2-05: Development of a Robust Yeast Biocatalyst for Low pH Lactic Acid and Cellulosic Ethanol Fermentation

Common characteristics are required for an economically viable biocatalyst for cellulosic ethanol and lactic acid production for commodity applications like Poly Lactic Acid (PLA). These include high yield, fast fermentation, robust growth in simple media, and tolerance to organic acids at low pH. Cargill started with non-conventional yeast naturally possessing some of these characteristics and successfully developed it to efficiently produce a new end product (lactic acid) or to ferment new sugars (pentoses). Replacing yeast’s ethanol pathway with lactic acid pathway was straight forward. Development of a strain capable of producing polymer grade lactic acid at commercially interesting titer, yield and productivity required concerted utilization of genome wide tools, targeted modifications, evolution and classical mutagenesis. The developed strain and low pH fermentation process offers considerable cost savings over conventional lactic acid processes.

Cargill has previously demonstrated efficient fermentation of xylose to ethanol in yeast (USPatentApp 10/554887). We are now combining our xylose fermentation technology into the acid tolerant yeast first developed for lactic acid production. Goals have been set for ethanol production from mixed sugars (dextrose, mannose, xylose, and arabinose) in the presence of 10 g/L acetate at 40^oC and at a pH less than 5.0. Under these conditions Cargill host can utilize 80 g/l of dextrose and 80 g/l of mannose in less than 36 hours, producing ~ 70 g/l ethanol. A xylose utilization pathway has been engineered into this host and efficient fermentation of xylose to ethanol demonstrated both in defined medium and in hydrolyzate.