Acetic Acid Removal from Corn Stover Hydrolysate Using Ethyl Acetate and The Impact on Saccharomyces cerevisiae Bioethanol Fermentation

Acetic acid is present in all pretreated lignocellulosic biomass and it has shown to have inhibitory effect on S. cerevisiae.  Many separation methods have also been used to remove the fermentation inhibitors.  Liquid-liquid extraction in particular have shown promising results in fermentation inhibitors removal but its impact on different yeast strains performance still needs further investigations.

The liquid-liquid extraction of acetic acid from biomass hydrolysate using ethyl acetate and butyl acetate has negative effect on fermentation of the raffinate stream.  Therefore the toxicity of ethyl acetate in presence of acetic acid was quantified at twelve different levels of ethyl acetate and acetic acid concentration combinations.  The synergistic inhibition study showed that ethyl acetate has higher inhibitory effect on the final ethanol concentration in presence of acetic acid.  2-stage liquid-liquid extraction with ethyl acetate removes 90 % of acetic acid from corn stover liquid hydrolysate and it reduces its content below the inhibition threshold but the ethyl acetate that remains in the aqueous phase is above the inhibition point.  Evaporation was used to both reduce the ethyl acetate content and make it available for reuse.  The fermentation results with NRRL Y-1546 show significant improvement after applying the extraction.  Specific production rate of ethanol, ethanol yield and its final concentration are increasing by 13.8, 10.6, and13.3 percent.

This study demonstrate that using ethyl acetate ,as an organic solvent to reduce the inhibitory effect on acetic acid, is a viable option to be incorporated in corn stover bioethanol production.