UC Riverside researchers have developed a streamlined process that could finally make the ethanol production cost from abundant “second generation” plant wastes competitive with “first generation” ethanol made from sugars.
A major historical barrier to low-cost production of ethanol from biomass is the low ethanol yields and titers that result from fermentation of biomass solids at high solids when compared with simple sugar fermentations. The UCR team showed that combining its cosolvent-enhanced lignocellulosic fractionation (CELF) pretreatment process with subsequent simultaneous saccharification and fermentation (SSF) can achieve similar high ethanol yields and titers that match that of separate pure glucose fermentations. The research is published in the Proceedings of the National Academy of Sciences (PNAS).
CELF augments dilute acid pretreatment with tetrahydrofuran (THF) as a water cosolvent. THF promotes delignification of biomass and hydrolysis of recalcitrant cellulose in water to produce a highly digestible glucan-rich solid.
The SSF strategy has been explored extensively before, as it results in a simpler process, and potentially reduces the enzymes needed to digest the solid material. However, ethanol yields from SSF strategies in the past have been too low, with a limited concentration of ethanol. Using CELF, researchers can pretreat biomass such as corn stover and produce a sugar-rich and highly digestible biomass that—using the SSF strategy—can be converted to ethanol while also maintaining high ethanol yields.
In the study, fed-batch glucose fermentations by Saccharomyces cerevisiae D5A revealed that this strain—which has been favored for SSF—can produce ethanol at titers of up to 86 g⋅L−1. Optimizing SSF of CELF-pretreated corn stover achieved unprecedented ethanol titers of 79.2, 81.3, and 85.6 g⋅L−1 in batch shake flask, corresponding to ethanol yields of 90.5%, 86.1%, and 80.8% at solids loadings of 20.0 wt %, 21.5 wt %, and 23.0 wt %, respectively.
Ethanol yields remained at more 90% despite reducing enzyme loading to only 10 mg protein⋅g glucan−1, revealing that the enduring factors limiting further ethanol production were reduced cell viability and glucose uptake by D5A and not loss of enzyme activity or mixing issues.
The UCR team achieved its maximum ethanol concentrations—similar to those produced from the expensive refined sugar of food crops—while saving more than 50% in enzyme costs over other SSF strategies.
The burden of further improving ethanol yields now depends on genetically modifying the yeast to tolerate higher concentrations of ethanol. The yeast dies from the high ethanol concentrations in this system.
This research was funded by the National Science Foundation and the BioEnergy Science Center through the Department of Energy’s Office of Science.
Thanh Yen Nguyen, Charles M. Cai, Rajeev Kumar, and Charles E. Wyman (2017) “Overcoming factors limiting high-solids fermentation of lignocellulosic biomass to ethanol” PNAS 114 (44) 11673-11678 doi: 10.1073/pnas.1704652114