Kinetic behavior of Candida tropicalis during xylitol production using semi-synthetic and hydrolysate based media

Abstract

The effects of the xylose concentration and the air supply on the production of xylitol from D-xylose by the yeast Candida tropicalis (AY 2007) using semi-synthetic and hydrolysate based media were investigated in four fermentation batch runs. The xylitol formation was favored under oxygen-limited conditions (2^nd experiment, air flow rate 0.3 vvm) with a maximum xylitol production (36.0gm/l) after 59h as compared with that obtained in the 1st experiment (30.99 g/L) after 66 h under relatively less oxygen limited condition (air flow rate 1.5 vvm). Increasing the initial xylose concentration from 45.5 g/L in the 2^nd experiment to 96.9 g/L in the 3^rd experiment led to a great decrease in the xylitol accumulation rate (Qp) from 0.506 to 0.134 g/L, xylitol yield coefficient (Yp/s) from 0.704 to 0.180 g/g and a mild increase in the xylose consumption rate (Qs) from 0.70 to 0.75 g/L/h. The semi-synthetic medium (1^st and 2^nd experiments) had a maximum xylitol yield coefficient (Yp/s) of 0.701 and 0.704 g/g, respectively which was less than that obtained in the hydrolysate experiment (0.783 g/g). This is probably due to the low hydrolysate initial xylose content (29.8 g/L).

Key words: Xylitol, Candida tropicalis, xylose, pH, aeration, kinetics