中文摘要：

直投式酸奶发酵剂因活性高、稳定性好、使用简便,已成为酸奶发酵剂的主要发展方向。嗜热链球菌（Streptococcus thermophilus,St）和保加利亚乳杆菌（Lactobacillus bulgaricus,Lb）经活化和纯化后,以存活率为指标,对混合发酵液的菌种混合比例、离心条件、预冻条件进行单因素试验。试验结果表明混合发酵液的最佳制备条件为：St∶Lb=1∶1（体积比）、离心速度3 000 r/min、离心时间10 min、预冻温度-80℃、预冻时间1 h;采用响应面法对冻干菌悬液的保护剂p H、保护剂和菌泥平衡时间、保护剂和菌泥混合比例进行优化试验,结果表明冻干菌悬液最佳制备条件为：保护剂p H6.6、平衡时间30 min、保护剂和菌泥混合比例2∶1（m L/g）,此时St存活率63.77%、Lb存活率66.03%,具有较好的市场前景和应用价值。

英文摘要：

Direct vat set（DVS） has become the main development direction of yogurt fermentation agent due to its high activity, good stability and easy to use. With the survival rate as the index, the single factor test of mix-ing ratio, centrifugal condition and pre-freezing condition were conducted after the activation and purification of Streptococcus thermophilus and Lactobacillus bulgaricus. The results showed that the optimizing fermentation conditions were：St：Lb=1：1 （volume ratio）, the centrifugal speed of 3000 r/min, centrifugation time of 10 min, pre-freezing temperature of-80℃, pre-freezing time of 1h;The pH of protective agent, the protec-tive agent, the bacterial mud balance time, the protective agent and the mixed proportion of the bacterial mire were optimized by response surface method and the results showed that the optimizing conditions of freeze dried suspension were：protective agent pH of 6.6, equilibration time of 30min, the mixing ratio of protective agent and bacterial mire of 2：1 （mL/g）. Under the optimized conditions, survival rates of St＆amp;Lb were 63.77%and 66.03%, respectively, which indicated a good market foreground and application value.