ARS | Publication request: Process design considerations for optimal production of ethanol from lignocellulose using available yeasts, including natural pentose-fermenting yeasts, and their derivatives

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: October 1, 2011
Publication Date: September 30, 2011
Citation: Slininger, P.J., Moon, J., Liu, Z. 2011. Process design considerations for optimal production of ethanol from lignocellulose using available yeasts, including natural pentose-fermenting yeasts, and their derivatives [abstract]. American Institute of Chemical Engineers. p. 40.

Technical Abstract: To expand the biomass to fuel ethanol industry, process strategies are needed to foster the production and utilization of microorganisms which can survive and ferment both hexose (C6) and pentose (C5) sugars while exposed to inhibitors (such as ethanol, furfural, and hydroxymethylfurfural, or HMF). Furfural and HMF are key byproducts of biomass pretreatment procedures to allow efficient enzymatic release of fermentable sugars during the hydrolysis of lignocellulose. The conversion of five-carbon xylose to ethanol and the inhibitor tolerance of the natural pentose-fermenting yeast Scheffersomyces (Pichia) stipitis NRRL Y-7124 has been optimized using a culture medium that supplied sufficient minerals and nitrogen as a mixture of urea and amino acids. During hydrolyzate fermentation, the switch from glucose to xylose uptake can result in diauxic lag unless steps are taken to prevent this. Priming yeast populations with a high xylose concentration was observed to induce faster fermentation rates in ethanol production fermentors and to eliminate diauxic lag during mixed sugar conversion by recycled S. stipitis populations. Ethanol concentrations circa 50 g/L have been found to repress induction of enzymes required for xylose-utilization. The process strategy of recycling xylose-primed cells was key to successful rapid utilization of high mixed sugar concentrations because specific enzymes for xylose utilization were induced before ethanol concentration climbed to an inhibitory level that would prevent xylose uptake. Using nutrition, culture priming, and cell recycle technologies, S. stipitis Y-7124 yields an economically recoverable 66 g/L ethanol in 48 h at a conversion efficiency of 88% (0.44 g ethanol/g sugar) from 95 g/L of glucose and 55 g/L xylose in defined media. Recent results of the application of these process strategies to the fermentation of lignocellulosic process streams in order to produce economically recoverable ethanol from biomass will be described.