Adaptation of Yeast to Inhibitors and Ethanol Fermentation of Lignocellulosic Substrates

Daehwan Kim, Purdue University

Abstract

Lignocellulosic ethanol production is considered to be a significant source of liquid fuels and is being developed as a renewable energy source for blending with gasoline, thereby reducing petroleum consumption and decreasing CO2 emissions. Since plant dry matter (i.e. corn stover, sugar cane bagasse and hardwood) consists of recalcitrant structural polymers including hemicellulose, cellulose, and lignin, pretreatment of biomass is an essential process to disrupt the cell wall structure and expose cellulose for subsequent hydrolysis. Pretreatment exposes or removes the closely associated components, hemicellulose and lignin, expose cellulose to enzyme digestion. The formed sugars are then fermented to ethanol. However, a diverse mixture of compounds are released during the pretreatment, and depending on substrate property and conditions of pretreatment, small molecules that cause severe enzyme inhibition or deactivation and/or decrease ethanol fermentations, may be released. Phenols and furan derivatives are well known inhibitors for enzyme hydrolysis and/or fermentation performance. Research has been performed to minimize these compounds, deveop strategies for minimizing enzyme inhibition, and develop microorganisms tolerance to soluble lignocellulose-derived inhibitors. This dissertation examines the mitigation of inhibition using three different approaches with corn stover, hardwood and corn pericarp, respectively. (1) For corn stover, the removal of inhibitors through a biological detoxification was carried out before the fermentation using fungus Coniochaeta ligniaria NRRL30616 which can metabolize and eliminate inhibitory compounds in the liquid hot water pretreated corn stover liquors. Alleviating toxic compounds (mainly phenols) followed with either enzyme or acid treatment for removing oligosaccharides (mainly xylo-oligomers) improved enzyme hydrolysis of the cellulose and fermentation performance. (2) For corn pericarp, fractionation and separation of the corn kernels in the enzyme-based concept were pursued to obtain the corn pericarp. The corn pericarp was utilized as a substrate to hydrolyze in the enzyme preparations to produce fermentable sugars and determine its fermentabilities. The formation of the inhibitory compounds from the fractionated corn pericarp was found to be closely related to its particle size, and the accumulation of total phenols resulted in the significant decrease in enzymatic hydrolysis. Moreover, potential inhibitory effects of hydrolysate solids found to be relieved by photochemical oxidation of the solids in the air. Successful ethanol production from the pericarp showed the design of process for substitutive glucose source that can convert to add-value molecules. (3) Adaptation of a Saccharomyces cerevisiae NRRL-Y1546 strain to inhibitory compounds medium (MS929, steam-pretreated hydrolysate solution, obtained from Mascoma Corporation) was used to determine whether adapted yeast had a tolerance to fermentation inhibitors, was able to ferment glucose to ethanol. The fermentations using the adapted yeast showed a greater resistance and a decreased fermentation lag phase compared to the results with non-adapted strain.

Degree

Ph.D.

Advisors

Ladisch, Purdue University.

Subject Area

Alternative Energy|Engineering|Agricultural engineering

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