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A Statistical Analysis of the Theoretical Yield of Ethanol from Corn Starch

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This paper analyzes the Illinois State Variety Test results for total and extractable starch content in 708 samples of 401 commercial varieties of corn. It is shown that the normally distributed extractable starch content has the mean of 66.2% and the standard deviation of 1.13%. The corresponding maximum theoretical yield of ethanol is 0.364 kg EtOH/kg dry corn, and the standard deviation is 0.007. In the ethanol industry units, this yield translates to 2.64 gal EtOH/nominal wet bushel, and the standard deviation is 0.05 gal/bu.

The U.S. ethanol industry consistently has inflated its ethanol yields by counting 5 volume percent of # 14 gasoline denaturant (8% of energy content) as ethanol. Also, imports from Brazil and higher alcohols seem to have been counted as U.S. ethanol. The usually accepted USDA estimate of mean ethanol yield in the U.S., 2.682 gal EtOH/bu, is one standard deviation above the rigorous statistical estimate in this paper.

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Notes

  1. Measuring extractable starch is a tricky problem (Eckhoff and others, 1996; Paulsen and others, 2003). Corn starch measurements in the Illinois data set are performed using the Corn Refiners Association (CRA) method. The extractable starch is from a 100-g test by Eckhoff (Eckhoff and others, 1996). Paulsen (Paulsen and others, 2003) uses the 100-g extractable starch test as a reference method for the NIR prediction of extractable starch. Sources: Darin Joos and Marvin Paulsen, 360-B Ag Engr. Sciences Bldg, 1304 W. Pennsylvania Ave, Urbana, IL 61801, 217-333-7926. Private communication, Feb. 8, 2006.

  2. Apparently, Farrell and others did not read this remark when they formulated their Response in Science (2006a, 2006b, 2006c): ``… Nonetheless, much of his analysis (i.e., this paper, TWP) seems to be rigorous in detail but erroneous overall. For instance, extractable starch only applies to wet milling, which presently produces approximately 30% of U.S. ethanol. Almost all new ethanol plants are dry mills, for which total fermentable starch is a better measure of ethanol yield, and that yield at least 5% more ethanol per unit mass of corn than wet milling."

  3. Marvin Paul Scott, USDA-ARS, Private communication, Feb. 1, 2006.

  4. David Andress of EIA has observed that the corn starch analysis in this paper considers all corn samples, but ethanol plant operators attempt to purchase high extractable starch corn. This can be accomplished by segregation or the use of inexpensive testing procedures. The difference in extractable starch is on the order of several percent. I will investigate his observation if I pursue this subject further (Source: private comm., 22 Feb. 2006.)

  5. This usually is done by dumping bad mash onto the surrounding land. As reported by PERRY BEEMAN of The Des Moines Register, 11 Sept 2005: ``Iowa plants - which produce a third of the nation's ethanol supply - have sent syrup, batches of bad ethanol and sewage into streams. As the pollutants decomposed, the waters lost oxygen, threatening fish. … Gieselman said inspectors discovered plant construction contractors were telling farmers the plants wouldn't discharge into waterways. ``They do," Gieselman said. The state has forced many of the plants to install holding ponds so the pollutants decompose or settle out before the water flows into streams used by fishing enthusiasts, canoeists and thirsty livestock."

  6. Under current rules, plants are classified as ``major sources" of pollution and forced to go through the more-cumbersome approval process if they emit in excess of 100 tons per year of a particular pollutant, such as nitrogen oxide, dust, or volatile organic compounds such as formaldehyde. The EPA this week proposed to boost that cap to 250 tons per year. The agency said the higher cap is justified because that is the limit for grain-processing facilities that make alcohol along with a range of food products. The typical ethanol plant in Iowa puts out 300 to 350 total tons of pollutants per year, including nitrogen oxide, sulfur dioxide and various volatile organic compounds, according to the Iowa Department of Natural Resources. Source: The Des Moines Register, Des Moines, IA. 6 March 2006.

  7. As of 7 Feb. 2006, the data shown in Table 2 are available at http://www.ksgrains.com/ethanol/useth.html. The newly redesigned website of the Renewable Fuels Association, http://www.ethanolrfa.org. no longer posts the corn bushels used to produce ethanol. Therefore Table 2 could not be extended beyond 2004.

  8. Between 1 January and 1 November 2004, the U.S. ethanol imports from Brazil were 334.531,283 kg or ∼0.142 billion gallons of denatured ethanol per year. These Brazilian imports accounted for ∼5% of ethanol produced in the U.S. in 2004. Source: Balan$cLo_Exp_Imp_Etanol_Brasil_EUA_1981-2004-DATAGRO.xls. an Excel spreadsheet with the U.S. - Brazilian balance of ethanol trade between 1981 and 2004, sent to me by Mr. Juan M. Granados of Biotrade USA, Inc., 17 March 2006.

  9. ASTM D5798-99 Standard Specification for Fuel Ethanol for Automotive Spark-Ignition Engines allows for up to 2% by volume of higher aliphatic alcohols (C3-C8), see Table 3 in http://www.nrel.gov/does/fy02osti/30849.pdf.

  10. States no longer will have to add corn-based ethanol or MTBE to gasoline to fight pollution---a requirement that costs as much as 8 cents a gallon---under rules announced on 15 February 2006, by the Environmental Protection Agency. They eliminate a mandate from the 1990 Clean Air Act that gasoline used in metropolitan areas with the worst smog contain 2% oxygen by weight. The law did not say which oxygenate must be used, but most refiners use either ethanol or methyl tertiary butyl ether, known as MTBE. Source: Associated Press, 15 Feb. 2006.

  11. (Source: Potential Supply Impacts of Removal of 1-Pound RVP Waiver, September 2002, Office of Oil and Gas of the EIA, Mary J. Hutzler (202-586-2222, mhutzler@eia.doe.gov). The EIA study was requested by Senator Jeff Bingaman, Chairman of the Senate Committee on Energy and Natural Resources).

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  1. Department of Civil and Environmental Engineering, 425 Davis Hall, MC 1716, University of California, Berkeley, CA, 94720, USA

    Tad W. Patzek

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Patzek, T. A Statistical Analysis of the Theoretical Yield of Ethanol from Corn Starch. Nat Resour Res 15, 205–212 (2006). https://doi.org/10.1007/s11053-006-9022-5

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