Comparative Response and Structural Characterization of Sugarcane Bagasse, Straw and Bagasse-Straw 1:1 Mixtures Subjected to Hydrothermal Pretreatment and Enzymatic Conversion

The change of manually harvested sugarcane, after crop burning, to mechanically harvest green cane allows straw and bagasse to be available for further processing via either chemical or biochemical routes, which increases the sector’s energy efficiency. In this study, sugarcane bagasse, straw and a bagasse-straw 1:1 mixture were subjected, under comparative conditions, to hydrothermal pretreatment at 195°C for 10 minutes and to enzymatic conversion. We evaluated the individual responses of the three different materials regarding the effect of the pretreatment on hemicellulose and lignin extraction, the formation of furfural and hydroxymethylfurfural and cellulose enzymatic digestibility. The morphological, chemical and physical properties of the raw and pretreated materials were analyzed by Scanning Electron Microscopy (SEM), infrared spectroscopy (FTIR) and X-ray Diffraction (XRD). We observed a higher hemicellulose extraction from straw (93.3%) in comparison to bagasse (83.7%), and the hemicellulose extract that was obtained from straw contained a higher concentration of inhibitors. Intermediate values for hemicellulose extraction (88.5%) and inhibitor formation were observed for the bagasse-straw 1:1 mixture. The cellulose enzymatic hydrolysis yield was higher for straw (90.5%) in comparison to bagasse (68.2%), whereas an intermediate yield of 73.3% was observed for the mixture. According to the SEM images, the pretreatment altered the native biomass at the level of the structure of the cell wall, and consequently, the arrangement of the macromolecular components of the cell wall was closely related to the high degree of hemicellulose removal. FTIR data indicated chemical changes mostly in OH, OCH3 and C=O groups; these changes were most noticeable in the pretreated straw. Adjusted data for the crystallinity index suggested that the pretreated materials had decreased crystallinity. All of the results showed that straw had a lower recalcitrance.