Study On Strain Screening And Xylitol Bioconvertion From Cotton Stalks Hydrolysate

To use the xylose in the hydrolysate of waste cotton stalk as raw materials, a new strain, X-Xol-1 was screened and could convert xylose to xylitol. The condition of xylose reductase expression and characteristic of xylitol fermentation of the strain was investigated. And the detoxification of cotton stalk hydrolysate was studied, and the detoxified hydrolysates were fermented for xylitol production. The results were as follows:Some strains screened from Shihezi area could growth well using xylose as the main carbon resource. From which the strain X-Xol-1 was screened and could effectively convert xylose to xylitol analysed by TLC and HPLC . The strain X-Xol-1 is a member of Candida lusitaniae, which was identified by the colony appearance and the sequence of 18SrDNA, and named Candida lusitaniae X-Xol-1, and got the GenBank accession EU562180.Research on the condition of xylose reductase expression showed that the optimal cultivation parameters for temperature and pH were determined as 35℃and 5.5, respectively; optimal nitrogen resource was yeast extraction, and the concentration of Mg2+ and K+ were 0.25g/L and 1.25g/L, respectively. The Kmxylose of xylose reductase was 0.179 mmol/L and Vmax was 1.705 mmol/(L·min). The KmNADPH was 0.573μmol/L and Vmax was 3.163μmol/(L·min).Investigation of the condition of cultivation and fermentation showed that the optimal parameters for temperature and pH were determined as 35℃and 5.0, and the inoculation volume was 5%, the culture volume was 40%. The shaking speed was 170rpm for biomass accumulation and 120rpm for xylitol converiton. The optimal nitrogen resource was yeast extraction with the concentration 0.5g/L. When the concentration xylose was 30 g/L, the yield of xylitol and volumetric productivity was 69.76% and 0.35 g/(L·h) at 60h, respectively.The effect of glucose on xylitol fermentation showed that glucose restrained the fermentation of xylitol. When there were glucose and xylose in fermentation broth, glucose was utilized firstly. Xylose was going to convert to xylitol only when glucose was consumed. The effect of Ca(OH)2, activated charcoal and activated clay on detoxification of cotton stalk hydrolysate showed that the best result of detoxification was obtained when pH was adjusted to10.0 with Ca(OH)2 and then to 5.5 with H2SO4. The optimal parameters of activated charcoal and activated clay for detoxification were solid-to-liquid ratio 1% and 0.5%, pH 5.5 and 5.5, temperature 70℃and 30℃, contact time 60min and 40min, respectively.To improve the adaptability of the strain to cotton stalk hydrolysate, C. lusitaniae X-Xol-1 was domesticated with hydrolysate as culture medium. It increased the yield of xylitol by 22.47%. When fermentation was taken place with the cotton stalk hydrolysate, xylitol conversion rate was low about 29.60 %. However, the yield could reach 42.51% with concentrated hydrolysate, which was lower than the yield with pure xylose as fermentation substrate.