Screening Of Xylitol-Producing Strains And Optimization Of Fermentation Conditions

Xylitol, a five-carbon sugar alcohol, occurs widely in nature such as fruits，vegetables and mushrooms and it is also an intermediate metabolite in the humancarbohydrate metabolism. The human body produces5-15g of xylitol a day duringnormal metabolism. Xylitol is one product that has attracted attention in recentyears because of its physic-chemical properties, including high sweeteningpower, low calorie and anticariogenic properties. At present, the industrialproduction for xylitol is based on xylose dehydrogenation in a nickel-catalysedprocess which cost higher and dangerous. As an alternative method, microbialproduction of xylitol is becoming more interesting and attractive which has theadvantages of low cost and mild process conditions. At present, the industrializationprocess of xylitol production by microbial fermentation technology has beenrestricted due to the lack of strains which have the ability of high-efficiencybiotransformation of xylose. Therefore, Screening excellent strains and optimizationof the fermentation process still are the focuses of the biotransformation of xylitolproduction.A yeast strain was isolated from soil samples in our laboratory, which couldtolerate high concentrations of xylose. The main product of strain Y-9was xylitolanalysed by HPLC and the infrared scan.According to the morphologicalcharacteristics, physiological and biochemical tests and26SrDNA sequencing of thestrain, the purpose of strains was initially identified as Candida tropicalis, namedCandida tropicalis Y-9.The culture medium and the fermentation condition of Candida tropicalis Y-9producing xylitol were optimized by single factor experiment. Based on the onevariable at a time experiment of fermentation medium, three mainsignificant factors,ammonium sulfate, yeast extract and zinc sulfate were screened by thePlacket-Burman design. Then, the optimal response surface area was approached bythe steepest ascent experiment. Finally, response surface analysis was performedbased on the principles of Box-Benhnken central composite design to obtain theoptimum concentrations of the three factors. The best medium consisted of200g/Lof xylose,4.3g/L of yeast extract,10mL/L of corn steep liquor,3.0g/L of(NH4)2SO4,0.1g/L of ZnSO4, and0.2g/L of MgSO4. The optimum fermentation condition was temperature30℃, initial pH6, rotation speed180r/min, inoculumsize4%, broth’s volume in shake flask40mL/250mL. Under optimum conditions，after96hours, the productivity reached to80%and the yield of xylitol was about163g/L.Based on the optimum flask shaking batch fermentation condition，the batchfermentation was investigated with strain Candida tropicalis Y-9in5liter fermentor.The fermentor conditions: medium volume3.5L, initial pH6.0, temperature30℃,the air pressure0.05MPa. Through the study of two-stage fermentation conditionswe finalized the rotation speed was500r/min before18h,300r/min after18h andthe fermentation period was96h.Finally the yield of xylitol reached to165g/L andthe conversion rate was above80%. The xylitol batch fermentation kinetics wasstudied based on the experimental data from the batch fermentation of5-literfermentor．The kinetic models of cell growth, product formation and substrateconsumption were constructed which could reflect the regularity of the process ofbatch fermentation production of xylitol.