Fermentation of Sweet Sorghum Syrup Under Reduced Pressure for Bioethanol Production

Production of bioethanol from sweet sorghum (Sorghum saccharatum) is a promising “green” energy source that can help to reduce energy dependence on petroleum products, to decrease greenhouse gas emissions, and fight environmental pollution. As an additional benefit, it can promote the exploitation of new uncultivated agricultural lands and favor establishing integrated agro-industrial energy independent enterprises. The alcoholic fermentation under reduced pressure may prevent the accumulation of high ethanol concentrations in the cultured broth and thus may create favorable conditions for the highest productivity of yeast Saccharomyces cerevisiae.

Elaboration of optimal conditions for sweet sorghum syrup fermentation under reduced pressure.

To determine the parameters of sweet sorghum syrup fermentation by S. cerevisiae under the conditions of constant and periodic reduced pressure for the highest bioethanol production efficiency.

The sweet sorghum was grown in a temperate continental climate region of Northern Ukraine. The parameters of diluted stem syrup and S. cerevisiae fermentation under reduced and atmospheric pressure were established and controlled by chemical, biochemical and physicochemical methods. The yeast cells were dyed with methylene blue and counted using a microscope and a Neubauer counting chamber. The obtained data have been statistically analyzed.

It has been established that a periodic vacuum extraction with short-term heating of the medium to the boiling point is the most promising procedure for bioethanol production. Periodically reduced pressure fermentation of sweet sorghum diluted syrup resulted in 55% higher bioethanol productivity compared to atmospheric pressure fermentation. Such treatment enables to maintain the concentration of ethanol in the medium below 5.5% vol., which does not significantly inhibit the productivity of industrial yeast strains and allows adding a nutrient with the subsequent continuation of the cultivation process. The resulting distillate requires less energy for further dehydration.

The sweet sorghum syrup does not contain substances that inhibit yeast cells although nitrogen and phosphorus supplements are required to support efficient S. cerevisiae growth. The optimal technology, elaborated in this research, consists of repeated cycles of fermentation under reduced pressure (to the level of vacuum) for boiling the cultured broth. This technology provides the highest bioethanol output, high efficiency, and productivity of the overall process.