Cold alkaline extraction as a pretreatment for bioethanol production from eucalyptus, sugarcane bagasse and sugarcane straw

• Mathematical approach to optimize the process of cold alkaline extraction.
• Hemicelluloses and lignin removal from biomasses by cold alkaline extraction.
• Higher xylan and lignin removal for straw during pretreatment.
• Formation of pseudo-extractives for eucalyptus during pretreatment.
• Higher ethanol production for pretreated sugarcane straw.

Optimal conditions for the cold alkaline extraction (CAE) pretreatment of eucalyptus, sugarcane bagasse and sugarcane straw are proposed in view of their subsequent bioconversion into ethanol through the semi-simultaneous saccharification and fermentation (SSSF) process (with presaccharification followed by simultaneous saccharification and fermentation, or SSF). The optimum conditions, which are identified based on an experiment with a factorial central composite design, resulted in the removal of 46%, 52% and 61% of the xylan and 15%, 37% and 45% of the lignin for eucalyptus, bagasse and straw, respectively. The formation of pseudo-extractives was observed during the CAE of eucalyptus. Despite the similar glucose concentration and yield for all biomasses after 12 h of presaccharification, the highest yield (0.065 gethanol/gbiomass), concentrations (5.74 g L−1) and volumetric productivity for ethanol (0.57 g L−1 h−1) were observed for the sugarcane straw. This finding was most likely related to the improved accessibility of cellulose that resulted from the removal of the largest amount of xylan and lignin.