Monday, April 30, 2007

Fermentation kinetics for xylitol production by a Pichia stipitis D-xylulokinase mutant previously grown in spent sulfite liquor

Rita de Cassia L.B. Rodrigues1, Bernice Lin2, Chenfeng Lu3, and Thomas W. Jeffries2. (1) Departamento de Biotecnologia, DEBIQ, Escola de Engenharia de Lorena, EEL, USP, Universidade de São Paulo, P.O Box 116, 12600-970, Lorena, SP, Brazil, (2) Forest Products Laboratory, USDA Forest Service, One Gifford Pinchot Dr, Madison, WI 53726-2398, (3) Department of Food Science, University of Wisconsin, Madison, WI 53706

Spent sulfite pulping liquor (SSL) contains lignin, which is present as lignosulfonate and hemicelluloses that are present as hydrolyzed carbohydrates. To reduce the biological oxygen demand (BOD) of SSL associated with dissolved sugars, we studied the capacity of Pichia stipitis FPL-YS30 (xyl3D) to convert these sugars into useful products.  FPL-YS30 produces a negligible amount of ethanol while converting xylose into xylitol.  This work describes the xylose fermentation kinetics. Yeasts were grown in YP medium (yeast extract 10 g/l, and Bacto peptone 20 g/l) supplemented with different carbon sources (30 g/l): glucose, xylose or 50% ammonia base SSL.  The SSL was diluted and the pH adjusted to 6.0 with NaOH. Xylitol production was carried out in 125 ml Erlenmeyer flasks containing 50 ml of YPX medium (xylose 80 g/l) and inoculum (0.2 g/l) on a rotary shaker at 200 rpm, 30°C. SSL and glucose acclimatized cells showed similar maximum specific growth rates (0.146 h-1). However, the maximum specific values of xylose consumption (0.19 gxylose/gcel.h) and xylitol production (0.059 gxylitol/gcel.h) were obtained with cells acclimatized in glucose. Xylitol production (31.55 g/l) was 19 and 8% higher than in SSL- and xylose-acclimatized cells, respectively.  We did not observe ethanol formation in any of the experiments. Maximum glycerol (6.26 g/l) and arabitol (0.206 g/l) production were obtained using SSL- and xylose-acclimatized cells, respectively.  The results showed that the medium composition used for the yeast pre-cultivation directly reflected their xylose fermentation performance and that SSL could be used as a carbon source for cell production.