Fermentation kinetics and ethanol production from different sweet sorghum varieties

In recent years bioethanol fuels derived from agricultural biomass resources or waste have been considered the cleanest liquid fuel alternative to fossil fuels. Ethanol consumption is expected to reach 11.2 billion gallons by 2012. Sweet sorghum, grown in the Southeast and Midwest states in US, contains 14%-20% fermentable sugar and is an ideal feedstock, owing to its ease of fermentation by yeast. The objectives of this research were to perform kinetic studies to determine the factors that may affect the rates of sugar consumption and ethanol production during fermentation of two varieties of sweet sorghum juice, Variety 1 (V-1) and Variety 2 (V-2), and to optimize the fermentation efficiency and ethanol production by varying strategies to process the juice before fermentation. Kinetic parameters of fermentation provide total sugar consumption rate of 3.4 g/(L·h) for V-1 juice and 2.2 g/(L·h) for V-2 juice, with ethanol production rates of 1.8 g/(L·h) for V-1 juice and 1.6 g/(L·h) for V-2 juice. Maximum ethanol production (V/V) was 8.5% for V-1 and 9.2% for V-2; ethanol yield was 0.065 g of ethanol/g of V-1 juice and 0.072 g ethanol/g of V-2 juice. Even though V-2 has a higher ethanol yield than V-1, V-1 has faster consumption and production rates due to its lower initial glucose and sucrose proportions in the juice, relative to V-2. Fermentation efficiency is greater than 90% for frozen, autoclaved juice and 25% sugar content juice. The lowest fermentation efficiency (79%) was for 30% sugar-content juice. These results can be used to optimize processing conditions of sweet sorghum juice during fermentation.