Optimization of Bioethanol Production after Enzymatic Treatment of Sweet Sorghum Stalks

Sweet sorghum stalks energy recovery is one of the most interesting alternatives to meet the current challenge of energy security. The usual ethanol production process from this biomass proves to be less profitable due to very expensive juice extraction step. This study aims to improve the performance and efficiency of this process experimenting new processing techniques of direct whole stem fermentation combined with enzymatic hydrolysis. Fermentation tests were carried out on shredded whole stem and pith (particle size <= 1 mm) and enzymatic process efficiency was compared to fermentation of juice obtained after mechanical pressing. Saccharification treatments of biomass fractions (whole stem and pith with particle size <= 0.5 mm) were performed using Celluclast 1.5 L and Viscozyme L, with three different enzymatic loads (15, 30 and 45 U/g DM; and 15, 30 and 45 FBGU/g DM) combined to three different biomass loadings (2, 5 and 10% w/v). Fermentation was carried out using Saccharomyces cerevisiae. Fermentation yields were 0.45; 0.50 and 0.49 g Eth/g of consumed sugars respectively for juice, marrow and whole stem. Saccharification with Viscozyme at 45 FBGU/g DM enzymatic load and 10% (w/v) of biomass load allowed an increase of 39.63% and 28.14% in ethanol production yield; and 27.50% and 26.42% for biomass conversion efficiency, respectively between treated and untreated whole stem fractions and between treated and untreated pith fractions. Sweet sorghum stalk is a viable and sustainable source of alternative energy feedstock. Direct fermentation of the whole stem treated with cellulolytic enzyme makes it possible to dispense the juice extraction step, improving energy bioconversion performance and efficiency.