Effects of Amylose, Corn Protein, and Corn Fiber Contents on Production of Ethanol from Starch-Rich Media

Cereal Chem. 83(5):569–575 The effects of amylose, protein, and fiber contents on ethanol yields were evaluated using artificially formulated media made from commercial corn starches with different contents of amylose, corn protein, and corn fiber, as well as media made from different cereal sources including corn, sorghum, and wheat with different amylose contents. Second-order response-surface regression models were used to study the effects and interactions of amylose, protein, and fiber contents on ethanol yield and conversion efficiency. The results showed that the amylose content of starches had a significant (P < 0.001) effect on ethanol conversion efficiency. No significant effect of protein content on ethanol production was observed. Fiber did not show a significant effect on ethanol fermentation either. Conversion efficiencies increased as the amylose content decreased, especially when the amylose content was >35%. The reduced quadratic model fits the conversion efficiency data better than the full quadratic model does. Fermentation tests on mashes made from corn, sorghum, and wheat samples with different amylose contents confirmed the adverse effect of amylose content on fermentation efficiency. Hightemperature cooking with agitation significantly increased the conversion efficiencies on mashes made from high-amylose (35–70%) ground corn and starches. A cooking temperature of ≥160°C was needed on highamylose corn and starches to obtain a conversion efficiency equal to that of normal corn and starch. A great amount of research recently has been conducted to increase ethanol yield and conversion efficiency from starch-rich sources. For example, plant breeders have made a great effort to develop new and improved corn hybrids with higher starch content to increase ethanol yields (Bothast and Schlicher 2005). Wang et al (1997, 1998) studied the saccharification and fermentation characteristics of rye and triticale for ethanol production. The saccharification and fermentation efficiencies of oats, barley, wheat, and pearl millet have also been investigated (Thomas and Ingledew 1990, 1995; Thomas et al 1995; Sosulski et al 1997; Wu et al 2006). These authors reported conversion efficiencies of starch to ethanol in the above-mentioned cereal grains were ≈90%. The effects of other factors such as fermentation temperatures, free amino nitrogen, nitrogen sources, bacterial contamination, and preprocessing of feedstock on ethanol fermentation have also been investigated (Thomas and Ingledew 1990; O’Connor-Cox et al 1991; Jones and Ingledew 1994a,b; Sosulski et al 1997; Narendranath et al 2000). But the relationships among the chemical composition of grains and ethanol production have not sufficiently been studied. The major components of cereal grains are starch, protein, fiber, and lipids. The bioavailability of starch may differ among grain cultivars and may affect the conversion rate and final yield of ethanol (Moorthy 2002). Starch is a polymer of glucose, composed of various genetically determined ratios of amylose and amylopectin. Amylose is basically a linear polymer with ≈200 to 6,000 glucose units (MW 10–10) linked mainly by α-1,4 bonds (≈99%) and few α-1,6 bonds (<1%). Amylopectin, on the other hand, is a much larger and highly branched polysaccharide with up to 3×10 glucose units and a MW of ≈5×10 and linked by ≈95% α-1,4, and 5% α-1,6 bonds. In general, normal cereal starches contain 20–30% amylose and 70–80% amylopectin. Starches with <5% and >35% amylose are defined as waxy and high-amylose starch, respectively (Tester et al 2004b). Cereal cultivars with various amylose contents have been developed in corn, rice, wheat, barley, and sorghum (Jacobs and Delcour 1998; Tester et al 2004a,b; Goesaert et al 2005). Many researchers have studied the structure and physical properties of high-amylose starches. High-amylose starches had higher gelatinization temperatures (Shi et al 1998) and formed stronger gels (Case et al 1998). Starch gels with different amylose contents had different continuous matrix structure (Leloup et al 1991). Higher cooking temperatures and branched starch molecules could retard the reassociation of starch molecules, phase separation, and network development processes during cooling (Case et al 1998; Klucinec and Thompson 1999). The resistance of high-amylose starches to α-amylase was also investigated (Sievert and Pomeranz 1989, 1990; Richardson et al 2000; Brumovsky and Thompson 2001; Evans and Thompson 2004). They reported that the residual resistant starches found after amylolytic hydrolysis of gelatinized starches consisted mainly of retrograded amylose. Reid et al (1998) reported that the amylose-to-amylopectin ratio of starches significantly affected its fermentation to fatty acid by Clostridium butyricum, especially after pancreatin digestion and retrogradation. But there is no information about the effects of amylose content in starches and grains on the production of ethanol and other bioproducts. The objective of this study was to determine the effects of amylose contents of starches, protein, and fiber contents, as well as their interactions, on yeast fermentation of starchy materials to ethanol. MATERIALS AND METHODS Starch and Cereal Samples The starch samples used in this study were Amioca (essentially pure amylopectin), Melojel (≈28% amylose), Hylon-V (≈50% amylose), and Hylon-VII (≈70% amylose), which were of corn origin. They were kindly provided by the National Starch and Chemical Co. (Bridgewater, NJ). High-amylose (corn-70, corn55, and corn-35), normal, and waxy corn samples were obtained from Mark Campbell’s 2004 summer breeding nursery at the Truman State University Agricultural Research Farm at Kirksville, MO. Corn-70 represents an S5 inbred line derived from the 1 Contribution No 06-173-J from the Kansas Agricultural Experiment Station, Manhattan, KS 66506. 2 Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506. 3 Corresponding author. Phone: 785-532-2919, Fax: 785-532-5825. Email: dwang@ ksu.edu 4 USDA-ARS Grain Marketing & Production Research Center, Manhattan, KS 66502. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 5 Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506. 6 Department of Agronomy, Kansas State University, Manhattan, KS 66506. 7 Science Division, Truman State University, Kirksville, MO 63501. DOI: 10.1094 / CC-83-0569 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. AACC International, Inc., 2006.