Detoxification of Dilute Acid Hydrolysates of Lignocellulose with Lime

Abstract

The hydrolysis of hemicellulose to monomeric sugars by dilute acid hydrolysis is accompanied by the production of inhibitors that retard microbial fermentation. Treatment of hot hydrolysate with Ca(OH)2 (overliming) is an effective method for detoxification. Using ethanologenic Escherichia coli LY01 as the biocatalyst, our results indicate that the optimal lime addition for detoxification varies and depends on the concentration of mineral acids and organic acids in each hydrolysate. This optimum was shown to be readily predicted on the basis of the titration of hydrolysate with 2 N NaOH at ambient temperature to either pH 7.0 or pH 11.0. The average composition of 15 hydrolysates prior to treatment was as follows (per L): 95.24 ± 7.29 g sugar, 5.3 ± 2.99 g acetic acid, 1.305 ± 0.288 g total furans (furfural and hydroxymethylfurfural), and 2.86 ± 0.34 g phenolic compounds. Optimal overliming resulted in a 51 ± 9% reduction of total furans, a 41 ± 6% reduction in phenolic compounds, and a 8.7 ± 4.5% decline in sugar. Acetic acid levels were unchanged. Considering the similarity of microorganisms, it is possible that the titration method described here may also prove useful for detoxification and fermentation processes using other microbial biocatalysts.