Exopolysaccharides By Lactic Acid Bacteria And Their Contribution To Fermented Milk

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.