Physical and Chemical Characterization of Fuel Ethanol Coproducts Relevant to Value-Added Uses

Cereal Chem. 87(5):439–447 One of the fastest growing industries in the United States is the fuel ethanol industry. In terms of ethanol production capability, the industry has grown by more than 600% since the year 2000. The major coproducts from corn-based ethanol include distillers dried grains with solubles (DDGS) and carbon dioxide. DDGS is used as a livestock feed because it contains high quantities of protein, fiber, amino acids, and other nutrients. The goal of this study was to quantify various chemical and physical properties of DDGS, distillers wet grains (DWG), and distillers dried grain (DDG) from several plants in South Dakota. Chemical properties of the DDGS included crude ash (5.0–21.93%), neutral detergent fiber (NDF) (26.32–43.50%), acid detergent fiber (ADF) (10.82–20.05%), crude fiber (CF) (8.14–12.82%), crude protein (27.4–31.7%), crude fat (7.4–11.6%), and total starch (9.19–14.04%). Physical properties of the DDGS included moisture content (3.54–8.21%), Aw (0.42–0.53), bulk density (467.7–509.38 kg/m), thermal conductivity (0.05–0.07 W/m·°C), thermal diffusivity (0.1–0.17 mm/sec), color L* (36.56–50.17), a* (5.2– 10.79), b* (12.53–23.36), and angle of repose (25.7–47.04°). These properties were also determined for DWG and DDG. We also conducted image analysis and size determination of the DDGS particles. Carbon group characterization in the DDGS and DDG samples were determined using NMR spectroscopy; O-alkyl comprised >50% of all DDGS samples. Results from this study showed several possibilities for using DDGS in applications other than animal feed. Possibilities include harvesting residual sugars, producing additional ethanol, producing value-added compounds, using as food-grade additives, or even using as inert fillers for biocomposites. The potential increase in the demand for ethanol as a fuel additive and as a source of alternate fuel has resulted in a radical transformation in agriculture throughout the United States. According to an RFA report, ≈15 billion bushels of corn was produced in 2009, out of which 4.2 billion bushels of corn went to the ethanol industry for bioethanol production (http://www.ethanolrfa.org). In 2009, 200 manufacturing plants in the United States had a total output production capacity of ≈34 billion L (9 billion gal) of ethanol (RFA 2009). Scientists estimated that >18 million metric tons of DDGS was produced in 2009. The amount of corn used for the ethanol production and the quantity of coproducts has increased 22-fold during past 20 years. Industrial processing of ethanol from corn is mainly classified into two types: wet milling and dry milling. Wet milling facilities are generally corporate-owned and have high operating costs. In these, starch is isolated in pure form due to fractionation of the corn kernel into starch, fiber, germ, and protein. Wet milling requires sophisticated equipment, high energy, and water consumption, and yields coproducts such as corn gluten feed (CGF), germ meal, corn gluten meal (CGM), and crude corn oil (Johnson and May 2003). The other process for obtaining ethanol from corn is dry milling. According to an RFA (2009) report, 85% production of ethanol comes from dry milling, while only 15% comes from wet milling. The dry milling process generally does not utilize fractionation (although that is beginning to change), and the primary coproduct is distillers dried grains with solubles (DDGS). DDGS is a dry granular form of the nonfermentable components after corn fermentation in bioethanol processing plants. The dry milling production process usually consists of several unit operations: grinding, cooking, liquefying, saccharifying, fermenting, and distilling the corn grain (Rosentrater 2006). More details about this process are available (Tibelius 1996; Weigel et al 1997; Jaques et al 2003). After distillation to remove the ethanol, the wet residuals are pressed or spun to remove excess water by centrifugation. Once a portion of the water is removed, the wet cake is mixed with condensed soluble materials and then dried. This final product is DDGS (Rosentrater 2006). The solubles are often referred to as “syrup” in the industry. This coproduct is high in vitamins, fat, and protein but low in fiber. Syrup yields a digestible energy value of ≈91% of that of raw corn (Buchheit 2002 [http://www.siu.edu/ ~readi/grains/factsheets/historyofethanolproduction.pdf]; Cruz et al 2005). It typically contains ≈28–46% dry matter, 6–21% (db) fat, 18–22% (db) protein, and 9–12% (db) minerals (Schingoethe 2001; Rosentrater and Muthukumarappan 2006). DDGS is used almost exclusively as livestock feed. Its nutritional components, product shelf-life, transportation, and flowability are vital considerations for overall feed quality. Changes in the final product quality affect the overall cost of DDGS and the economic viability of each ethanol plant. Research has been related to nutritional properties (Spiehs et al 2002), physical properties (Rosentrater 2006), and flowability properties of DDGS (Ganesan et al 2008a,b). DDGS has also been investigated as a protein-rich ingredient for aquafeeds (Chevanan et al 2007, 2008). Additionally, sorption isotherms for varying soluble solid levels and humidity levels were developed; this study observed that modified Halsey and modified exponential models performed well for isotherm data; however, the GMR model followed by a new modified exponential model were the best fit for DDGS (Ganesan et al 2007). Researchers have also worked with using flow agents in DDGS to minimize flow restrictions due to caking of particles (Ganesan et al 2008b). Thus various studies have been conducted on DDGS yet there are other areas in which DDGS could be used as a value-added product, in addition to its use as animal feed, e.g., removal of fiber from DDGS, biodiesel production from corn oil, biomass gasification, cellulosic degradation of DDGS (Bals et al 2006) for further ethanol production. However, to address these new areas, a complete understanding of physical and chemical properties of DDGS is required. Consequently, the objective of this study was to quantify various physical and chemical properties of DDGS, DWG, and DDG to establish a thorough understanding of these coproducts, which will produce novel uses for these materials. 1 South Dakota State University, Ag and Biosystems Engineering, Brookings, SD. 2 Agricultural and Bioprocess Engineer, North Central Agricultural Research Laboratory, USDA-ARS, Brookings, South Dakota. 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. 3 Corresponding author. Phone: 605-693-5248. Fax: 605-693-5240. E-mail address: [email protected] doi:10.1094 / CCHEM-02-10-0014 © 2010 AACC International, Inc.