Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/17/2008
Publication Date: 10/22/2008
Citation: Saha, B.C., Cotta, M.A. 2008. Fuel ethanol production from agricultural residues: current status and future prospects [abstract]. Journal of Biotechnology. 136(S):S285-S286. Interpretive Summary:
Technical Abstract: In 2007, about 6.5 billion gallons of ethanol were produced from corn starch in the U.S. Various agricultural residues such as corn stover, wheat straw, rice straw, and barley straw can serve as low-cost lignocellulosic feedstocks for conversion to fuel ethanol. These residues contain both cellulose and hemicellulose as complex carbohydrates. The conversion of these feedstocks to ethanol involves four process steps: feedstock pretreatment, enzymatic saccharification, fermentation, and ethanol recovery. Any lignocellulosic biomass is resistant to enzymatic saccharification in its native state. These residues need to be pretreated to facilitate the decrystallization of cellulose. Dilute acid pretreatment at a high temperature has become a state-of-the-art technology to pretreat any lignocellulosic feedstock. It helps to solubilize and breakdown hemicellulose to mainly pentose sugars and to decrystallize cellulose to a certain extent which can then be broken down to glucose enzymatically. However, it also produces compounds inhibitory to fermentative microorganisms depending upon the severity of pretreatment. Other pretreatment options include autohydrolysis, alkali, and alkaline peroxide pretreatment. Each of these pretreatment methods has its own distinct advantages and disadvantages. A number of enzymes are involved in the saccharification of cellulose and hemicellulose to simple sugars. Product inhibition, low specific activity, slow rate of reaction, and high cost of cellulases are some of the barriers to commercial development of enzymatic hydrolysis of cellulose. Research has been carried out to develop efficient pretreatment methods which can help enzymes breakdown the complex carbohydrates to sugars without generating compounds inhibitory to fermentative microorganisms and to develop improved enzymes for use in saccharification. Upon hydrolysis, any lignocellulosic biomass generates a mixture of sugars such as glucose, xylose, arabinose, and galactose. Efficient conversion of all these sugars to ethanol is essential in order to reduce the production cost. Conventional yeast (Saccharomyces cerevisiae) can ferment glucose to ethanol very efficiently but it cannot convert pentose sugars to ethanol. During the last 20 years, a number of recombinant microorganisms have been developed for simultaneous conversion of both hexose and pentose sugars to ethanol. Integration of process steps in a simultaneous enzymatic saccharification and fermentation (SSF) is also critical for process economics. It is possible to produce about 60-80 gallons of ethanol per ton of agricultural residues. In this presentation, current state of technology development and prospects for commercial production of ethanol from lignocellulose will be discussed.