A major problem associated with the fermentation of hydrolysates of lignocellulosic biomass is the presence of a broad range of degradation and products (nonsugar) of hemicellulose hydrolysis that inhibit fermenting microorganisms (Ezeji et al. 2007a, b). These inhibitors can be divided into three groups (Figure 3.2a, b) on the basis of origin: compounds released from the hemicellulose component of lignocellulosic biomass (e. g., acetic, ferulic, glucuronic acid); lignin degradation products (e. g., syringaldehyde, syringic acid, and phenolic com­pounds); and sugar degradation products (e. g., furfural, HMF, formic, and levulinic acid; Ezeji et al. 2007a, b). The type, number, and concentrations of these inhibitory compounds vary depending on the type of lignocellulosic biomass and pretreatment conditions. For effi­cient utilization of lignocellulosic hydrolysates by fermenting microorganisms, hydrolysates must be detoxified to rid or reduce concentrations of these inhibitors to tolerant amounts prior to fermentation. Villarreal et al. in 2006 evaluated five different detoxification procedures
to remove microbial inhibitors from eucalyptus hemicellulose hydrolysates prior to xylitol production by Candida guilliermondii. The detoxification methods employed in the investiga­tion include treatment with activated charcoal and four different resins (cationic and anionic) connected in sequence. Among the detoxification methods employed, ion exchange resins were more efficient in the removal of all three major groups of inhibitory compounds without sugar loss than activated charcoal (Villarreal et al. 2006). Marchal et al. (1986) found hydro­lysates obtained by enzymatic saccharification of wheat straw or corn stover pretreated by steam explosion in acidic conditions could not be fermented into ABE. A simple treatment involving heating the hydrolysates in the presence of calcium or magnesium compounds such as Ca(OH)2 or MgCO3 at neutral pH values restored normal fermentation of these hydroly­sates to ABE. Qureshi et al. (2008a) , during evaluation for use of corn fiber as a substrate for ABE production by C. beijerinckii BA101, reported a maximum cell concentration of 0.65g/L when dilute acid pretreated corn fiber hydrolysates (54.3g/L total sugar) were the substrate compared to a maximum cell concentration of 3.37g/L obtained from the control (pure mixed sugars). C. beijerinckii BA101 grown in dilute acid pretreated corn fiber hydro­lysates experienced a longer lag phase than the control, and the culture was unable to make a transition from the acidogenic to solventogenic ABE production phase, consistent with it being inhibited by lignocellulosic degradation products. When dilute acid pretreated corn fiber hydrolysates were treated with XAD-4 resin prior to fermentation, growth and ABE production by C. beijerinckii BA101 increased by approximately 300% and 500%, respec­tively. Soni et al. (1982), in addition, reported that bagasse and rice straw hydrolysates were inhibitory to Clostridium saccharoperbutylacetonicum.

A fungus, Coniochaeta ligniaria NRRL30616, which metabolizes furfural, 5-HMF, alde­hydes, aromatic, and aliphatic acids, was recently isolated (Nichols et al. 2008). The inves­tigators found that C. ligniaria NRRL30616 grew in corn stover dilute-acid hydrolysates, and compounds representing all of the three groups (aromatic and aliphatic acids, aldehydes, and phenolic compounds) of inhibitory products were removed during the course of fungal growth. Fungal laccase and peroxidase enzymes, in addition, have been used to detoxify wood hydrolysates (Jonsson et al. 1998- Martin et al. 2002). Furthermore, Larsson et al. (2001) expressed laccase in Saccharomyces cerevisiae and the mutant had increased resistance to phenolic compounds. These developments show that biological inhibitor abatement for reduc­ing or eliminating inhibitory compounds from biomass hydrolysates appears to be a promising method for detoxification but drawbacks in terms of cost-effectiveness and competition for substrate could be a problem. The development of fermenting strains that can tolerate greater concentrations of inhibitory compounds generated during acid pretreatment and hydrolysis of lignocellulosic biomass remains a priority.