Process Optimization Of Xylitol Fermentation By Xylose Using Candida Tropicalis W103

Corncob is a kind of cheap renewable lignocellulosic resource.The bioconversion of hemicellulosic hydrolysate to xylitol by microorganisms could be a good alternative to the traditional chemical process, bacause it is a simple process, with great specificity and low energy requirements.In this study, Candida tropicalis W103 was used as yeast strain to produce xylitol by xylose fermentation,the effects of fermentation operation parameters on fermentation process and xylitol synthesize were discussed,such as the culture time of inoculum,the cuture method of inoculum, inoculum size, initial pH value,the culture media and so on. The results showed that both the biomass and content of xylitol were maximum when xylitol was used as a carbon source.The optimum fermentation conditions in shaking flask were as follows:inoculum size,10% ; inoculum age,18h ; xylose 40g/L,nitrogen source,(NH4)2SO4 10g/L,3g/L Bacto-yeast extract and 5g/L Bacto-tryptone;Mg2+ and Ca2+ were added to culture medium;initial pH value 4.5;temperature was 35℃.Under this condition,the xylitol yield and xylitol productivity were 72%and 1.13g/(Lh).Optimized the fermentation operation parameters of Candida tropicalis W103 in 1L fermentor,the result showed that both the biomass and content of xylitol had improved when ventilation was 0.6vvm,pH=4.5,stirrer speed was 550r/min and adding a certain of xylose during the middle period of logarithmic growth.Under batch fermentation,the maximum xylitol concentration of 52.47g/L,the xylose utilization efficiency of 90.42%,the xylitol yield of 74% and the xylitol productivity of 1.22g/(Lh) were obtained respectively.The xylitol production was also studied in a 1L fermentor with fed-batch culture.The results showed that 0h-24h was the cell growth stage.It was helpful to the cell mass accumulation by increasing aeration and stirring rate.The culture anaphase was the main production stage of xylitol.Appropriate decrease of aeration and stirring rate could effectively improve fermentation efficiency.Under optimal conditions,the xylitol concentration of 194.10g/L,the xylitol yield of 70% and the xylitol productivity of 1.62g/(Lh) were obtained respectively.The reaearch work also includes the preparation,detoxification of corncob hemicellulosic hydrolysate,and xylitol fermentation from corncob hemicellulosic hydrolysate.The optimum hydrolysate preparation process is:the corncob with a ratio of 1:10 w/v,and hydrolyzed in an autoclave at 120℃for 1.5h using 1% H2SO4. Then the detoxification of hydrolysate was used as fermentation substrate.The congcentration of xylitol was 60.56g/L in the fermentation liquid. The xylitol yield and xylitol productivity were 67% and 0.88g/(Lh).The total polysaccharide utilization concentration was 74g/L in the fermentation process.This process can effectively reduce the pretreatment of costs,showing broad prospects of applications.The research work developed an environmental-friendly and economical xylitol production by fermentation on corncob hemicellulosic hydrolysate.This thesis developed an effective and economical xylitol fermentation process using corncob henmicellulosic hydrolysate in stead of xylose.The technology is of important social value and economic value.