【作者】 马静；

【导师】 张柏林；

【作者基本信息】 河北农业大学 ， 农产品加工及贮藏工程， 2004， 硕士

【摘要】 乳酸菌胞外多糖（LAB EPS）是指乳酸菌在其生长、代谢过程中分泌到细胞外的粘液或荚膜多糖。它的成分和结构非常庞杂，可分为同源多糖和异源多糖。在乳制品行业中，乳酸菌胞外多糖可作为增稠剂、稳定剂、乳化剂、胶凝剂、填充剂和持水剂应用。目前，有关胞外多糖的生理作用尚不完全清楚，通常认为胞外多糖的主要生理功能是防护作用，包括维持适度的水分、吸取金属离子、抑制溶菌酶以及防御噬菌体的吞噬等作用。另外，荚膜多糖还具有一定的抗肿瘤作用。本项研究旨在探索乳酸菌产胞外多糖的机理，优化其胞外多糖的发酵条件，初步应用乳酸菌产胞外多糖菌株到不同固体物含量的发酵乳生产中，为乳酸菌胞外多糖的研究和开发工作提供参考依据。 研究了嗜热链球菌ST、MY800、MYE和保加利亚乳杆菌Y-5四株乳酸菌随时间产胞外多糖的情况，结果表明，37℃培养24h时，四株乳酸菌胞外多糖的产量最高。碳源是影响乳酸菌胞外多糖产量的显著因素，嗜热链球菌ST、MY-800、MYE和保加利亚乳杆菌Y-5分别以葡萄糖、乳糖、半乳糖、蔗糖和果糖作为碳源时，ST、MYE和Y-5菌株的最佳碳源为果糖，MY-800菌株的最佳碳源为蔗糖；在同一碳源条件下不同乳酸菌菌株其胞外多糖的产量相差较大。氮源试验表明，嗜热链球菌ST、MY-800、MYE和保加利亚乳杆菌Y-5分别以胰蛋白胨和鱼蛋白胨作为氮源时，ST、MY-800、MYE菌株的最佳氮源是胰蛋白胨，Y-5菌株在以鱼蛋白胨作为氮源时胞外多糖的产量较高。碳氮比试验表明，嗜热链球菌ST、MY-800、MYE和保加利亚乳杆菌Y-5在碳氮比分别为1：1、1：2、2：1、2：3的情况下，ST和MY-800菌株的最适碳氮比为1：2，MYE菌株的最适碳氮比为2：3，而Y-5菌株在碳氮比2：1时胞外多糖的产量最高。 应用正交试验确定了四株乳酸菌产胞外多糖的最佳条件。ST菌株胞外多糖产量的最佳条件为：培养温度为40℃，发酵时间为24h，初始pH值为7.0，葡萄糖添加量为2％；MY-800菌株胞外多糖产量的最佳条件为：培养温度为40℃，发酵时间为16h，初始pH值为6.0，葡萄糖添加量为6％；MYE菌株胞外多糖产量的最佳条件为：培养时间为24h，培养温度为40℃，初始pH值为6.0，葡萄糖添加量为2％；Y-5菌株胞外多糖产量的最佳条件为：培养温度为40℃，发酵时间为16h，初始pH值为6.0，葡萄糖添加量为2％。 金属离子对乳酸菌胞外多糖水解酶的影响试验表明，K⁺和Mg²⁺对胞外多糖水解酶的活性有促进作用，可以降低乳酸菌胞外多糖的产量；而Cu²⁺和Mn²⁺这两种金属离子对胞外多糖水解酶的活性有强烈的抑制作用，可以提高乳酸菌胞外多糖的产量。 研究表明，乳酸菌胞外多糖具有较好的持水能力，以6%固体物含量的乳进行发酵，仍能生产出质地状态较好的酸奶，并长时间维持发酵乳高的活菌数和有效抑制后酸化。更多还原

【Abstract】 Exopolysaccharides (EPS) by lactic acid bacteria (LAB), excreted in growth medium or attached to bacterial cell wall, are divided into homopolysaccharide and heteropolysaccharide due to complex composition and structure. EPS by LAB can serve as viscosifying, stabilizing, emulsifying, gelling, or water-binding agents in food, pharmaceutical and petrochemical industries. Now the physiological function of EPS is under study. The purposes of this study were to: (1) examine the ability of LAB strains to produce EPS; (2) optimize the growing process of 4 EPS-producing lactic bacteria for the maximum production of EPS; (3) deal with the factors affecting the production of LAB EPS; and (4) add screened EPS-forming strains to pasteurized milk and evaluate their effect in improving the viscosity of fermented milk.The formation of EPS by 4 LAB strains reached the highest after incubation at 37C for 24h in MRS or M17 medium. LAB strains studied demanded carbohydrates to produce EPS, but there was a big variation in the utilization of sugar resources including glucose, lactose, galactose, saccharose, or fructose. Fructose was found to be the optimal carbon resource for the formation of EPS by Streptococcus thermophilus strains ST and MYE as well as Lactobacillus delbrueckii subsp. bulgaricus strain Y-5. When saccharose was used as carbon resource, the EPS-producing strain of S. thermophilus MY-800 formed the largest EPS than other strains did.Nitrogen resource was of importance in affecting the formation of LAB EPS. As far as tryptone or polypeptdn was concerned, tryptone might the best nitrogen resource for strains ST, MY-800 and MYE to produce EPS. Polypeptdn might be the best nitrogen resource for strain Y-5 to form EPS;The ratio of carbon to nitrogen resources was an important factor in determining the production of LAB EPS. Such ratios of carbon to nitrogen as 1:1, 1:2, 2:1 or 2:3 were examined to evaluate the ability of LAB strains to form EPS. It was seen that when the ration of carbon to nitrogen was 1:2, the strains ST and MY-800 formed thehighest level of EPS than other strains did. 2:3 of carbon to nitrogen might be the best for strain MYE to produce EPS, and 2:1 the best for strain Y-5.Orthogonal test with 4 factors by 3 levels was used to deal with the best conditions for EPS production by four EPS-producing strains. An environment that the medium with initial pH of 7.0 contained 2% glucose and fermentation time lasted for 24h at 40C favored the largest EPS production by strain ST. Strain MY-800 required the following conditions to form the most EPS, that is, incubation of 40C for 16h, medium containing 6% glucose and an initial pH of 6.0. Such environment as incubation of 40C for 24 h, medium containing 2% glucose and an initial pH of 6.0 would help strain MYE produce the most EPS. The conditions like incubation temperature of 40C, fermentation time of 16h, initial pH of 6.0 in medium containing 2% glucose supported the largest formation of EPS by lactobacillus strain Y-5.Metal cations affected the final EPS production due to disturbance of glycohydrolase. It was seen that K+ and Mg2+ awoke the activity of glycohydrolase, reducing the EPS production by LAB strains. In contrast, Cu2+ and Mn2+ strongly depressed the enzyme glycohydrolase, improving the production of LAB EPS.Several studies showed that LAB EPS had good abilities to bind water and subsequently promote the consistence of fermented milks. Additions of EPS-producing strains to the total solid of skim milk that was reduced to 6% produced very thickening and good viscosity fermented milk. Clearly, good EPS-producing lactic bacteria promoted the texture of fermented milk in the presence of low total solids like 6%. Furthermore, formation of EPS protected the live cells of lactic acid bacteria from destroying by post-acidification in fermented milk, producing a good viability of LAB cells.更多还原