Oxygen uptake rate in production of xylitol by Candida guilliermondii with different aeration rates and initial xylose concentrations.

The global oxygen uptake rate (OUR) and specific oxygen uptake rates (SOUR) were determined for different values of the volumetric oxygen mass transfer coefficient (15, 43, and 108 h(-1)), and for varying initial xylose concentrations (50, 100, 150, and 200 g/L) in shaking flasks. The initial cell concentration was 4.0 g/L, and there was only significant growth in the fermentation with the highest oxygen availability. In this condition, OUR increased proportionally to cell growth, reaching maximum values from 2.1 to 2.5 g of O2/(L x h) in the stationary phase when the initial substrate concentration was raised from 50 to 200 g/L, respectively. SOUR showed different behavior, growing to a maximum value coinciding with the beginning of the exponential growth phase, after which point it decreased. The maximum SOUR values varied from 265 to 370 mg of O2/(g of cell x h), indicating the interdependence of this parameter and the substrate concentration. Although the volumetric productivity dropped slightly from 1.55 to 1.18 g of xylitol/(L x h), the strain producing capacity (Y(P/X)) rose from 9 to 20.6 g/g when the initial substrate concentration was increased from 50 to 200 g/L. As for the xylitol yield over xylose consumed (Y(P/S)), there was no significant variation, resulting in a mean value of 0.76 g/g. The results are of interest in establishing a strategy for controlling the dynamic oxygen supply to maximize volumetric productivity.

Influence of oxygen availability on cell growth and xylitol production by Candida guilliermondii.

Oxygen availability is the most important environmental parameter in the production of xylitol by yeasts, directly affecting yields and volumetric productivity. This work evaluated the cell behavior in fermentations carried out with different dissolved oxygen concentrations (0.5-30.0% of saturation), as well as a limited oxygen restriction (0% of saturation), at several oxygen volumetric transfer coefficients (12 < or = kLa < or = 70 h(-1)). These experiments allowed us to establish the specific oxygen uptake rate limits to ensure high yields and volumetric productivity. When oxygen availability was limited, the specific oxygen uptake rate values were between 12 and 26 mg of O2/of g cell x h, resulting in a yield of 0.71 g of xylitol/xylose consumed, and 0.85 g/[L x h] for the volumetric productivity. According to the results, the effective control of the specific oxygen uptake rate makes it possible to establish complete control over this fermentative process, for both cell growth and xylitol production.