stored at4℃when their pH values arrived to4.5.The fermentation time was monitored as well as pH value, titratable acidity, syneresis, free amino nitrogen, living bacterial counts and texture profile under different storage time. The sensory characteristics of these fermented milk samples after stored for1d at4℃were also evaluated. The main conclusions are as follows:(1)10%reconstituted whole milk were fermented by S. thermophilus strains at42℃and the fermentation time was recorded when their pH values arrived to4.5. Sixty three S. thermophilus strains were screened out from two hundred and eighty-nine S. thermophilus strains.(2) Twelve S. thermophilus strains were screened out from sixty three S. thermophilus strains by comparing with the fermentation characteristics of yoghurt with S. thermophilus Yc. The storage characteristics of these fermented milk samples were also evaluated after stored for12h at4℃. As a result, the best pH value, titratable acidity, syneresis value and viscosity of fermented yoghurts were respectively belong to S. thermophilus G81-1, G80-3, D4-7and G80-1(P<0.05) and the pH value, titratable acidity, syneresis values and viscosity of the other single fermented yoghurt groups were no significant different P>0.05).The urease activity of D4-7,G80-1and G80-5strains performance positive urease activity and the other yoghurt groups performance negative urease activity.(3) The selected twelve S. thermophilus strains and L. bulgaricus ND02were combined for milk fermentation with evaluation for their fermentation characteristics, sensory characteristics, storage characteristics and texture profile. Fermented milk by two combination (S. thermophilus C24-3and L. bulgaricus ND02, S.thermophilus S13-4and L.bulgaricus ND02), with higher fermentation speed and sensory scores and lower△pH and△TA (after21d storage) than the S. thermophilus Yc and L.bulgaricus ND02group, the△FAN of these two combination groups were higher than the S. thermophilus Yc and L.bulgaricus ND02group during21d storage period.As for texture, no significance between these combined groups were found. Considering all these, the S. thermophilus C24-3and L. bulgaricus ND02group and S.thermophilus S13-4and L.bulgaricus ND02group had potential to used as starter culture for yoghurt with fast fermentation time, favorable sensory score, stable storage characteristics and texture.