EXOPOLYSACCHARIDE PRODUCTION BY SELECTED LACTIC ACID BACTERIA ISOLATED FROM FERMENTED VEGETABLES

Lactic acid bacteria (LAB) play a key role in the food fermentation process since they contribute to the texture, flavor, quality and conservation of the fermented products. Several LAB strains have been shown to produce exopolysaccharides (EPS), with potential applications in food industry, since they can act as natural thickeners that improve the texture of the final product, decrease syneresis and reduce the fat levels in fermented foods. In situ production of EPS by LAB to get a desired texture and mouthfeel of some fermented products is being explored, in order to replace polysaccharides from plants or animals, currently in use. Moreover, it has been suggested that some EPS produced by LAB have prebiotic activity, contributing to the promotion of human gastrointestinal health. During this study, five new EPS-producing LAB strains have been selected from 21 strains isolated from fermented vegetables. The mucoidness/ropiness of the colonies developed on MRS agar media containing different carbon sources (glucose, sucrose, fructose, galactose, lactose, or xylose) was firstly observed. EPS presence in the culture supernatant was detected through gel permeation chromatography (GPC). The EPS material was isolated from these strains by acetone precipitation, then dialysed, dried and weighted. The molecular mass was estimated by the same GPC method, while the monomer composition was determined by automated thin layer chromatography (TLC), after hydrolysis with 8N HCl. One of the positive strain, Leuconostoc mesenteroides/pseudomesenteroides 406 has been shown to produce large amounts of EPS, of about 15 g/L and two strains, Leuconostoc citreum/lactis/garlicum 167 and Leuconostoc sp. 208 were able to produce around 6 g/L of EPS. All isolated EPS have a high molecular mass, of above 1400 KDa, and a monomer composition dominated by the presence of glucose. The influence of the growth medium composition and incubation temperature on the EPS biosynthesis was also investigated. Three LAB strains, that were shown to produce high amounts of EPS, have been selected to be used in this study. It was shown that some mild stress conditions might stimulate, in some cases, the EPS-production.