Acid hydrolysis of Eucalyptus grandis chips for microbial production of xylitol

Acid hydrolysis of the hemicellulosic fraction of eucalyptus chips was influenced by the following parameters: temperature, acid concentration, residence and immersion time. The concentrations of sugars (glucose, xylose, arabinose) and acetic acid varied systematically for all hydrolysis conditions tested. The fermentation of different types of hydrolysate showed distinct behaviour due to the differences in initial concentrations of sugars and acetic acid. Smaller amounts of xylose and acetic acid were obtained under conditions of low temperature and low concentration of sulphuric acid. With respect to xylose/xylitol bioconversion, the xylose consumption rate ranged from 80% to 94%, irrespective of the initial concentration of this sugar in the medium. However, the amount of xylitol produced varied significantly. The highest xylitol concentration (29.7 g/litre) was found in hydrolysate obtained at the following hydrolysis conditions: temperature (140 degrees C), sulphuric acid concentration (0.5%), residence time (10 min) and immersion time in acid solution 24 h and treated with CaO. The pH rise during the fermentation of the eucalyptus hydrolysate might be related to acetic acid consumption by the yeast Candida guilliermondii FTI 20037. (C) 1998 Elsevier Science Ltd.