Primary Studies On Microbial Conversion Xylose For Xylitol Production

Xylitol is a polyol that has some interesting properties.It has found increasing use in the food industry as sweetener because of its anticariogenic and cariostatic properties. Xylitol is currently manufactured by the chemical reduction of xylose present in hemicellulose hydrolysates.The production xylitol by chemicial methods has been found to be expensive because of difficult separation and purification steps. It has been necessary and worthwhile to explore new methods for the effective production of xylitol using microbial conversion. The microbial conversion may overcome the shortcoming of chemical synthsis method. In order to raise the yield and volumetric productivity of xylitol using microbial conversion,xylitol production by microbial conversion was studies ststemically in the dissertation. This discussion mainly focused on screen the strain for high yield, the basic ingredient of culture medium, the processing parameter of conversion.The studies incolve: Screening the high strain yeast for xylitol production through the thin layer chromatography. The important operational factors, such as nitrogen source and carbon source, dissolved oxygen and pH, metal-salt , affecting the microbial conversion of xylose into xylitol by the yeast was studied. Different sorts of carbon and nitrogen resources,metal ions of inorganic salt were studied in the signle factor experiments. Two ingredients (xylose and glucose) ratio and three ingredients(yeast extract, peptone, KH2PO4)of media were optimized through Respone Surface Analysis. The optimal concentrations of the factors were obtained: xylose 9.8g/L, glucose 9.3 g/L, yeast extract 0.8, peptone 2g/L, KH2PO41g/L.The cultivation conditions of the yeast strain was studied and the optimal conditions were as followed: the inoculation size 10%, a 250mL flask containing 40 mL medium, the speed was 200rpm, the initial pH value was 6. The microbial concersion conditions was studied and the optimal conditions of the yeast cell were as followed: yeast culture time was 12h, a 250mL flask containing 60 mL medium, the speed was 200rpm,Fed-batch was adopted to relieve the inhibit function of high osmotic pressure to microbial cell.Xylose reductases of the yeast cell can keep the more high activity.The yeast cell was immobilized by encapsulated in hydrogel beads prepared with calciumalginate-gelatin.The ability of conversion was as same as the native yeast cell.The conversion was repeated for five times. Using Hansenula anomala as starting strain,mutagenized with UV and NTG. Obtainsed a mutant UN89 which has much higher productive qualities than its parental strain.Its genetic quality was proved to be stable by experiments.