COFERMENTATION OF GLUCOSE AND XYLOSE TO ETHANOL BY A RESPIRATORY-DEFICIENT MUTANT OF SACCHAROMYCES-CEREVISIAE CO-CULTIVATED WITH A XYLOSE-FERMENTING YEAST

As a part of the alcoholic conversion of lignocelluloses, fermentation of a glucose-xylose mixture by a co-culture process was investigated in oxygen-limited conditions. In batch mixed cultures of Saccharomyces cerevisiae CBS 1200 and Candida shehatae ATCC 22984, ethanol was produced only from glucose. During the fermentation by S. cerevisiae consuming glucose, the fermentation and growth activities of the xylose-fermenting yeast were extremely low, although an optimal condition of oxygen transfer rate in the co-culture was used. The use of a respiratory-deficient mutant of S. cerevisiae CBS 1200 allows significant cell growth of C. shehatae in a batch culture under a favourable oxygen condition. The growth of C. shehatae, however, results in the utilization of glucose, due to the catabolic repression of glucose on the xylose consumption. When the two yeast strains were co-cultivated in a continuous culture, the simultaneous conversion of glucose and xylose was obtained: conversion yields of glucose and xylose were respectively 100% and 27% at a dilution rate of 0.02 h-1. When the mutant of S. cerevisiae was co-cultivated with Pichia stipitis NRRL Y11545, a rapid xylose-fermenting yeast, the co-fermentation of glucose and xylose was enhanced: ethanol was produced with a yield of 0.42 g of ethinol/g of consumed sugars and the respective yields of glucose and xylose conversions were 100% and 69% at the tested dilution rate of 0.02 h-1. The advantages of the co-cultivation of a respiratory-deficient mutant of hexose-fermenting and a xylose-fermenting yeast are discussed.