The corn wet milling process technology separates corn into its four basic components, namely: starch, germ, fiber, and protein. There are eight basic steps involved in accomplishing this corn refining and alcohol fermentation process.
1. The corn is visually inspected and cleaned. Refinery people inspect incoming corn shipments and clean them two or three times to remove cob, dust, chaff, and any other foreign materials before the next process technology stage, steeping. Effective screening processes can save a great deal of trouble in the subsequent stages.
2. The corn is steeped to initiate bond breaking of starch and protein. Steeping is typically carried out in stainless steel tanks. Each steep tank (or steeping tank) holds about 3000 bushels of corn soaked in water at 50°C for 3040 h. During steeping, the kernels absorb water, thereby increasing their moisture levels from 15 to 45% by weight and also more than doubling in size. The addition of 0.1% sulfur dioxide (SO2) to the water suppresses excessive bacterial growth in the warm 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 releases the starch. A bushel is a unit of volume measure used as a dry measure of grains and produce. A bushel of corn or milo weighs 56 lb, a bushel of wheat or soybeans weighs 60 lb, and a bushel of sunflowers weighs 25 lb. That is, a U.S. bushel is equivalent to 35.23907 l as a volume unit.
3. A coarse grind separates the germ from the rest of the kernel. Germ is the embryo of a kernel of grain. This 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. The germs, which contain about 85% of corns oil, are pumped onto screens and washed repeatedly to remove any starch left in the mixture. 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 feeds. Both corn oil and germ residue 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 (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 feeds. The starch-gluten suspension, called mill starch, is piped or sent to the starch separators.
5. Starch is separated from the remaining slurry in hydrocyclones. By centrifuging mill starch, the gluten is readily spun out owing to the density difference between starch and gluten. Starch is denser than gluten. Separated gluten can be used for animal feeds. The starch, with just 1 to 2% protein remaining, is diluted, washed 8 to 14 times, rediluted and rewashed in hydrocyclones to remove the last trace of protein and produce highquality starch, typically more than 99.5% pure. Some of the starch is dried and marketed as unmodified cornstarch, some is modified into specialty starches, but most is converted into corn syrups and dextrose. Cornstarch has a variety of industrial and domestic uses. All these are important byproducts of the process, which improve the corn distillers profitability.
6. The cornstarch then is converted to syrup, and this stage is called starch conversion step. Starch-water suspension is liquefied in the presence of acid and enzymes. Enzymes help convert the starch to dextrose, which is soluble in water as an aqueous solution. Treatment with another enzyme is usually carried out, depending on the desired process outcome. The process of acid and enzyme reactions can be stopped or terminated anytime throughout the process to produce a right mixture of sugars such as dextrose and maltose for syrups to meet desired specifications. 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 cases, however, the starch conversion process is allowed to proceed until the syrup becomes nearly all dextrose. After this conversion process, the syrup is refined in filters, centrifuges or ion-exchange columns, and excess water is evaporated, producing concentrated syrup. Syrup can be sold directly as is, crystallized into pure dextrose, or processed further to produce high-fructose corn syrup.
7. Syrups can be made into several other products through a fermentation process. Dextrose is one of the most fermentable forms of all the sugars. Dextrose is also called corn sugar and grape sugar, and dextrose is a naturally occurring form of glucose. Dextrose is better known today as glucose. Following the conversion of starch to dextrose, corn refiners pipe and send dextrose to fermentation units and 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 h before the distillation process 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. Ethanol separation 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, and nutrients still remaining in the broth afterfermentation are used as components of animal feed ingredients. These by-products also help 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, because these products are not only valuable but also targeted in the master plan of corn distillers.
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