Study On The Xylitol Fermentation From Corn Cob Hemicellulosic Hydrolysate

The research work in this thesis includes: the comparison of the xylitol fermentation strain, strain adaptation, preparation and detoxification of corn cob hemicellulosic hydrolysate and xylitol fermentation from corn cob hemicllucellulosic hydrolysate, etc.It was found that Candida sp. is a good strain for xylitol production. The optimum fermentation conditions in shaking flask were as follows: pH 5.7, xylose 109.09 g/L, culture medium 115 ml in 250 ml flask, inoculum age 3 d, inoculum size 10%; nitrogen source, 3 g/L Bacto-yeast extract and 5 g/L Bacto-tryptone; The rotation rate was 200 rpm in the first 24 h, than changed to 140 rpm for 48 h. Under this condition, the xylitol yield and xylitol concentration were 85% and 92.73 g/L, respectively.The adaptation of Candida sp. strain effectively enhanced its resistance to the inhibitors in corn cob hemicellulose hydrolysate. The xylitol yield was more than fourfold and the cell mass was more than threefold after strain adaptation.The optimum hydrolysate preparation process was: the corn cob with a ratio of 1:6 w/v (dry basis) was hydrolyzed in an autoclave at 107-110 癈 for 3 h using 0.92% (w/v) H2SO4 (prepared with the wash water of filtration residue),. The hydrolysate was concentrated by vacuum evaporation to 1/3 of its original volume. Then it was treated with Ca(OH)2 until pH 10.0, filtered and acidified to pH 7.0 with concentrated H3PO4. The precipitate was removed by centrifugation.Ammonia steeping before acid hydrolyzation could effectively improve xylitol fermentation from the corn cob hydrolysate. However, a part of hemicellulose was lost in this process.In xylitol fermentation of corn cob hemicellulosic hydrolysate, the optimum conditions were as follows: inoculum size, 10%; inoculum age, 22 h; nitrogen source. 1.5 g/L Bacto-yeast extract and 2.5 g/L Bacto-tryptone; culture medium, 120 mL in 250 mL flask; intitial pH value, 7.0; temperature. 30 癈 ; the rotation rate . 200 rpm during the first 0~24 h , then 140 rpm during 24-70 h and 60 rpm during 70-74 h.,The xylitol yield of 67.5%, the xylitol concentration of 74.0 g/L and the xylose utilization efficiency of 80.6% were obtained respectively, under this condition.The xylitol production on hemicellulosic hydrolysate was also studied in a 3 L fermentor. The results showed that the culture prophase 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 yield of 51.4%, the xylitol concentration of 60.3 g/L and the xylose utilization efficiency of 72.1% were obtained respectively.The research work of this thesis developed an effective and economical xylitol fermentation process using com cob hemicellulosic hydrolysate in stead of xylose. The technology is of important social value and apparent economic value.