Corn grain contains about 70% starch, 10 to 11% crude protein, 4.5 to 6.0% oil, 6% hemicellulose, 2 to 3% cellulose, 1% lignin, and 1% ash and is the dominant choice for current fuel ethanol production by dry or wet milling operations in the United States. In a typical dry mill, grain is milled to a powder, heated with water addition to about 85°C, mixed with alpha-amylase enzyme, held for up to an hour, heated further to 110 to 150°C to liquefy the starch and reduce bacteria levels, cooled back to about 85oC, and held with more alpha amylase for about 1 h. Then, the stream is cooled further, and gluco – amylase enzyme added to complete conversion to sugars known as dextrose. This overall saccharification operation occurs by the hydrolysis reaction:
in which C6H12O6 is a glucose sugar molecule formed when the alpha bonds linking n units of C6H10O5 in long chains of starch are broken and combined with n molecules of water, H2O. Yeast then ferment glucose to ethanol and carbon dioxide, just as in Eq. (1). The fermentation can be in batch operations held until the fermentation is complete in about 48 h or by continuous processes with ongoing sugar addition and takeoff of fermented broth known as beer. The beer containing about 12 to 14% or more ethanol is typically distilled to around 95% purity, close to the azeotropic composition at which further enrichment by ordinary distillation ceases, and this mixture is dehydrated with molecular sieves to anhydrous ethanol. The solids containing the remaining protein, oil, and fiber are dried to a 27% protein product known as distillers dried grains with solubles (DDGS) or just distillers dried grains (DDG), depending on whether process syrup is combined with the solids or not, and used in animal feeds. The carbon dioxide can be sold for carbonated beverages, dry ice, and other applications. Overall, about 460 to 490 liters (110 to 118 gallons) of ethanol weighing 365 to 390 kg (730 to 780 lbs), 357 to 335 kg (714 to 670lbs) of DDGS, and 350 to 370 kg (700 to 740 lbs) of carbon dioxide result from each metric tonne (per ton in parentheses) of corn processed—close to equal masses of each coproduct. Figure 2 presents a simplified block flow diagram for dry milling.
Although the greatest number of facilities are dry mills run by smaller producers, more ethanol is made in larger wet milling operations generally owned by larger companies. In wet mills, corn is first steeped and milled to separate the starch, protein, fiber, and germ, again as summarized in Fig. 2. Corn oil is removed from the germ for food, and the remaining germ meal is combined with the fiber and water from steeping as a 21% protein animal feed known as corn gluten feed. A portion of the corn protein is sold as corn gluten meal, a 60% protein product added to chicken and other animal feeds. Starch can be used directly for industrial uses and food or broken down by enzymes as for dry milling to make dextrose that can be enzymatically converted to high fructose corn syrup for a beverage sweetener or fermented to ethanol. One metric tonne (1 ton in parentheses) of corn can yield approximately 34 kg (67lbs) of oil, 52 kg (105 lbs) of corn gluten meal, 283 kg (567lbs) of corn gluten feed, 470 liters (112 gallons) or 370 kg (740 lbs) of ethanol, and 354 kg (708 lbs) of carbon dioxide. Again, all corn components are utilized, with protein sold for animal feed, the major market for corn.