Production of highly concentrated xylitol by Candida magnoliae under a microaerobic condition maintained by simple fuzzy control

Microbial production of xylitol from xylose was investigated using Candida magnoliae. In particular, the effect of the oxygenation condition on the xy litol production yield was examined and the significance of maintaining a m icroaerobic condition was demonstrated. A simple system of fuzzy logic cont rol (FLC) was devised to maintain the microaerobic condition in the xylitol production phase by regulating the proportion of air (air flow rate) suppl ied to the fermenter. The input variables to the fuzzy control system were the dissolved oxygen (DO) concentration in the culture broth and the CO2 co ncentration in the exit gas. A batch cultivation test using the FLC system confirmed the importance of maintaining a constant microaerobic condition t hroughout the xylitol production phase, and indicated it would be advantage ous for this phase to be prolonged. An intermittent fed-batch culture was t herefore carried out. The FLC system allowed a constant microaerobic condit ion to be maintained, resulting in minimal cell mass production and constan t xylitol accumulation in the culture medium. As a consequence, a very high xylitol concentration of 356 g/dm(3) could be attained. The xylitol yield in the fed-batch culture was 0.75, which corresponded to 82% of the theoret ical yield.