Solid-state fermentation of sugars in sweet sorghum

Abstract

The sugars in unsterilized sweet sorghum solids fermented readily during anaerobic solid-state fermentation (SSF). Products, based on 100 g hexose, were: 36.7 g volatiles (mass loss), 32.8 g ethanol, 2.8 g glycerol, 12.7 g lactic acid, 2.0 g acetic acid, 4.8 g dextrans, and 4.2 g mannitol, presumably from fermentation with naturally occurring heterolactic bacteria, such as Leuconostoc dextranicum, as well as wild yeasts. Inoculation of unsterilized sorghum solids with 0.007% to 0.20% Saccharomyces cerevisiae increased fermentation rate, gas evolution, and yields of ethanol, glycerol, and acetic acid, while other yields decreased. With 0.2% yeast, yields were 40.9 g ethanol, 4.8 g glycerol, and 45.5 g gases. The volatiles from all fermentations had a strong odor of acetaldehyde. Compared with yields expected from the Pasteur equation, the ratio of mass loss to ethanol yield was 17% greater and glycerol yield was 50% greater, which suggests that significant amounts of acetaldehyde evaporated with CO 2 during SSF and Neuberg's second form of fermentation occurred to a much larger extent during SSF than in liquid phase fermentations.