[Screening and identification of exopolysaccharide-producing yeasts]

Most of microbial extracellular polysaccharide (EPS) has the favorable functionality and peculiarity, but little research has been done about EPS that were synthesized by yeasts. Screening of EPS-producing yeasts and studies on the optimal culture composition are the main purpose of this study.

Yeasts were obtained by screening from natural samples using spread-plate method, Yield of EPS synthesized by yeasts was estimated by phenolsulfate acid method, Based on 5.8S rDNA sequence determination, three strains were identified and selected for the optimization of fermentative medium.

132 yeast strains were isolated from grapes, preserves and topsoil. Three yeast strains were proved to be as high exopolysaccharide-producing yeasts. The results of 5.8S rDNA sequence determination showed that similarities of two yeasts isolated from "Zuoyouhong" grape (Z14 and Z20) were over 99% compared with Issatchenkia orientalis, meanwhile another yeast L25 isolated from topsoil of Larix gmelini was identified as Cryptococcus humicolus, with the similarity of 98.8%. The optimal medium for strain Z20 for producing EPS was as followed: 8% glucose, 0.2% (NH4)SO4, 0.1% KH2PO4, 0.1% yeast extract powder and 0.01% CaCl2. Under the condition of initial pH 6.0, 28 degrees C and 160 r/min for 4 d, yield of EPS could reach to 2.046 g/L, which was 79.9% higher than the control (1.137 g/L).

Reports that yeasts belong to some genera possessed the abilities to synthesize EPS could be seen in some related references, according to the study, we could get the conclusion that yeasts belong to Issatchenkia can synthesize EPS and EPS yield of stain Z20 could be increased by changing culture composition.