Optimisation of alkaline reagent based chemical pre-treatment of Brewers spent grains for bioethanol production

Abstract

The pre-treatment step of lignocellulosic ethanol production is recognised as the most energy intensive part of the overall process. Key objectives are thus that pre-treatments should operate effectively at low to moderate temperatures and at high percentage solids, in order to minimise energy inputs.Chemical pre-treatment of BSG using alkaline reagents, at moderate process temperatures, was optimised in terms of sugar release following enzymatic saccharification of the pre-treated residues. An excess of Celluclast® cellulase enzyme was used to compare various pre-treatments in terms of the % of theoretical sugars that could be liberated. Using alkaline peroxide (5% H2O2/NaOH at pH 11) up to 87% of the theoretical glucose yield (based on composition of the pre-treated residue) was attained following pre-treatment of BSG at 25% (w/v) solids loading for 5h at 100°C. Pre-treatment using 5% NaOH at 25% (w/v) solids loading for 12h at 50°C liberated 92% of theoretical glucose This pre-treatment remained effective with process times as short as 2h (ca. 70% of theoretical glucose). For NaOH pre-treatments the extent of lignin removal from pre-treated residues broadly correlated with the efficacy of the pre-treatment in terms of final saccharification yields. However, for alkaline peroxide pre-treatments no such correlation was observed.Saccharified BSG was prepared following treatment with the optimised NaOH or alkaline peroxide pre-treatments and the resultant hydrolysates fermented with Saccharomyces cerevisiae. Fermentation performance was superior for the NaOH pre-treated samples relative to alkaline peroxide generated feedstocks with shorter lag phases and faster attenuation rates. Mean ethanol concentrations of ca. 17.3g/L and 8.4g/L were achieved for the NaOH and alkaline peroxide fermentations respectively which corresponded to 81% and 43% of theoretical ethanol yields.