| Yeast exopolysaccharide is a bioactive substance secreted by yeasts outside the cell during growth and metabolism,and has good application prospects in many fields such as food,pharmaceutical,chemical and biological engineering.In this study,the yeast strains with high yield of extracellular polysaccharides were isolated and screened from nature,and their structures were initially identified.On this basis,the fermentation process was optimized,and the effects of carbon sources and other nutrients on the fermentation process,the structure and biological activity of exopolysaccharides were explored in depth.The main results are as follows:(1)Nine yeast strains were isolated and purified from the traditional fermented food dairy fan by using plate and phenol/sulfate method.The purified strains were analyzed through morphological,physiology characteristics and molecular biology identification of 26S rDNA sequencing.The strains mainly belonged to Rhodotorula mucilaginosa,Candida albicans,Candida zeylanoides and Papiliotrema aurea.Antioxidant activities of EPS from P.aurea DF-12 were evaluated,and the results indicated that the EPS had good scavenging ability on DPPH free radical,hydroxyl free radical and total reducing power.(2)The optimal conditions of the fermentation process of exopolysaccharide produced by P.aurea DF-12 were investigated by single-factor experiments and response surface methodology.The result showed that the optimal fermentation conditions were inoculation amount of 2%,200 r·min-1 pH 6.0 for 7 day with sucrose 50 g·L-1,mixed nitrogen source(yeast extract:ammonium sulfate=1:2)1.8 g·L-1,as the carbon,nitrogen of exopolysaccharide produced by P.aurea DF-12,respectively.The optimal conditions were confirmed by response surface methodology(RSM)as follows:sucrose 52.08 g·L-1,the volume of liquid 27.91 mL,and mixed nitrogen source 1.57 g·L-1,result in an exopolysaccharide yield of 4545.92 mg·L-1.After experimental verification,the actual yield is basically consistent with the theoretical value.(3)Using Logistic,Luedeking-Piret,and Herbert-pirt models to fit the dynamic changes of P.aurea DF-12 fermentation process under different carbon source conditions,and the structural differences of the EPS were studied by high performance liquid chromatography(HPGPC),infrared spectra(IR),scanning electron microscopy(SEM)and gas chromatography(GC).The results showed that when glucose was used as carbon source,the maximum specific growth rate reached 0.15 h-1,which was higher than that of other carbon sources.However,when sucrose was used as carbon source,the maximum specific product formation rate was higher,which was reached 11.95 h-1.cultured with different carbon sources showed remarkable different antioxidant activities.Interestingly,the surface morphology of different EPS components obtained from glucose and sucrose as carbon source was different,but there was no significant difference in molecular weight,functional groups and chemical bonds.Especially,the EPS obtained from different carbon sources were composed of fucose,rhamnose,mannose,glucose,but the molar ratio was different.(4)The effects of different pretreatments on the fermentation of P.aurea DF-12 in molasses and the treatment of extracellular polysaccharides were investigated,such as boiling,activated carbon,CaO,sulfuric acid,phosphoric acid,and calcium phosphate method.The results showed that at the end of fermentation of different molasses media,the production of exopolysaccharides was:CaO method>activated carbon method>phosphate flocculant method>calcium phosphate method>boiling method>phosphoric acid method>sulfuric acid method>no treatment.Among them,the combined CaO method is most beneficial to increase the production of exopolysaccharides. |
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