Evaluation of xylose-fermenting yeasts for ethanol production from spent sulfite liquor

Abstract

Thirty strains of xylose-fermenting yeasts were screened for their ability to produce enhanced ethanol yields in the search for an economically viable fermentation process, utilizing both the pentose and hexose sugars in spent sulfite liquor (SSL). Among the four Pichia and Candida strains selected for further evaluation, Pichia stipitis CBS 5773 was found to have the best ethanol yield and the highest optimum fermentation temperature. The fermentation parameters of Pichia stipitis CBS 5773 were optimized in batch experiments and confirmed by continuous culture on SSL. At 30°C, pH 5.7, and moderate aeration, the hexoses and xylose were completely converted with an ethanol yield of 0.38 g g −1 based on consumed sugar. In comparative tests, the amount of ethanol produced was significantly greater with Pichia stipitis CBS 5773 than by conventional hexose fermentation with Saccharomyces cerevisiae. Due to the relatively high fermentation pH resulting from the need to neutralize toxic acids, some of the continuously fermented SSL samples became contaminated by lactic acid bacteria.