(741e) Glucose Fermentation Under Carbon Dioxide Pressure

Among other issues, the development of second-generation processes for the production of bio-ethanol from biomasses has to face and solve the problem of reducing the energy duty required to obtain the desired product purity (i.e. 99.8% ethanol).

Distillation of the fermentation broth, coupled to dehydration by adsorption or membrane separation, is the main responsible for energy consumption. One possible alternative is extraction of ethanol by supercritical carbon dioxide, which is known to be able to break the water-ethanol azeotrope if suitable operative conditions are used (Budich and Brunner, J. of Supercritical Fluids, 2003, 25, 45). In addition, carbon dioxide is also made available in the process itself, being produced by the fermentation reaction at a weight ratio 0.96:1 with respect to ethanol.

On the other hand, the pressure needed to carry out supercritical CO2 extraction of ethanol is quite high, thus resulting in higher compression costs that make such an idea practically useless. A possibility which deserves some attention could be the development of a fermentation process able to work under CO2 pressure, in order to reduce substantially the compression work downstream. In addition, as CO2 is more soluble in organic solvents than in water, fermentation under CO2 pressure could be useful to extract ethanol as soon as it gets formed, thus maintaining low its concentration in the solution and reducing product inhibition effects on the yeasts.

In view to exploit this possibility, our work is aimed to investigate the feasibility of glucose fermentation under CO2 pressure. Quite a long time ago, this idea was already proposed. In particular, Thibault et al. (Biotechnol. Bioeng., 1987, 30, 74) studied the effect of hyperbaric CO2 near supercritical conditions (up to 84 bar) and reported that a fermentation activity could be maintained anyway in spite of the harsh process conditions. However, in the meantime it has been shown that a suitable CO2 pressure is effective in the deactivation of microorganisms, including yeasts (Bertucco et al., Italian J. Food Technol., 2007, 165). So, the question whether ethanol can be produced under CO2 pressure is still open.

In our work the effect of CO2 on commercial and genetically modified S.cerevisiae yeast strain viability and the rate of ethanol production have been studied in batch cultures under different values of CO2 pressure.

The results show that the effect of CO2 pressure on ethanol productivity strongly depends on the temperature, glucose and biomass concentration used for fermentation.

The fermentation was obtained with a high starting concentration of biomass at low and moderate CO2 pressure, even though no growth was observed during the process at 10 bar or more. Inositol addition, in millimolar concentrations, greatly influenced the viability of yeast cells. The feasibility of ethanol fermentation was also demonstrated in a pilot fermentation unit.