Bioethanol from Brewer’s Spent Grains: Acid Pretreatment Optimization

Abstract

This study performs a parametric study aiming at the optimization of the acid pretreatment step of brewer's spent grains (BSG) simultaneously with the enzymatic hydrolysis for conversion into simple sugars fermentable to bioethanol. For this purpose three acids and five enzymes were tested, by adding each two acids (HCl with H2SO4 or HCl with HNO3) either in mixture (in one step) or sequentially (in two steps), to 25 g of dry BSG, together with varying quantities of the enzymes. Results show that when using Viscozyme L or the mixture of Cellulase and Hemicellulase by action of two acids in mixture, the total sugars conversion ranges between 20-27 wt%, in which the mixture of HCl and H2SO4 promotes a greater release of glucose plus maltose, while the mixture of HCl and HNO3 promotes the release of higher amount of xylose and arabinose. Results also show that when Glucanex 100g and Ultraflo L are used simultaneously with the sequential addition of HCl and H2SO4, the highest total sugars conversion (54.5 wt%) is obtained using 2.30 mL of Ultraflo L and 1.67 g of Glucanex 100g. Furthermore, by increasing the amount of Glucanex 100g (from 1.67 to 2.48 g) to the same amount of Ultraflo L (2.30 mL) the total sugars conversion decreased from 54.5 wt% to 40.5 wt%. Moreover, a greater release of glucose was verified by increasing the amount of Ultraflo L (from 1.75 mL to 2.30 mL), while by increasing the amount of Glucanex 100g (from 1.67 to 2.48 g) the release of arabinose and maltose was enhanced. Also, when using Glucanex 100g and Ultraflo L simultaneously with the acids HCl and HNO3, the best method to obtain high conversions of sugars is by the sequential addition of the acids, instead of in mixture. In this work, it resulted in the best conversion of BSG to simple sugars (72.1 wt%), corresponding to about 720 g of sugars per kg of dry BSG.