Xylitol Production from D ‐Xylose at Different Oxygen Transfer Coefficients in a Batch Bioreactor

Conversion of D ‐xylose to xylitol by Candida boidinii NRRL Y‐17213 was studied under anaerobic and oxygen limited conditions by varying the oxygen transfer coefficient k L a . Shake flask experiments were used to provide the preliminary information required to perform experiments in a bioreactor. The yeast did not grow under fully anaerobic conditions, but anaerobic formations of xylitol, ethanol, ribitol, and glycerol were observed as well as D ‐xylose assimilation of 11 %. In shake flasks, with an initial D ‐xylose concentration of 50 g/L, an increase in k L a from 8 to 46 h –1 resulted in a faster growth, higher rate of substrate uptake and lower yields of products. The highest xylitol productivity (0.052 g/L h) was attained at k L a  = 8 h –1 . At k L a  = 46 h –1 , 98.6 % of D ‐xylose was consumed and mainly converted to biomass. Using 130 g/L D ‐xylose, k L a was varied in the fermenter from 26 to 78 h –1 . The percentage of consumed D ‐xylose increased from 31 % at k L a  = 26 h –1 to 93–94 % at all other aeration levels. Biomass yield increased with k L a , whereas ethanol, ribitol, and glycerol yields exhibited an opposite dependence on the oxygenation level. The most favorable oxygen transfer coefficient for xylitol formation, in the fermenter, was k L a  = 47 h –1 when its concentration (57.5 g/L) surpassed ethanol accumulation by 3.6‐fold, and the glycerol plus ribitol by 10‐fold. Concurrently, xylitol yield and productivity reached 0.45 g/g and 0.26 g/L h, respectively. The volumetric xylitol productivity was affected more by changes in the aeration than the corresponding yield.