Optimization of Water-Soluble Carbohydrate Extraction from Sweet Sorghum and Sweet Pearl Millet Biomass

The interest for sweet sorghum and sweet pearl millet use as potential feedstocks for bioethanol production has markedly increased in the last few years due to the high concentration in fermentable carbohydrates in their juice. The bottleneck of this production is the carbohydrate extraction process that needs to be optimized. The main objective of this study was therefore to determine the effect of operating conditions on water-soluble carbohydrate (WSC) extraction from the biomass of both species. For this purpose, two chopping modes of the biomass (coarse and fine) were investigated. Furthermore, the biomass was subjected to a second pressing after impregnation with water. In this context, the effects of four water:bagasse ratios (0.5, 1, 1.5, and 2) and three water temperatures (25, 45, and 65 °C) were evaluated. The initial biomass WSC concentration was 180 g kg−1 DM for sweet sorghum and 125.7 g kg−1 DM for sweet pearl millet. After the first pressing, these concentrations decreased by 63 and 56%, respectively. For the second pressing, the efficiency of WSC extraction was more influenced by the water:bagasse ratio than by the chopping mode and the water temperature. For an economic and energetic sustainable extraction process of carbohydrates, a coarse chopping, and a second pressing after impregnation with a water:bagasse ratio of 1 at 25 °C are therefore recommended to improve residual carbohydrate extraction. Under these conditions, the WSC extraction yields reached 83 and 78% for sweet sorghum and sweet pearl millet biomass, respectively.