Two-stage alkaline and acid pretreatment applied to sugarcane bagasse to enrich the cellulosic fraction and improve enzymatic digestibility

Abstract

Sugarcane bagasse (SB) is made up of cellulose (32-43%), hemicellulose (19-34%) and lignin (14-30%). Due to high recalcitrant nature of SB, pretreatment is required to deconstruct its structure and enrich the cellulosic fraction. A two-stage NaOH and maleic acid pretreatment was applied to SB to enrich its cellulosic fraction. SB used in the present study is composed of cellulose (40.4 wt%), hemicellulose (20.9 wt%), lignin (22.5 wt%) and ash (4.0 wt%). After one-stage NaOH pretreatment, its cellulosic fraction increased to 61.8 wt% and later increased to 80.1 wt% after the second-stage acid pretreatment. Lignin fraction decreased to 3.0 wt% after one-stage NaOH pretreatment and remained unaffected after the acid pretreatment step. Hemicellulose fraction decreased substantially after the second-stage pretreatment with maleic acid. Pretreated SB displayed high crystallinity index and improved enzymatic digestibility. Hydrolysates of pretreated SB contained very low amount of xylose and subsequent fermentation by Saccharomyces cerevisiae -IQAr/45-1 resulted to ethanol level of 8.94 g/L. Maximal ethanol yield of 0.49 g/g (95.8% of theoretical yield) and productivity of 0.28 g/L/h was attained. At the same time, biomass yield and productivity of 0.47 g/g and 0.27 g/L/h respectively were obtained. Two-stage NaOH and maleic acid pretreatment led to ~ two-fold increase in cellulosic fraction and enhanced the enzymatic digestibility of SB up to 70.4%. The resulted enzymatic hydrolysate was efficiently utilized by S. cerevisiae -IQAr/45-1 to produce high yield of ethanol. Thus, optimization of enzymatic hydrolysis at low enzyme loading is expected to further improve the process and reduce cost.