SHORT CONTRIBUTION S. Sa´nchez Æ V. Bravo Æ E. Castro Æ A. J. Moya F. Camacho The production of xylitol from D-xylose by fermentation with Hansenula polymorpha Received: 26 March 1998 / Received revision: 30 June 1998 / Accepted: 2 July 1998 Abstract We have analysed the influence of the initial pH of the medium and the quantity of aeration provided during the batch fermentation of solutions of D-xylose by the yeast Hansenula polymorpha (34438 ATCC). The initial pH was altered between 3.5 and 6.5 whilst aera- tion varied between 0.0 and 0.3 vvm. The temperature was kept at 30 °C during all the experiments. Hansenula polymorpha is known to produce high quantities of xylitol and low quantities of ethanol. The most favour- able conditions for the growth of xylitol turned out to be: an initial pH of between 4.5 and 5.5 and the aeration provided by the stirring vortex alone. Thus, at an initial pH of 5.5, the maximum specific production rate (l m ) was 0.41 h )1 , the overall biomass yield Y G x=s  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 of ethanol Y G E=s  and xylitol Y G Xy=s  were 0.017 and 0.61 g g )1 respectively. Both q s and q Xy decreased during the course of the experiments once the exponential growth phase had finished. Introduction The yeasts most commonly used in fermentation pro- cesses tend to be selected from the genera Saccharomyces and Schizosaccharomyces and fundamentally ferment hexoses, whilst being incapable of transforming pent- oses. Nevertheless, to take full advantage of lignocellu- lose residues, of which D-xylose is the main pentose in the hemicellulose fraction, we need some alternative to enable us to transform this fraction into some industri- ally useful bioproduct. It should be borne in mind that the hemicellulose fraction may account for between 15% and 35% of the dry residue (Detroy et al. 1982). In previous works we have studied the ethanolic fermentation of D-xylose with yeasts other than those traditionally used, such as Pachysolen tannophilus (Bra- vo et al. 1995a). In this paper we report our findings on the fermentation of D-xylose using another yeast, Hansenula polymorpha, which has been reported as be- ing capable of fermenting both D-glucose and D-xylose. Very little information is available about the influ- ence of the operating variables upon the growth of H. polymorpha. Some authors who have used this species to ferment D-glucose (Levine and Cooney 1973; Denenu and Demain 1981) have chosen an initial pH of 5.5 as being the most suitable. As far as aeration is concerned, those authors who have worked with this yeast have provided no aeration as such in their experiments, but by using relatively small, magnetically stirred culture vessels they must have created a vortex through which an in- determinate flow of air would have entered. Denenu and Demain (1981), for example, experimented with quan- tities of 50 ml culture in 250-ml Erlenmeyer flasks with a stirring speed of 220 rpm. Materials and methods Microorganism We used the yeast H. polymorpha 34438, supplied by the American Type Culture Collection. Experimental device All our experiments were carried out at the laboratory scale using a batch-culture installation, as described elsewhere (Bravo et al. 1995b). This installation consists basically of three temperature- controlled, magnetically stirred fermenters with a usable volume of 2 l and delivery to the culture medium of bubble air flow when Appl Microbiol Biotechnol (1998) 50: 608–611 Ó Springer-Verlag 1998 S. Sa´nchez (&) Æ E. Castro Æ A. J. Moya Department of Chemical Engineering, Faculty of Science, Jae´n, E-23071, Spain V. Bravo Æ F. Camacho Department of Chemical Engineering, Faculty of Science, Granada, E-18071, Spain