Comparison Study on the Biomass Recalcitrance of Different Tissue Fractions of Sugarcane Culm

Conversion of sugarcane bagasse to bioethanol is hindered due to intrinsic biomass recalcitrance that is related to its chemical composition and physicochemical properties. Furthermore, biomass heterogeneity interferes with process effectiveness. To compare sugarcane culm recalcitrance epidermis, internode and node fractions were individually investigated by acid pretreatment, enzymatic hydrolysis and thermal analysis/degradation. The epidermis fraction was found to be the largest mass fraction of the sucrose-free sugarcane culm followed by the epidermis-free internode and node fractions, comprising 65, 19 and 15.5 %, respectively. In comparison to the internode and node acid pretreatment solubilized a lower level of xylose from the epidermis resulting in higher mass recovery of water-insoluble solids (WIS) demonstrating its higher resistance to acid pretreatment. Enzymatic digestion showed that the epidermis is least susceptible to hydrolysis followed by the node and internode: 18.6, 56.5 and 75.9 %, respectively. In agreement with the enzymatic hydrolysis yield the internal/external surface area was lower for the epidermis than for the node and internode. Scanning electron microscopy (SEM) showed the epidermis exhibited less structural damage after enzymatic hydrolysis. Moreover, the epidermis required a higher start temperature for degradation (330 °C) and exhibited a higher heating value (4,236 cal/g). The internode and node required a degradation start temperature of 288 and 265 °C and had heating values of 4,098.9 and 3,998.76 cal/g, respectively. Taken together the results of this study demonstrate that the epidermis is more resistant to pretreatment, to thermal and enzyme degradation than are the internode and node. The separation of the epidermis from the culm could provide a new perspective on proper use/conversion of the most recalcitrant fraction of the sugarcane. In fact, separation of fractions decreases the biomass heterogeneity with a positive impact on the effectiveness of the conversion process for selecting material more susceptive to pretreatment or optimizing the process for each fraction.