For the past two centuries in the United States, corn refiners have been devel­oping, improving, and perfecting the process of separating corn into its com­ponent parts to create a variety of value-added corn products and by-products. The corn wet milling process separates corn into its four basic components, viz., starch, germ, fiber, and protein. There are eight basic steps involved to accom­plish this corn refining and alcohol fermentation process [20].

1. First, the incoming corn is visually inspected and cleaned. Corn refin­ers use #2 yellow dent corn, which is removed from the cob during harvesting. One bushel of yellow dent corn weighs about 56 pounds on average. Refinery people inspect arriving corn shipments and clean them two or three times to remove cob, dust, chaff, and any other foreign unwanted materials before the next processing stage of steeping. An effective screening process can save a great deal of trouble in the subsequent stages. The inspected and screened corn is then conveyed to storage silos holding up to 350,000 bushels.

2.

Second, it is steeped to initiate polymeric bond cleavage of starch and protein into simpler molecules. Steeping is typically carried out in a series of stainless steel tanks. Each steep (or steeping) tank may hold about 2,000-13,000 bushels of corn soaked in water at 50-52°C for 28-48 hours. During steeping, the kernels (as shown in Figure 3.5) absorb water, thereby increasing their moisture levels from 15% to 45% by weight and also more than doubling in size by swelling [20]. The addition of 0.1% sulfur dioxide (SO2) to the water suppresses excessive bacterial growth in the warm water environment. As the corn swells and softens, the mild acidity of the steeping water begins to loosen the gluten bonds within the corn and eventually release the starch [20]. A bushel is a unit of volume measure used as a dry measure of grains and produce. A bushel of corn or milo weighs about 56 pounds, a bushel of wheat or soybeans weighs about 60

FIGURE 3.5

Corn kernel.

pounds, and a bushel of sunflowers weighs about 25 pounds. Or, a U. S. bushel is equivalent to 35.23907 liters as a volume unit.

3. The third step is the germ separation. It starts with coarse grind­ing of the corn in the slurry to separate/break the germ from the rest of the kernel. The germ is the embryo of a kernel of grain, as shown in Figure 3.5. This germ separation is accomplished in cyclone separators, which spin the low-density corn germ out of the slurry. Therefore, this cyclone separator is called a germ separa­tor. It is also called a degerminating mill. The germs, which contain about 85% of corn’s oil, are pumped onto screens and washed repeat­edly to remove any starch left in the mixture [20]. A combination of mechanical and solvent processes extracts the oil from the germ. The oil is then refined and filtered into finished corn oil. The germ resi­due is saved as another useful component of animal feed. Both corn oil and germ residues are important by-products of this process.

4. As the fourth step, the remaining slurry, consisting of fiber, starch, and protein, is finely ground and screened to separate the fiber from the starch and protein. After the germ separation step described in Step 3, corn and water slurry goes through a more thorough grind­ing in an impact or attrition-impact mill to release the starch and gluten from the fiber in the kernel. The suspension of starch, glu­ten, and fiber flows over fixed concave screens, which catch fiber but allow starch and gluten to pass through. The fiber is collected, slur­ried, and screened again to reclaim any residual starch or protein, then piped or sent to the feed house as a major ingredient of animal feed. The starch-gluten suspension, called mill starch, is piped or sent to the starch separators [20].

5. Fifth, starch is separated from the remaining slurry in hydrocyclones. By centrifuging mill starch, the gluten is readily spun out due to the density difference between starch and gluten. Starch is denser than gluten. Separated gluten, a type of protein composite, can be used for animal feed. Corn gluten meal (CGM) is a by-product of corn processing and is used as animal feed. CGM can also be used as an organic herbicide. The starch, now with just 1-2% protein remain­ing, is diluted, washed 8 to 14 times, rediluted and rewashed in hydrocyclones to remove the last trace of protein and produce high — quality starch, typically more than 99.5% pure. Some of the starch is dried and marketed as unmodified cornstarch, another portion is modified into specialty starches, but most is converted into corn syrups and dextrose [20]. Cornstarch has a variety of industrial and domestic uses. All these are important by-products of the process that contribute to the corn distillers’ profitability.

6. Sixth, the cornstarch then is converted to syrup (corn syrup) and this stage is called the starch conversion or starch-to-sugar conversion step. The starch-water suspension is liquefied in the presence of acid or enzymes. Enzymes help convert the starch to dextrose that is soluble in water as an aqueous solution. Treatment with another enzyme is usually carried out, depending upon the desired process outcome. The process of acid and enzyme reactions can be stopped or terminated at key points throughout the process to produce a proper mixture of sugars such as dextrose (a monosaccharide, C6H12O6) and maltose (a disaccharide, C12H22O11) for syrups to meet desired specifications [20]. For example, in some cases, the conver­sion of starch to sugars can be halted at an early stage to produce low-to-medium sweetness syrups. In other situations, however, the starch conversion process is allowed to proceed until the syrup becomes nearly all dextrose. After this conversion process, the syrup is then refined in filters, centrifuges, or ion-exchange columns, and excess water is evaporated to result in concentrated syrup. Syrup can be sold directly as is, crystallized into pure dextrose, or processed further to produce high-fructose corn syrup (HFCS). Across the corn wet milling industry, about 80% of starch slurry goes to corn syrup, sugar, and alcohol fermentation.

7. Seventh, the concentrated syrups can be made into several other products through a fermentation process. Dextrose is one of the most fermentable forms of all of the sugars. Dextrose is also called corn sugar and grape sugar, and dextrose is a naturally occurring form of glucose, that is, D-glucose. Dextrose is better known today as glucose. Following the conversion of starch to dextrose, dextrose is piped and sent to fermentation reactors/units/facilities where dextrose is con­verted to ethanol by traditional yeast fermentation. Using a continuous process, the fermenting mash is allowed to flow, or cascade, through several fermenters in series until the mash is fully fermented and then leaves the final tank. In a batch fermentation process, the mash stays in one fermenter for about 48 hours before the distillation process for alcohol purification is initiated. Generally speaking, a continuous mode is more effective with a higher fermenter throughput, whereas higher-quality product may be obtained from a batch mode.

8. As the eighth step, ethanol separation or purification follows the fermentation step. The resulting broth is distilled to recover etha­nol or concentrated through membrane separation to produce other by-products. Carbon dioxide generated from fermentation is recap­tured for sale as dry ice and nutrients still remaining in the broth after fermentation are used as components of animal feed ingredi­ents. These by-products also contribute significantly to the overall economics of the corn refineries.

Even though the term "by-product" was used throughout the process description, "coproduct" may be a better term in corn refining technol­ogy, inasmuch as these products are not only valuable but also targeted in the master plan of corn distillers. The corn-to-alcohol process detailed above can be summarized in a schematic process diagram, as shown in Figure 3.6.