产胞外多糖的嗜热链球菌ST冷冻干燥制剂的研究

从当地市售的传统酸奶中分离出一株产胞外多糖的链球菌，经生理生化和16S rDNA序列分析，将该菌株确定为嗜热链球菌ST。在补充了20g·L-1的葡萄糖，初始pH7.0的培养基上，40℃，培养24h后，其胞外多糖产量为55.62mg·L-1。进一步的结构分析显示，此多糖的主要单糖组成为葡萄糖。在生产嗜热链球菌ST冷冻干燥制剂时，其最佳的增殖培养基组成为脱脂奶粉100.0g·L-1，酵母膏3.0g·L-1，碳酸钙9.0g·L-1和乳清粉20.0g·L-1，活菌数达到1.05×109CFU·mL-1。由脱脂奶粉160.0g·L-1、甘油30.0g·L-1谷氨酸钠30.0g·L-1和5.0g·L-1吐温80构成的保护剂能明显改善菌体细胞在冷冻干燥条件下的存活能力。在发酵温度37℃、搅拌转速90r·min-1和pH5.9的条件下培养，经冷冻干燥获得的直投式发酵剂产品的活菌数量可达1.7×1011CFU·g-1。当采用固形物仅为80.0g·L-1的还原乳做为原料时，与不产胞外多糖的菌株生产的酸乳相比，采用该产品制备成的产品乳清析较少。

Exopolysaccharide production and lyophilization preparation from Streptococcus thermophilus ST

ST strain, isolated from yogurt purchased from a local market and identified as Streptococcus thermophilus based on physiological-biochemical characteristics and 16S rDNA sequence, produced 55.62mg·L-1 of exopolysaccharides (EPS). The EPS level produced by strain ST depended upon incubation time (24h), temperature (40℃), initial pH (7.0) of the medium and supplemented glucose (20g·L-1). The major monosaccharide of EPS formed by this strain was confirmed to be glucose. Moreover, data from the lyophilization preparation of strain ST showed that the optimal medium containing NFMS (100.0g·L-1), yeast extract (3.0g·L-1), CaCO3(9.0g·L-1) and whey powder (20.0g·L-1) promoted its viable cells to a level of 1.05×109CFU·mL-1. The combination of NFMS (160.0g·L-1), glycerol (30.0g·L-1), sodium glutamate (30.0g·L-1) and Tween 80 (5.0g·L-1) was proved to be a good cytoprotectant for the protection of strain ST against the stresses of freeze-drying. Temperature, pH and rotation speed were important factors for gaining high viable counts in pilot-plant production of strain ST lyophilizator. The optimal combination including fermentation temperature (37℃), rotation speed (90r·min-1) and pH (5.9), resulted in a direct-vat-inoculation preparation of strain ST that contained viable cells of 1.7×1011CFU·g-1 after lyophilization. Use of lyophilized strain ST preparation to directly ferment 80.0g·L-1 reconstituted skim milk to yogurt produced lower whey separation than that of yogurt made with non-EPS strains used as controls.