Alkali Treatment

Alkali treatments, particularly the treatment with calcium hydroxide (overliming), have been widely used for improving the fermentability of ligocellulose hydrolysates. In an overliming process, lime is added to the hydrolysate, resulting in the formation of insoluble salts. Larsson et al. (1999) compared the effects of 12 detoxification methods, including alkali treatments with sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2), on the chemical composi­tion and fermentability of a hydrolysate from spruce pretreated with dilute acid. Overliming was more efficient than NaOH in terms of removal of inhibitors under similar conditions (Larsson et al. 1999) . One drawback with overliming is the formation of calcium sulfate precipitate (gypsum). In addition, if the treatment is done at high pH and temperatures, a considerable degradation of fermentable sugars occurs (Cheng et al. 2008). The treatment has to be optimized to maximize the fermentability and lower sugar degradation.

Martinez et al. )2001) employed a titration method to predict the optimal amount of Ca(OH)2 for overliming at 60°C using 15 different batches of bagasse hemicellulose hydro­lysate. All 15 overlimed hydrolysates exhibited the same trend, despite differences in the amount of added Ca(OH)) . Total furans were reduced by 51% and soluble phenolic com­pounds were reduced by 41%. Presumably, these furans and phenolic compounds were converted to less toxic products by overliming. Total sugars were reduced by 8.7%. Although common and effective, overliming does not remove acetic acids, which are known to inhibit ethanol production at concentrations greater than 2g/L (Berson et al. 2006).

Ammonium ions do not form poorly soluble salts, and treatment with ammonium hydrox­ide compared favorably with overliming (Horvath et al. 2005) . NaOH would be another option, but under similar conditions NaOH treatment has so far been less efficient than over­liming. Optimal conditions were found to be in a range around pH 9.0/60°C for NH. OH treatments and in a narrow area stretching from pH 9.0/80°C to pH 12.0/30°C for NaOH treatments (Horvath et al. 2005).