GENOMICS AND ENGINEERING OF STRESS-TOLERANT MICROBES FOR LOWER COST PRODUCTION OF BIOFUELS AND BIOPRODUCTS
Location: Crop Bioprotection Research
Title: Lignocellulosic biomass conversion to ethanol by Saccharomyces
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 4, 2007
Publication Date: February 1, 2008
Citation: Liu, Z., Saha, B.C., Slininger, P.J. 2008. Lignocellulosic biomass conversion to ethanol by Saccharomyces. In: Wall, J., Harwood, C., Demain, A., editors. Bioenergy. Chapter 4. Washington, DC: ASM Press. p. 17-36.
As interest in alternative energy sources rises, the concept of agriculture as an energy producer has become increasingly attractive (Outlaw et al. 2005). Renewable biomass, including lignocellulosic materials and agricultural residues, are low-cost materials for bioethanol production (Bothast and Saha 1997; Wheals et al. 1999; Zaldivar et al. 2001). In the U.S., the production of corn grain based ethanol reached five billion gallons in 2006, a fraction of the 140 billion gallons of transportation fuel used annually. The goal is to displace 30% of the Nation’s 2004 motor gasoline use with ethanol by 2030; and this will require production levels equal to roughly 60 billion gallons a year. If all corn grain now grown in the U.S. is converted to ethanol, it can satisfy approximately 15% of current gasoline needs. Thus, developing ethanol as fuel, beyond its current role as fuel oxygenate, will require developing lignocellulose as feedstock because of its abundance. In particular, various agricultural residues (corn stover, wheat straw, rice straw), agricultural processing byproducts (corn fiber, rice hulls, sugar cane bagasse), and energy crops (switchgrass) can be used as low-cost sources of sugars for biofuel production. At present, conversion of lignocellulosic biomass to fermentable sugars represents significant technical and economic challenges; and its success depends largely on the development of effective pretreatment as well as highly efficient and cost-effective enzymes for conversion of pretreated lignocellulosic substrates to fermentable sugars. Lignocellulosic biomass generates a mixture of hexose and pentose sugars upon pretreatment itself or in combination with enzymatic hydrolysis. Although traditional S. cerevisiae ferments glucose to ethanol rapidly and efficiently, it cannot ferment pentose sugars (xylose, arabinose) to ethanol. Some yeasts (Pachysolen tannophilus, Pichia stipitis, Candida shehatae) have the capability to ferment xylose to ethanol (Bothast and Saha, 1997; Du Preez, 1994; Hahn-Hägerdal et al. 1994; Jeffries and Jin, 2004; Prior et al. 1989; Slininger et al. 1987). These yeasts have low ethanol tolerance and slow rates of fermentation. Xylose can be converted to xylulose using the enzyme xylose isomerase, and S. cerevisiae can ferment xylulose to ethanol (Gong et al. 1981). However, the process is not cost-effective. S. cerevisiae has been characterized as the best yeast for fermentation of hexose sugars present in lignocellulose-derived hydrolysates due to its ethanol-producing capacity and high inhibitor tolerance. It has been used extensively to test the fermentability of hydrolysates. For future sustainable and cost-efficient lignocellulosic biomass conversion to ethanol, there exists two major challenges, heterogeneous sugar utilization and stress tolerance in engineering microbial catalytic fermentors for bioethanol production. In this chapter, the authors review the current knowledge on the composition and structure of lignocellulosic biomass, pretreatment and enzymatic saccharification to simple sugars, and strain development of Saccharomyces cerevisiae for efficient fermentation of the biomass driven sugars to ethanol.