Low severity dilute-acid hydrolysis of sweet sorghum bagasse

Sweet sorghum is emerging as a promising energy crop in India, with potential for ethanol production both from the extracted juice and the bagasse residue. This work investigates the release of fermentable total sugars (TS) and formation of total degradation compounds (TDC) by dilute sulphuric acid hydrolysis of sweet sorghum bagasse from the genotype SSV 84. Hydrolysis was conducted at moderate temperatures (100 °C and 121 °C). The effect of pretreatment severity (measured by the combined severity factor, CS), particle size and substrate loading on the hydrolysate composition was examined. The kinetics of xylose, acetic acid and furfural formation was also studied. The maximum theoretical yield is 29.9 g C5 sugars/100 g bagasse on dry weight basis. Under the conditions of this study, 10% substrate loading in the CS range of 1.64–1.93 was optimal with C5 yield >22.3 g/100 g bagasse and efficiency (TS/TDC ratio) >8. Bagasse particle size reduction also resulted in a higher efficiency. SEM and FT-IR analysis of the solid residue confirmed disruption of the biomass structure and removal of functional groups associated with hemicelluloses and lignin. This study demonstrates that acid hydrolysis at moderate temperature has an advantage in terms of high sugars concentration vis-a-vis low degradation compounds formation.