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 kLa. 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 kLa 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 kLa = 8 h–1. At kLa = 46 h–1, 98.6 % of D‐xylose was consumed and mainly converted to biomass. Using 130 g/L D‐xylose, kLa was varied in the fermenter from 26 to 78 h–1. The percentage of consumed D‐xylose increased from 31 % at kLa = 26 h–1 to 93–94 % at all other aeration levels. Biomass yield increased with kLa, 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 kLa = 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. Over the last decade, considerable efforts have been focused on the microbial production of xylitol from D‐xylose derived mainly from wood hydrolysates. The ultimate goal of all these investigations related to the model system of D‐xylose fermentation was to acquire knowledge for establishing an efficient process for xylitol production using the pentose fraction of the lignocellulosic hydrolysates, instead of pure D‐xylose.