Influence of Juice Preparations on the Fermentability of Sweet Sorghum

Abstract

Agricultural biomass has shown potential to aid in the solution to the rising need for biofuels and other biobased products. This study focused on sweet sorghum, a crop that produces relatively high quantities of easy to process, directly fermentable aqueous sugar. Sweet sorghum is desirable because of the reduced steps and inputs required for complete conversion to ethanol compared to starch and lignocellulosic biomass, as well as its adaptability to temperate regions, drought tolerance, and photosynthetic efficiency. Laboratory fermentation studies were conducted to evaluate the processing parameters for ethanol production on fresh and frozen sorghum juice. The effects of pH level, yeast type, incubation time, and sterilization were investigated. Ethanol and individual sugar concentrations (sucrose, fructose, and glucose) were measured over time. Fresh sorghum juice stocks on average contained 50% w/w sucrose, 24% w/w glucose, and 26% w/w fructose of total initial sugars, while frozen sorghum juice stocks contained 27% w/w sucrose, 37% w/w glucose, and 36% w/w fructose of total initial sugars. Ethanol yields were higher for sterile treatments, with 7.86% v/v ethanol (62 g L-1) and 6.24% v/v ethanol (49.3 g L-1) within 3 days on average for fresh sterile and non-sterile treatments, respectively. Frozen juice samples resulted in 7.81% v/v ethanol (61.6 g L-1) and 7.10% v/v ethanol (56.1 g L-1) for sterile and non-sterile treatments, respectively, after 3 days of incubation. Sterile treatments typically resulted in >84% conversion of sugars to ethanol, while non-sterile treatments ranged between 60% and 75% conversion. Overall, sweet sorghum juice is capable of supporting yeast fermentation with minimal input, making it a suitable substrate for development of on-farm and large-scale industrial ethanol conversion.