Process optimization to convert forage and sweet sorghum bagasse to ethanol based on ammonia fiber expansion (AFEX) pretreatment

With growing demand for bio-based fuels and chemicals, there has been much attention given to the performance of different feedstocks. We have optimized the ammonia fiber expansion (AFEX) pretreatment and fermentation process to convert forage and sweet sorghum bagasse to ethanol. AFEX pretreatment was optimized for forage sorghum and sweet sorghum bagasse. Supplementing xylanase with cellulase during enzymatic hydrolysis increased both glucan and xylan conversion to 90% at 1% glucan loading. High solid loading hydrolyzates from the optimized AFEX conditions were fermented using Saccharomyces cerevisiae 424A (LNH-ST) without any external nutrient supplementation or detoxification. The strain was better able to utilize xylose at pH 6.0 than at pH 4.8, but glycerol production was higher for the former pH than the latter. The maximum final ethanol concentration in the fermentation broth was 30.9 g/L (forage sorghum) and 42.3 g/L (sweet sorghum bagasse). A complete mass balance for the process is given.