Screening Of Productive Exopolysaccharide Strains Of Lactic Acid Bacteria And Application In Yogurt Production

Yogurt manufacturers are often faced with fracture of the gel, low-viscosity and wheying-off. Using polymer producing strains of lactic acid bacteria can reduce considerably the syneresis of yogurt.The appearance of EPS is beneficial to the rheological and texture properties of yogurt. In this study one high ropy strain was selected from 16 lactobacillus, then the mutation techniques improved in order to modify its producing properties and to make yogurt, lactose utilization ratio, wheying off, viscosity and acceptability of the ultimate products were investigated systematically.Six lactobacillus preserved in the laboratory and ten isolated from kefir grains were fermented in the optimum conditions. TCA and ethanol were used to precipate protein and EPS respectively. Total sugar concentration was determined by the phenol-sulfuric method using glucose as a standard. EPS quantity and cultured broth viscosity were parameters, strain 7 was selected and EPS was 265.93mg/L.Strain 7-12 was used as original strain through primary selection on plate. U\^ DES and NTG were choosed to continue to mutate them for 6 generations respectively. Productive EPS 7F379 was obtained by primary and repeated selection from approximately 2000 colonies and productivity 581.5 mg/L, increased by 129.4% over the original strain. After continued 6 generations , EPS productivity became unstable. It showed NTG is the best mutagenic , DES is better and UV is worse. The optimum dose of single mutagenic is UV for 3min, NTG 300ug/mL for 40min, DES (0.2M) 1 L / L cultured broth.Both polymer and non-polymer forming starter were used in the yogurt.Cultured broth was digested for 16h at pH 7.5, 30 with Pronase, precipitated with 95% ethanol, centrifugation (12000*g, 20min)and dialyzation, thus get the crude EPS, HPLC was choosed to determine the EPS composition in the cultured broth. It showed that lactose utilization ratio and galactose synthesis ratio of mutated 7F379 strain were faster than others. After 24h of incubation.about half of the lactose was used. Galactose concentrations increased during the incubation cycle even after 10h of incubation. EPS composition fermented with strain 7 and Streptococcus thermophilus together was stable and consists of glucose, galactose and rhamnose, but EPS with normal starters of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus does not consist of rhamnose. The yogurt fermented with mutanted ropy strain and normal starters was little wheying-off. Fermented milk with ropystrain was greatly accepted through organoleptic evaluation.