Corn (Zea mays) is the most employed feedstock in the world to produce starch either for the food industry or for ethanol production. Corn is a crop whose origin is in the Americas. Its stalks can reach 4 m in height and does not have branches. Its seeds are the structure with the highest value of starch in this plant and are the base for human food in many communities in Central and South America. The starch accumulates in the endosperm of corn seeds. Corn requires a temperature of 25 to 30°C and an important degree of sunlight for its growth. Corn can toler­ate minimum temperatures down to 8°C. At 30°C, the adsorption of nutrients and water can be difficult. This crop is exigent in water, but adapts itself very well to all kind of soils preferring those with a pH of 6 to 7. Similarly, corn requires deep soils rich in organic matter (Infoagro, 2007).

Corn is the most cropped grain in the world, followed by wheat and rice. The United States is the major corn producer, followed by China and Brazil (FAO, 2008), as shown in Table 3.7. The United States reported the planting of 35.02

million ha in 2007; China, 28.07 million ha; and Brazil, 13.82 million ha. The U. S. yield is very high (about 9.48 ton/ha), although Kuwait has the maximum yield (21.0 ton/ha), but it has a very small annual production: 1,050 ton in 2007 (FAO, 2008). In the United States, both private and public sectors have been sys­tematically investing in research and development of corn cropping oriented to the production of sweeteners for the food sector and ethanol for transportation sector. In addition, the U. S. government has granted certain tax exemptions to farmers in order to support corn production, which is considered a strategic crop. It should be noted that the powerful lobby of corn producers has confronted the big oil industry supporting the implementation of renewable fuels from corn. In fact, most ethanol-producing plants in the United States employ corn as a feedstock. In 2007, 22.5% of U. S. corn production was dedicated to ethanol production (FAO, 2008; Renewable Fuel Association, 2009; Sanchez and Cardona, 2008b). China also has been increasing its ethanol production from grains including corn. This fact, along with the expected increase in ethanol production in the United States, has been pushing corn prices upward. This has had direct effects on the costs of corn-containing foods in such importing countries, such as Mexico.

Starch is the corn component that is directly used for ethanol production. Likewise, starch is corn’s main component, as seen in Table 3.8 from data com­piled by Cardona et al. (2005). There exist two technologies for using corn starch as a feedstock for fuel ethanol production. One technology employs the separation of starch from all the other components of the corn kernel and this is called wet milling. In this way, during the wet milling process, the corn kernel is separated into its components: starch, fiber, gluten, germ, and oil. The other technology does not involve the separation of starch. By contrast, the whole milled grain enters into the ethanol production process directly (dry milling).

The corn wet milling process allows the production of a series of value-added co-products that offset to a certain degree the fuel ethanol production costs. These co-products are directly related to the structure of corn grain. Starch represents 60% of the grain and is located in the endosperm. Grain proteins are concen­trated in the gluten. Corn oil is located in the germ and represents 4% of the grain

(Gulati et al., 1996; U. S. Grains Council, 2007). The overall wet milling process is depicted in Figure 3.5. The first step in the corn wet milling process is the steeping of the grains in large tanks that can process from 1,500 to 6,000 bushels of corn, according to the typical volumes of starch industry in the United States (U. S. Grains Council, 2007). Steeping is carried out during 30 to 50 h at 49 to 54°C in water containing 0.1 to 0.2% sulfur dioxide. The sulfurous acid that is formed contributes to the separation of the starch and insoluble proteins, break­ing the protein matrix of the endosperm through the destruction of disulfur bonds in gluten proteins. During this step, about 6% of dry matter is dissolved in the liquid containing the corn steep liquor. These dissolved components provide the nutritional value to the corn extractives that are condensed and fermented after the partial dehydration of steeping in water.

After grain steeping, the swelled corn grain contains about 45% water. This soft grain is ground and the germ is removed by flotation. The germ is cracked and its oil is extracted using hexane as a solvent. The corn oil is refined while the solid residue of the germ is dried to prepare the corn germ meal. Once the germ is removed, the resulting material undergoes milling that crushes starch particles and releases the fiber. This fiber is separated from the starch and gluten proteins by using a screen. The thick mixture of starch and gluten is pumped to a rotary disk column where these two components are separated by applying centrifugal force. In this centrifugal separator, the product that is obtained contains approxi­mately 60% protein. This product is concentrated, filtered, and dried to obtain the corn gluten meal. The starch is separated again to reduce its protein content down to 0.3% (U. S. Grains Council, 2007). The fiber in the first part of milling is added to the evaporated corn steep liquor. The stream from this process evaporates in order to produce the corn gluten feed, as illustrated in Figure 3.5.

The co-products obtained are mostly intended for animal feed. The corn gluten meal has a high protein and energy content and is a good source of the amino acid methionine. This co-product is used for cattle feed as a protein supplement (about 60% protein) as well as for feeding poultry and pigs. Corn gluten feed contains about 20% protein and a reduced amount of oil and fats. This material is also used as a protein supplement, although it has a lower nutritive value in comparison to corn gluten meal. It can be used for poultry and pig feed and even for ruminants due to its significant content of digestible fiber. Corn oil is utilized for human food. In general, from each 100 kg of corn processed by this technology, 2.87 kg corn oil, 4.65 kg corn gluten meal, and 24.09 kg corn gluten feed can be obtained. According to the U. S. Department of Agriculture, the average price per tonne of these products is as follows: US$357 for corn gluten meal, US$88 for corn gluten feed, and US$662 for corn oil (Bothast and Schlicher, 2005).