Biological production of industrial chemicals, i.e. xylitol and ethanol, from lignocelluloses by controlled mixed culture systems

The paper describes the utilization of the mixed cultures concept for the production of ethanol from a mixture of cellulosic glucose and hemicellulosic xylose and for the production of xylitol from xylose present in a hemicellulosic sugars mixture. For ethanol production, the process was studied in continuous aerated conditions with a respiratory deficient mutant of Saccharomyces cerevisiae CBS 1200, associated with Pichia stipitis NRRL 11545 and using a microfiltration membrane assisted bioreactor. Using a synthetic medium (35 g/l glucose, 15 g/l xylose), the highest fermentative performances were obtained at D = 0.1 h(-1). Under these conditions, ethanol was produced with a yield of 0.43 g/g and a volumetric rate of 2 g/l/h. The glucose and xylose conversion yields were, respectively, 100 and 60%,, giving an overall substrate conversion yield of 88%. At D = 0.13 h(-1) with the medium containing aspen wood hydrolysate as carbon source (glucose 41 g/l, xylose 9 g/l), ethanol was produced with a volumetric rate of 2.9 g/l/h, a yield of 0.46 g/g and the substrate conversion yield was 94%. For xylitol production, the utilization of Lactobacillus reuterii in association with the xylitol producing yeast, Candida guilliermondii permits to reduce the accumulation of arabinitol from arabinose and to produce xylitol from a synthetic medium (xylose 40 g/l, glucose 10 g/l, arabinose 10 g/l), with a purity up to 98%. Similar fermentative parameters were obtained when the couple of microorganisms was grown on a wheat straw hemicellulosic hydrolysate containing 39 g/l of xylose, 5.9 g/l of glucose, and 7.9 g/l of arabinose. (C) 1998 Elsevier Science B.V.