Process strategies to maximize lipid accumulations of novel yeast in acid and base pretreated hydrolyzates

Oleaginous yeasts can accumulate up to 70% of cell biomass as lipid, predominantly as triacylglycerols.  Yeast lipid fatty acid profiles have been reported to be similar to that of vegetable oils and consist primarily of oleic, palmitic, stearic and linoleic acids. This capability provides the opportunity to use yeasts to produce bio-based fuels and chemicals from agricultural residues and bioenergy crops, such as corn stover and switchgrass, respectively.  Successful cultivation of these yeasts on inhibitory lignocellulosic hydrolyzates is uncertain because little has been published regarding their application to bioconversion of plant biomass.  Novel yeasts producing promising lipid concentrations (>10 g/L) from enzyme hydrolyzates of acid and base pre-treated lignocellulosic biomass were discovered in a prior screening of the ARS Culture Collection (NCAUR, Peoria, IL).  Process strategies for maximizing lipid production in one of the more benign hydrolyzates, AFEX-pretreated corn stover, and one of the more toxic hydrolyzates, dilute acid-pretreated switchgrass, respectively, were employed.  Hydrolyzates were prepared at up to 20% solids loading to provide over 100 g/L sugars (at ~60:40 mass ratio glucose:xylose).  Variables examined included hydrolyzate concentration, nitrogen source, C:N ratio, pH, temperature and process train design.  Depending upon plant biomass yields, we estimated that oleaginous yeasts have the capability of producing ~48 and 190 gal oil per acre from corn stover and switchgrass, respectively.  As a frame of reference, ~68 gallons of oil per acre are produced from processing soybeans.  Results are discussed with respect to advances made and limitations still to be addressed in future work.