Study On Yeast Strain Screening And Evaluation Of Xylitol Production From Corn Cob Hemicellulose Hydrolysate

Xylitol, a five-carbon sugar alcohol, can be applied to many fields of the society and has important social and economic value. The xylitol is usually produced by chemical method. With this way, the environments are polluted and the costs are expensive. Xylitol can be deoxidized from xylose by some yeasts and moulds. The fermentative process of producing xylitol is a promising process. There are many kinds of microorganism that can produce xylitol, in which yeast is the best producer of xylitol. Therefore, it's necessary to select microbiology which ferment xylitol effectively for xylitol production and application. The costs of xylitol production are so expensive if xylose is used for fermentation that xylose is unfit for xylitol production. The corn cob is a widespread agricultural residue and contains much carbohydrate. The circumstances will be polluted if the corn cob is threw away or fired. But, the corn cob can be converted to xylose and glucose if it is deal with correctly. To save the costs of production and increase the value of the corn cob, we study the hydrolyze and xylitol production of the corn cob.The research work in this thesis includes :1. The ability of xylitol production of four different yeast strains were compared by determining the cell mass, the residual xylose, the xylitol yields and the activity of the key enzymes. The results showed that C.tropicalis is superiorthan other yeast strains for xylitol production. The concentration of xylitol after three days of fermentation is 9.459 g/L; the relative xylitol yield is 0.473 g per g substrate consumed; and the xylose reductase activity is the best.2. Optimization of the xylitol fermentation by the C.tropicalis. The optimum fermentation conditions in shaking flask were as follows: temperature 28 °C, the rotation rate was 200 rpm> pH 5,culture medium 50 mL in 100 mL flask, inoculun size 10%, the ratio of glucose to xylose, 1:6. Under this condition, the xylitol yield and xylitol concentration were 70.91% and 14.182g/L, respectively.3. 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 120 for 2h using 1.5%(w/v) H2SO4. Then the concentration of sugar is 52.936 g/L. The relative sugar yield is 0.3176 g per g substrate consumed. The hydrolysate was concentrated by vacuum evaporation to 1/2 of its original volume. Then it was treated with NaOH pellets until pH 10.0, filtered and acidified to pH 5.5 with concentrated H2SO4. The precipitate was removed by centrifugation. Xylitol production from corn cob hemicellulose by C.tropicalis was 16.89 g/L.4. In xylitol fermentation of corn cob hemicellulosic hydrolysate, the optimum conditions were as follows: inoculum size 10%, nitrogen source,culture medium 70 mLin 100 mL flask; initial pH 7.5; temperature 28°C,the rotation rate 200 rpm.