ARS | Publication request: Characterization of Corn Grains for Ethanol Production

Submitted to: Journal of ASTM International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 18, 2009
Publication Date: February 1, 2010
Citation: Srichuwong, S., Gutsea, J., Blanco, M.H., Duvick, S.A., Gardner, C.A., Jane, J. 2010. Characterization of Corn Grains for Ethanol Production. Journal of ASTM International. 7(2):1-10.

Interpretive Summary: Corn is the primary crop used for ethanol production in the United States. The production of ethanol involves dry-grinding of corn kernels which breaks the seed coat (pericarp) and disrupts the protein matrix, releasing free starch granules. The dry-grind starch is cooked, and treated with enzymes and yeast to convert (ferment) sugars to ethanol. Corn varieties are known to differ in starch properties such as granule structure and composition which influence ethanol content. Knowledge of starch structural attributes that influence ethanol yield and conversion efficiency of sugars to ethanol is important in order to develop varieties for improved ethanol production. Four GEM inbred lines were analyzed for starch composition, structural and gelatinization properties, and enzymatic digestibility as measured by conversion of starch to glucose. Differences among the four inbred lines were found for the starch traits studied. The inbred with the highest ethanol yield also had the highest starch content, and the smallest protein and lipid content associated with starch granules. Low gelatinization temperature and shorter amylopectin (starch molecule) branch length resulted in greater conversion efficiency of starch to ethanol. The impact of this research may be valuable to plant breeders for identifying and utilizing germplasm for starch traits which support increased ethanol production, and for enhancing the germplasm base.

Technical Abstract: Objectives of this study were to understand how the composition of corn kernels and starch structure affect enzyme hydrolysis of starch in dry-grind corn and ethanol yield from yeast fermentation. Four selected corn inbred lines were used in this study. Starch in uncooked dry-grind corn samples showed higher enzyme digestibility than did the uncooked starch isolated from the same source by wet-milling process. The increased digestibility of starch in uncooked dry-grind corn correlated with physical damage of starch granules. In contrast, starch in cooked dry-grind corn samples displayed less enzyme digestibility than did the cooked, isolated-starch samples. The difference could be attributed to interference caused by non-starch components in dry-grind corn. Entrapment of starch in the protein matrix and the formation of amylose-lipid helical complexes and/or retrograded starch may decrease the enzymatic digestibility of starch in cooked, dry-grind corn. Lab-scale ethanol production showed that ethanol yield after 72 h fermentation of the four corn inbred lines ranged between 34.3 and 38.0 g ethanol/100g dry-grind corn. The conversion efficiency at 72 h of fermentation ranged between 86.8 and 90.3% of the theoretical ethanol yield. The highest ethanol yield was found in the corn line with the highest starch content and the lowest lipid and protein content.