(331d) Effect of Headspace Gas Composition on Mixed Acid Fermentations of Corn Stover for Fuels and Chemicals

Mixed acid fermentations employ a mixed culture of anaerobic microorganisms that economically ferments lignocellulose to carboxylate salts (C₂-C₈), which can be subsequently chemically converted to chemicals and liquid fuels. They do not need any external enzymes, genetically modified organisms or sophisticated controls, and are inexpensive and robust. This research investigates the effect of headspace gas composition, specifically hydrogen and carbon dioxide, on the yields and composition of the carboxylic acid spectrum obtained from the mixed acid fermentations of corn stover. A 2 bar hydrogen pressure in the headspace resulted in 13% higher total acetic acid equivalents (aceq.) as compared to N₂ headspace (control). This was mostly due to higher production of short chain carboxylates like acetate (C₂) and butyrate (C₄), 20% and 21% increase respectively, whereas the longer chain carboxylates like caproate (C₆) increased only by 6%. A 2 bar pressure of CO₂ in the headspace resulted in same total aceq. as the control (N₂) but the carboxylate spectrum shifted more towards the longer chain carboxylates. There was a 98% increase in the caproate production. A 2 bar headspace gas composition of H₂:CO₂ (50:50) yielded 37% higher aceq. than the control (N₂) with the carboxylate spectrum shifting even more towards the longer chain carboxylates. There was a 180% increase in caproate production. The results of this study show the benefits of controlling the headspace atmosphere used for anaerobic mixed acid fermentations and can be used for producing higher carboxylic acids with a larger fraction of long chain carboxylates.