Effects of Acid and Alkali Pretreatments on Carbohydrate Release from Sweet Sorghum and Sweet Pearl Millet Bagasse for Bioethanol Production.

Sweet sorghum and sweet pearl millet are considered promising alternative feedstocks for bioethanol production. Water soluble carbohydrate (WSC) extraction from the stems for first generation ethanol production is practically well mastered. However, structural carbohydrate (SC) release from the bagasse for cellulosic ethanol production still needs to be more understood and improved, especially for sweet pearl millet. In this study, the effects of two bagasse particle sizes (4.5–9 mm and 1 mm) and two pretreatment solutions (NaOH and H2SO4) at two concentrations (1% and 3%) on the release of SC from sweet sorghum and sweet pearl millet bagasse were investigated. After the extraction of juice, the bagasse pretreatment was carried out at 121 °C for 60 min followed by an enzymatic saccharification. Results indicated that hemicellulose and lignin were better solubilized when the bagasse from both crops was pretreated with NaOH than with H2SO4. The highest glucose concentration (320 and 249 g/kg DM of bagasse for sweet sorghum and sweet pearl millet, respectively) was reached after enzymatic saccharification of the bagasse chopped to 4.5–9 mm particle size and pretreated with 3% NaOH. Under these conditions, cellulose saccharification efficiency reached 88% and 73% for sweet sorghum and sweet pearl millet, respectively, while that of hemicellulose saccharification approached 100% for sweet sorghum and reached 90% for sweet pearl millet. Potential first generation ethanol of 80.3 and 60.7 L/kg DM of biomass and cellulosic ethanol of 138.8 and 115 L/kg DM of biomass were estimated for sweet sorghum and sweet pearl millet, respectively. [ABSTRACT FROM AUTHOR]