Review paper
Fermentation of lignocellulosic hydrolysates. I: inhibition and detoxification

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Abstract

The ethanol yield and productivity obtained during fermentation of lignocellulosic hydrolysates is decreased due to the presence of inhibiting compounds, such as weak acids, furans and phenolic compounds formed or released during hydrolysis. This review describes the effect of various detoxification methods on the fermentability and chemical composition of the hydrolysates. Inhibition of fermentation can be relieved upon treatment with the ligninolytic enzyme laccase, pre-fermentation by the filamentous fungus Trichoderma reesei, removal of non-volatile compounds, extraction with ether or ethyl acetate, and treatment with alkali or sulfite. Various fermentation strategies can also be used to improve yield and productivity in lignocellulosic hydrolysates. Batch, fed-batch, and continuous fermentation are discussed in relation to inhibition of fermentation in lignocellulosic hydrolysates.

Introduction

Lignocellulosic materials such as wood provide abundant and renewable energy sources. Lignocellulosics contain sugars polymerised to cellulose and hemicellulose which can be liberated by hydrolysing the material, and subsequently fermented to ethanol by microorganisms, such as Saccharomyces cerevisiae. Lignocellulose-derived ethanol can be used as an environmentally friendly liquid fuel, the exhaust carbon dioxide being taken up by growing biomass and therefore not making a net contribution to the atmosphere. However, rapid and efficient fermentation of the hydrolysates is limited because in addition to monomeric sugars a range of toxic compounds is generated during steam pretreatment and hydrolysis of lignocellulosics (for a review, see Palmqvist and Hahn-Hägerdal, 1999). The inhibiting compounds are divided in three main groups based on origin: weak acids, furan derivatives, and phenolic compounds. In the present article detoxification methods and strategies to enhance the efficiency of the fermentation process are reviewed.

Except where explicitly stated, S. cerevisiae has been used for fermentation of lignocellulose-derived sugars and in model fermentations in all work reviewed herein. S. cerevisiae has been shown to be the best yeast for the fermentation of hexose sugars present in lignocellulose-derived hydrolysates due to its ethanol-producing capacity and high inhibitor tolerance Hahn-Hägerdal et al., 1991, Olsson and Hahn-Hägerdal, 1993.

Section snippets

Detoxification

Biological, physical, and chemical methods have been employed for detoxification (i.e., the specific removal of inhibitors prior to fermentation) of lignocellulosic hydrolysates (for a recent review, see Olsson and Hahn-Hägerdal, 1996). In the following, detoxification methods reported in the literature and their effects on the composition and fermentability of the hydrolysates used will be discussed in relation to the identity of compounds mainly responsible for inhibition. Different

Acknowledgements

This work was supported by grants from the Swedish National Board for Industrial and Technical Development (NUTEK), Swedish Natural Science Research Council (NFR), the Knut and Alice Wallenberg Foundation, and the Carl Trygger Foundation.

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