초록
Xylitol is an important commercial sweetener that can be produced by fermentation. Previous studies of xylitol production have not been able to combine high average productivity and yield in a single process. Benzoate is a highly chaotropic stressor and has been found to stimulate the fermentation metabolism of yeasts at low concentrations. Therefore, in the current work, it was hypothesized that benzoate increases xylitol production, because it is a concurrent kosmotropic/compatible solute under aerobic conditions. Shake flask experiments without control of dissolved oxygen revealed that sodium benzoate added at a concentration of up to 150ppm stimulated the fermentation of xylose to xylitol by Candida mogii. Sodium benzoate at a concentration of >200ppm had a clear inhibitory effect on the xylose metabolism. Controlled batch fermentations carried out in bioreactors with and without sodium benzoate (150ppm) were used to further assess its potential for improving the xylitol production. Under highly aerobic conditions (dissolved oxygen concentration >75% of air saturation), the presence of sodium benzoate (150ppm) increased both specific xylitol productivity and yield. The specific xylitol productivity increased by >2-fold and the yield increased by ~30%, relative to control. However, benzoate increased the xylitol yield only slightly under oxygen limiting conditions. The volumetric xylitol productivity previously obtained for most potential strains was highest under microaerobic conditions. However, in the current study, the xylose consumption was remarkably enhanced under aerobic conditions in the presence of sodium benzoate, which makes an increase in the volumetric productivity of xylitol feasible for industrial applications. This method is readily applicable to previously developed xylitol processes by simply adding a suitable amount of sodium benzoate. The findings of this study devise new interventions for microbial processes in industrial reactors to expand the microbial tolerance of chaotropic stressors and, hence, the biotic windows for such processes.