A Comparison of Dilute Acid- and Alkali-Catalyzed Hydrothermal Pretreatments for Bioethanol Production from Brewers' Spent Grains

Acid- or alkali-catalyzed hydrothermal (121°C, 30 min) processing of brewers' spent grains were evaluated as pretreatments for subsequent bioethanol production. At 1% (w/v), HCl was the most effective acid and NaOH the most effective alkali pretreatment. However, ∼3% (w/v) NaOH was required to generate saccharification yields similar to the 1% HCl system following enzymatic digestion. Each such pretreatment could be operated at 25% solids loading (pretreatment) with high theoretical glucose yields (86% of maximum with 1% HCl hydrothermal treatment; 70% with 3% NaOH) when solid residues were digested with excess Celluclast enzyme. Enzyme hydrolysates resulting from both pretreatments were fermented using Saccharomyces cerevisiae, attaining 70% of theoretical ethanol production within 18 hr. The solid residue following fermentation contained ∼25% (w/w) protein (dry basis). the preferred pretreatment was 1% HCl reagent when considering overall sugar yield, including those in the liquor generated during pretreatment. It was further shown that this liquor could be fermented directly, thus enhancing overall process yields.