**1. Introduction**

We face many challenges in our society, due especially to growing population pressures and increased economic mobility. These can result in increased demands for food, clothing, housing, and consumer goods. Additionally, there has been a growing need for energy during the last several decades, which historically has been met primarily by use of fossil fuels. In the U.S., transportation fuels generally account for about 1/3 of all energy consumed. Of this, about 90% comes from fossil sources. Between 1/2 and 2/3 of the total U.S. demand for petroleum has been met by imports during the last 30 years [1]. Many argue that this scenario is not sustainable in the long run, and other energy alternatives are needed. During 2005-2010, the U.S. experienced some of the highest growth rates ever seen in the domestic biofuels industry.

The U.S. biofuels industry has recently grown as a response to increasing energy needs and energy prices. Bio-based fuels can theoretically be manufactured from many biological materials. Biochemical conversion of carbohydrates into ethanol and lipids into biodiesel are the two most common routes at this point in time. This includes most cereal grains (e.g., corn, barley, wheat, sorghum, rice, etc.), oilseeds (e.g., soybeans, sunflowers, flax, rapeseed, and others), native prairie grasses (including miscanthus, switchgrass, prairie cord grass, reed canary grass, and other grasses), agricultural residues (including corn stover, rice husks, wheat straw, and other crop residues), algae, municipal solid wastes, food processing wastes, and other biological wastes and substrates [2, 3, 4, 5, 6, 7, 8, 9, 10]. Right now, however, the most heavily used feedstock for biofuel production in the U.S. is corn grain. Industrial-scale alcohol manufacturing from corn starch is readily accomplished, and at a low cost (generally about \$1/gal), which is considerably lower than other biological material conversions in the U.S.

In recent years, the corn-based fuel ethanol industry in the U.S. has reached a scale which can impact the nation's supply of transportation fuels. Only during the last decade, however, has

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this industry become visible to the average citizen. This has been due, in part, to the growing demand for domestic transportation fuels for national security, escalating prices at the fuel pump, positive economic effects throughout rural America, as well as questions and contro‐ versies surrounding the production and use of corn ethanol. While fuel is the main aim of the ethanol industry, nonfermentable coproducts are also generated during manufacturing. The primary coproduct is distillers dried grains with solubles (DDGS) (Figure 1), which is the focus of this chapter.

**Figure 1.** Corn-based Distillers Dried Grains with Solubles (DDGS) is currently the most common coproduct available from U.S. fuel ethanol plants (Photo courtesy of Rosentrater).

To help meet the increasing demand for transportation fuels, and to meet federal U.S. man‐ dates, the number of ethanol plants has rapidly increased in recent years, as has the quantity of fuel ethanol and coproducts produced (Figure 2). As of 2015, the RFS (Renewable Fuel Standard) mandates the use of 15 billion gal/y (56.8 billion L/y) of corn-based ethanol in the U.S. [13]. At the beginning of 2015, there were 213 fuel ethanol plants in the U.S., which produced nearly 56.8 billion L/y (15.0 billion gal/y) [12, 15]. As the biofuels industry continues to evolve, cellulosic and other bio-based fuels will gain prevalence in coming years.

Most new ethanol plants have been dry-grind facilities (Figure 3), which will be discussed subsequently. As production volume has increased, the processing residues, known collec‐ tively as "distillers grains" – have increased concomitantly (Figure 2). Between 35 and 40 million metric tonnes (t) of distillers grains (both wet and dry) will be produced each year by the U.S. fuel ethanol industry over the next several years. In recent years there has actually been a slight decline in distillers grains production, due to evolving processes which now extract oil from the coproduct streams. This will be discussed at a later point in the chapter.

As the industry has grown over the last few decades, the consumption of corn has grown as well (Figure 4). In recent years, over 30% of the U.S. corn crop is used to produce fuel ethanol. When examining these data, however, it is important to note several key points: most corn in the U.S. is not human food-grade corn, exports have been relatively constant over time, there has only been a slight decline in the corn used for animal feed, but the overall quantity of corn which is produced by U.S. farmers has been greatly increasing over time. The corn used to manufacture fuel ethanol has arisen from the growing U.S. supply of corn. Furthermore, and more importantly, the corn which has been used to produce ethanol (instead of animal feed) is generally replaced by DDGS and other ethanol coproducts in these animal feeds. Ethanol coproducts (primarily DDGS and DWG) are key to the long-term viability of the fuel ethanol industry as well as the various livestock industries. Thus feed and fuel can be produced simultaneously, contrary to what may be discussed in the media.
