Gevo, a NARA affiliated organization, has developed a fermentation and process technology that converts biomass sugars into isobutanol and further into biojet fuel through chemical processing. To convert the simple sugars found in wood to isobutanol, they rely on a technology called GIFT®, Gevo Integrated Fermentation Technology. In GIFT®, specialized yeast are used as a biocatalyst designed to ingest the simple sugars, produce isobutanol and then excrete the isobutanol into a broth. Natural yeast primarily produce ethanol during fermentation. The production of isobutanol over ethanol provides commercial advantages for fuel refiners.
Read Gevo’s latest white paper, Isobutanol for Transportation Fuels here.
Gevo’s work to producing isobutanol from wood sugars has been a multi-step process. The first task was to characterize the wood derived sugar solutions (commonly called hydrolysate) fed to the yeast biocatalysts. Liberating simple sugars from softwoods requires a pretreatment and enzymatic hydrolysis step. Pretreatment is intended to break apart the wood fibers so that the sugar polymers are exposed. The sugar polymers are then decomposed to sugar monomers by addition of commercial enzymes. Wood is naturally resistant to breakdown. Consequently, the pretreatment process involves harsh conditions that can produce a variety of compounds. Some of these compounds are toxic to yeast and can inhibit yeast growth and productivity.
Gevo scientists measured the amount and type of simple sugars and inhibitors present in hydrolysate from multiple Douglas-fir and forest residue samples subjected to different pretreatment protocols. Turns out that sugar and inhibitor yields vary significantly depending on the quality of the wood feedstock and pretreatment protocols.
Measuring yeast biocatalyst growth and isobutanol production
After the various hydrolysates were characterized, Gevo measured the growth and isobutanol production rates for their yeast biocatalyst at increasing concentrations of hydrolysate. Their results indicated that the pretreatment protocol has an effect on growth and isobutanol production rates and established the initial hydrolysate concentrations for optimal productivity. These results also contributed to NARA’s downselect decision to use the mild bisulfite pretreatment protocol over other pretreatments as the preferred pretreatment process to convert forest residuals into biojet fuel.
Adapting the yeast biocatalyst to Douglas-fir hydrolysate.
Taking advantage of yeast’s rapid growth rate and of the process of natural selection, Gevo is currently adapting their yeast biocatalyst to the Douglas-fir residual hydrolysate pretreated by the mild bisulfite protocol. The adaptation process involves isolating the best performing yeast strains over multiple generations. In the next two years, Gevo will contribute to NARA and scale-up and optimize the GIFT® fermentation process on Douglas-fir hydrolysate using the adapted yeast biocatalyst strains.
View a research poster on Gevo’s contribution to the NARA project here.