A Study On Xylitol Fermentation From Commercial Xylose And Hydrolysate Of Rice Straw By Candida Tropicalis As2.1776

Xylitol production from commercial xylose and hydrolysate of rice straw by Candida tropicalis As2.1776is investigated.In the fermentation of commercial xylose, the influence of culture conditions, including initial xylose concentration, aeration, nitrogen source and nutritious salts on fermentation was evaluated. On the base of this, the fermentation medium was optimized by use of Response Surface Analysis.Calcium alginate entrapped yeast cells were also used to ferment the optimized medium to produce xylitol. Because of the presence of some components causing Ca2+to fall off in the synthetic medium, the cell beads could be used just for one fermentation period, and dissolved into the medium when inoculated for the second time. By addition of calcium chlorine to the medium, the life time of the beads was prolonged greatly and the xylitol yield was increased as well. But compared with the fermentation by free cells, the immobilized cells obtained lower xylitol yield and productivity. In the fed-batch fermentation by immobilized cells,243g/L of xylitol was accumulated.Two operations were used to enhance the fermentability of rice straw hydrolysate. One was ammonia pretreatment prior to rice straw hydrolysis, which resulted in66.1%lignin was removed. In the resulted hydrolysate, the quantity of phenolics compounds decreased greatly, and acetic acid was absent. The other operation was to detoxify the hydrolysate directly. Among several steps, Ca(OH)2overneutrilization combined with S-8macroporous resins or activated charcoal adsorption was selected as the detoxification routes. As a result, Ca(OH)2overneutrilization combined with S-8macroporous resin adsorption removed monomeric89.7%phenolics,82.7%polymeric phenolics,57.1%furfural and18.9%acetic acid, respectively. The corresponding result for Ca(OH)2overneutrilization combined with activated charcoal was96.1%,94.0%,65.7%and11.2%, respectively.The fermentations of hydrolysates resulted from the obove operations were carried out. Supplement of nutritions to hydrolysate increased the xylitol volumetric productivity. The initial pH of hydrolysate-based medium of ammonia steeped rice straw should be adjusted to4-5, while6-7for those detoxified by S-8macroporous resin and activated charcoal.1.22g/L of intial cell concentration (dry weight) was feasible for fermentation. Under the feasible fermentation conditions of each, the hydrolysate of ammonia steeped rice sraw showed the best fermentability and resulted in highest xylitol yield and volumetric productivity. The hydrolysate detoxified by S-8macroporous resins and activated charcoal showed the similar fermentation results.Highly concentrated hydrolysates of ammonia steeped rice sraw, with99.2g/1of initial xylose, were fermented by free cells in flasks and in21bioreator. In flasks,0.746g/g of xylitol yield and0.686g/1.h of volumetric productivity were obtained. In bioreactor, the corresponding values were0.689g/g and0.697g/(1.h), respectively. Highly concentrated hydrolysates of ammonia steeped rice sraw, with104.2g/L of initial xylose, were fermented by immobilized cells in flasks. The immobilized cells could be used repeatedly for5batches, aquiring0.737g/g of xylitol yield and0.536g/(L.h) of volumetric productivity, repectively.