As indicated above, conversion of cellulosic biomass to butanol requires pretreatment employing dilute sulfuric acid or dilute sodium hydroxide, or alkaline peroxide. During the pretreatment and neutralization process, inhibitors such as salts (sodium acetate, sodium chloride, and sodium sulfate), and chemicals including furfural, hydroxymethyl furfural (HMF), syringealdehyde, and acids (acetic, glucuronic, ferulic, and p-coumaric) are produced. Some of these chemicals are toxic to the culture. In a recent study it was observed that sodium sulfate [18], sodium chloride [59], glucuronic, ferulic, and p-coumaric acids, phenol, and syringealdehyde [18] were toxic to a butanol producing culture. Ferulic acid, at a concentration as low as 0.3 g/L, was a strong inhibitor to both cell growth and fermentation. On the contrary, syringealdehyde (0.3-1.0 g/L) was not so toxic to the cell growth; how­ever, it resulted in complete arrest of ABE production [18]. As expected, phenol was found to be inhibitory to both cell growth and ABE production.

In order to produce ABE from agricultural residues successfully, inhibitors present in the hydrolyzates must be removed prior to fermentation. In recent studies, Qureshi et al. [61, 62] attempted to produce butanol from untreated and treated BSH and CSH. ABE was successfully produced from the treated hydrolyzates, however, strong inhibition of cell growth was still observed. In case of BSH and CSH, 0.80 and 0.77 g/L cell mass was obtained as compared to 2.66 g/L in the control experiment in which glucose was used. It is likely that chemical inhibitors were removed from the hydrolyzates by overliming leaving behind salts that were generated during neutralization. In order to reduce or eliminate cell growth inhibition, it is recom­mended that salts also should be removed from the medium by electrodialysis [59] followed by fermentation. Possibly, removal of salts could improve both cell growth and productivity.