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Effects of selected by-products of an acid hydrolyzate on cell growth and ethanol fermentation by Saccharomyces cerevisiae.

Article Excerpt
ABSTRACT

Acid hydrolysis of lignocellulosic biomass results in the release of various sugars as well as inhibitory compounds such as furfural, 5-hydroxymethylfurfural (HMF), vanillin, and vanillic acid that could inhibit microbial growth and ethanol fermentation. During the process of acid hydrolysis of pinewood sawdust conducted in this study, heavy metals such as chromium (Cr) were released from the extruder of the processing reactor. The purpose of this research was to study the effect of selected by-products in an acid hydrolysate on cell growth and ethanol fermentation of Saccharomyces cerevisiae. Furfural, HMF, vanillin, vanillic acid, and Cr were added at different concentrations to fermentation media. S. cerevisiae growth and ethanol yield were measured after 18 hours of incubation. The results showed that individual presence of furfural, HMF, vanillin, and vanillic acid at concentrations as low as 1.0, 2.0, 2.0, and 0.5 mg/ml, respectively, significantly inhibited ethanol production, while Cr did not show any inhibitory effect at concentrations up to 20 ppm. Effects of the mixture of furfural, HMF, vanillin, vanillic acid, and Cr on ethanol yield and cell growth were also studied. Results showed that at low concentration such as 0.5 mg/ml, the by-products stimulated not only the S. cerevisiae growth but also the ethanol production. At 1.0 mg/ml, ethanol production and cell growth were significantly inhibited. Co-existence of Cr did neither enhance nor reduce the inhibitory/stimulating effect.

INTRODUCTION

There is a growing demand for the already depleting energy resources for fuel. One of the alternative solutions to the problem is ethanol production, which is a cost effective and environmentally friendly process of producing fuel (Luo et al., 2002). Processing method and feedstock are the determining factors in ethanol production costs. Currently in the U.S., corn is the primary feedstock for ethanol production and accounts for 60% of the total production costs (Kim and Dale, 2002). The use of feedstock, such as lignocellulosic biomass that has no or low commercial value, could significantly reduce ethanol production costs. The processing of lignocellulosic biomass to obtain the monomeric sugars needed for fermentation is one of the major factors contributing to the non-competitive costs of lignocellulose derived ethanol. The conversion of the polysaccharides in lignocellulose to monosaccharides can be achieved by dilute or concentrated acid hydrolysis. The drawback of acid hydrolysis is that it produces a product (acid hydrolysate) that contains not only the sugars needed for fermentation but also furan and phenolic compounds as well as weak acids that could inhibit fermentation and microbial growth.

The known by-product compounds found in acid hydrolysate include furfural and 5-hydroxymethyl furfural (HMF) formed from high temperature degradation of pentose and hexose sugars respectively (Larsson et al., 1999; Palmqvist et al., 1999; Taherzadeh et al., 1997, 2000). Others include phenolics formed from the partial breakdown of lignin (Ando et al., 1986; Jonsson et al., 1998; Larsson et al., 1999) and weak organic acids such...

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