Toxic compounds in lignocellulosic hydrolysates can be divided into four major groups: sugar degradation products, lignin degradation products, compounds derived from lignocellulose structure, and heavy metallic ions (Mussatto and Robert 2004). Typical inhibitory compounds include carboxylic acids, aldehydes, furans, and phenolics (Zaldivar and Ingram 1999; Palmqvist, and Hahn-Hagerdal 2000b; Oliva et al. 2003; Varga et al. 2004). The first step in the conversion of biomass to fermentable sugars is pretreatment. A pretreatment process uses mainly physical and chemical methods to increase surface area; decrystallize cellulose;

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 233 © 2010 Blackwell Publishing. ISBN: 978-0-813-80252-7

remove sheathing on cellulose by hemicellulose and lignin; alter lignin structures; and some­times partially remove lignin to improve both the rate of enzymatic hydrolysis and the yield of monosaccharides (Mosier et al. 2005a). Different pretreatment methods, including steam explosion (Ohgren et al. 2005) , dilute sulfuric acid (Lloyd and Wyman 2005) , hot water (controlled pH) (Mosier et al. 2005b), ammonia fiber/freeze explosion (AFEX; Teymouri et al. 2005), ammonia recycle percolation (ARP; Kim and Lee 2005), lime (Kim and Holtzapple 2005), ionic liquid (Dadi et al. 2006, 2007), and peroxide (Saha and Cotta 2006), among others, have been proposed and tested for deconstruction of a variety of lignocellulosic biomass. The pretreatment of lignocellulosic biomass by acids or water often results in the degradation of cellulose, hemicellulose, and lignin (Figures 11.1-11.3). These degradation compounds exert inhibitory effects on fermentation microorganisms.

Enzymatic hydrolysis of lignocellulosic biomass may also release inhibitors from biomass

OH

Figure 11.4. Inhibitors from linkages between hemicellulose and lignin (Klinke et al. 2004).

arabinoxylan, coumaric acid, and ferulic acid are significant components of solubilized corn stover hemicellulose produced during simultaneous saccharification and fermentation (SSF). These acids from biomass structure are toxic to fermentation microorganisms. When 0.3 g/L of p-coumaric and ferulic acids were incorporated into a fermentation medium, the acetone — butanol-ethanol (ABE) production by Clostridium beijerinckii BA101 decreased significantly (Ezeji et al. 2007).

Heavy metal ions (iron, chromium, nickel, and copper) can originate from corrosion of hydrolysis equipment (Figure 11.5). Although they are not always produced in large quanti­ties, they can have some toxic effect on the alcoholic fermentation microorganisms (Mussatto and Robert 2004).

The toxic effects of various inhibitory compounds on the growth response of ethanolo — genic Escherichia coli were found to be in the following order: aldehydes > organic acids > alcohols (Zaldivar and Ingram, 1999; Zaldivar et al. 1999, 2000). When plotting series of separate functional groups of phenol, aldehydes, ketones, and acids, a correlation between the hydrophobicity and inhibition of volumetric ethanol productivity was reported.

Figure 11.5. Inorganic salts and heavy metal ions (Klinke et al. 2004; Mussatto and Roberto 2004).

The more hydrophobic the compound was, the more the inhibition was evident. The phe — nolics were reported to be among the most toxic compounds to fermentation microorganisms (Larsson et al. 2000; Klinke et al. 2004).

Detoxification Methods

To overcome the toxic effects of inhibitory degradation products of lignocellulosic biomass, many detoxification methods have been investigated. The focus has been placed on removing the inhibitory compounds from the hydrolysates, modifying the inhibitory compounds, or improving the resistance of the fermenting microorganisms to the toxic effects of the inhibi­tory compounds (Palmqvist and Hahn-Hagerdal 2000a; Pienkos and Zhang 2009). The detox­ification methods can thus be generally divided into three categories: chemical, physical, and biological methods (Table 11.1).