The production of xylitol from D-xylose by fermentation with Hansenula polymorpha

We have analysed the influence of the initial pH of the medium and the quan tity of aeration provided during the batch fermentation of solutions of D-x ylose by the yeast Hansenula polymorpha (34438 ATCC). The initial pH was al tered between 3.5 and 6.5 whilst aeration varied between 0.0 and 0.3 vvm. T he temperature was kept at 30 degrees C during all the experiments. Hansenu la polymorpha is known to produce high quantities of xylitol and low quanti ties of ethanol. The most favourable conditions for the growth of xylitol t urned out to be: an initial pH of between 4.5 and 5.5 and the aeration prov ided by the stirring vortex alone. Thus, at an initial pH of 5.5, the maxim um specific production rate (mu(m)) was 0.41 h(-1), the overall biomass yie ld (Y-x/s(G)) was 0.12 g g(-1), the specific D-xylose-consumption rate (q(s )) was 0.075 g g(-1) h(-1) (for t = 75 h), the specific xylitol-production rate (q(Xy)) was 0.31 g g(-1) h(-1) (for t = 30 h) and the overall yields o f ethanol (Y-E/s(G)) and xylitol (Y-Xy/s(G)) were 0.017 and 0.61 g g(-1) re spectively. Both q(s) and q(Xy) decreased during the course of the experime nts once the exponential growth phase had finished.