INFLUENCE OF OXYGEN ON ETHANOL AND XYLITOL PRODUCTION BY XYLOSE FERMENTING YEASTS

The behaviour of Pichia stipitis, Pachysolen tannophilus, Candida sheh atae and Candida parapsilosis was investigated to select the most suit able yeast to convert xylose either to ethanol or to xylitol, with lit tle or no formation of by-products. The aeration rate was used as a va riable parameter. P. stipitis and C. parapsilosis were the most effect ive producers of ethanol and xylitol, respectively, both reaching prod uctivities at very low levels of oxygenation. With P. stipitis, better ethanol productivity was attained under microaerobic conditions (K(L) a=4.8 h(-1)) while with C. parapsilosis high yields and rates of xylit ol production were detected at K(L)a values of about 16.3 h(-1). P. ta nnophilus and C. shehatae showed lower performances under all conditio ns used while changes in oxygenation modified the ratio of ethanol to xylitol produced by these yeasts, suggesting that they are more depend ent on the oxygen power input than P. stipitis and C. parapsilosis. Th e influence of oxygen transfer rates on ethanol and xylitol formation with the best producers is discussed.