Study On Xylitol Fermentation Using Microencapsulated Cells

Microcapsule technology, as a new cell-immobilizing method, was employed to embed Candida mogii and applied in the xylitol fermentation.The preparation process of chitosan-alginate(ACA) microcapsule was studied firstly. 0.6% chitosan solution (dissolved in 1% HAc, pH 5.5) and a simple droplet generator were employed to form chitosan-alginate microcapsules with diameter of about 2.5mm and mechanical strength of about 13 g/L. The best reaction time of the membrane construction of microcapsules was found to be 15mm, and the best reaction time of the liquefying of microcapsules to be 30mm. Some possible improvement methods on the mechanical strength were also studied, and it was found that surface decoration with glutaraldehyde could not enhance the membrane strength and multi-membrane reduced the membrane strength.The effect of pH value on the membrane strength was found light, and pH 5 was chosen since it was the best pH of fermentation. The permeability of microcapsules was researched, too. It was found surprisingly that the permeability of xylitol (the proportion between intralextern-capsule concentration of xylitol became 1:1 in 60mm) was better than that of xylose(the proportion between intralextern-microcapsule concentration of xylose became 0.34:1 in 150mm).ACA microcapsule was applied in xylitol batch fermentation. The microcapsules were used repeatedly in four batches. It was found that the fermentation with microcapsules had better and fixed conversion (65%), comparing with free-cells fermentation (changed from 63% to 50%) and Ca-algmate immobilizing fermentation (changed from 38% to 59%), while the end time of microcapsule fermentation was enhanced and fixed to SOhr.By the theoretic analysis of the fermentation mechanism, the reason that led to the differences between microcapsule fermentation and Ca-alginate fermentation was explained as the oxygen-transfer, and it was better oxygen-transfer ability that caused the better efficiency of microcapsules fermentation.ACA microcapsules were finally applied in high cell-concentration fermentation. Firstly, by means of Uniform Design and stepwise regression, the culture medium of high cells-concentration was optimized to be: yeast extracts 6.8g/L, with a better conversion of 88%. Then high cells-concentration fermentation based on ACA microcapsules was conducted. The results showed a steady conversion of 60~70% and that the production process of xylitol approximated to a beeline, which means the fermentation could be controlled easily.