Wet Milling of Cereal Grains

Robert L. Zipf.

Orlando Jones, an Englishman, was a pioneer whose work in 1840 founded a multimillion-dollar industry that touches all our daily lives. Every early starch producer used his process, in which an alkali was used to recover the starch. The process worked so well that the industry grew by leaps and bounds until, by 1880, this country's yearly consumption was estimated at 200 million pounds. Thus the cornstalk industry was born.

Wet milling, which has been considerably modified from Jones' original method, now consists of grinding the soaked grain and then separating the starch with water. Eleven companies were operating 14 wet-milling plants in 1950. They used about 6 percent of our corn crop, or 140 million bushels. Three of the plants have occasionally wet milled grain sorghums since about 1945. Another firm has announced plans for wet milling about 2 million bushels (56,000 tons) of grain sorghum annually. Wheat starch and rice starch have been made by various wet-milling procedures. Starch also has been obtained from barley and oats in the laboratory.

This use of the cereal grains has given the starch industries the advantage of a raw material high in starch, easy to store, and available in quantity. Every cereal grain can be wet milled by modification in equipment or processing. Because of the differences in the physical properties of starch from various sources, the starches may not be interchangeable for some purposes. Therefore, the production of starch from cereal or tuber crops depends upon the demands of industries for products most satisfactory for their purposes.

Cereal grains can be roughly classed as waxy and nonwaxy, or common. The terms do not imply the presence or absence of wax, but are merely descriptive of the physical appearance of the inside of the whole kernel. Waxy varieties are found in corn, grain sorghum, barley, rice, millet, and Job's tears. Commercially, only small amounts of the waxy corn and sorghum have been processed for starch.

Because sound grain yields the most starch with the least processing difficulty, wet mills as a rule process grain of grade No. 2 or 3. Only corn and sorghum are wet milled today.

THE OLDEST WET-MILLING METHOD for extracting wheat starch is the Halle or fermentation process, in which the grain, softened by steeping in water, is ground and made into a mash with water and allowed to ferment a week or two. The modified gluten can then be washed out of the starch. A second method, known as the Alsatian process, consists of steeping, grinding, and washing out the starch, without previous fermentation. The methods have been replaced by the Martin process and the batter process, both of which are wet-separation, not wet-milling, methods. Wheat starch was also made during the Second World War by a modification of the corn wet-milling process developed at the Northern Regional Research Laboratory.

Rice starch is obtained by steeping the grain for 24 hours in 0.3 percent caustic soda solution, then grinding it with additional caustic soda. The starch is obtained after screening, sedimentation, and centrifuging. Sulfur dioxide has also been used instead of the caustic soda.

The process as described for extracting starch from corn applies generally for grain sorghum.

MOST STARCH FACTORIES start with shelled corn delivered in railroad boxcars, each holding 1,200 to 1,800 bushels. A sample is collected from each car and analyzed. The grain is unloaded by means of power shovels, air conveyors, or tilters. It is weighed and then may go to either storage bins or a cleaner to remove dust, chaff, cobs, stones, rodent excreta, insects, broken grain, and other foreign material. In some plants the cleanings, except the larger particles, are conveyed to the feed system and mixed to form part of the gluten feed.

The cleaned grain is stored in bins in an amount sufficient to fill a steep tank. Some of the larger wet-milling plants are equipped with grain driers to dry high-moisture grain. Corn with a moisture content under 16 percent can usually be stored safely during the winter, but corn that contains more than 16 percent moisture is turned in the bins periodically.

Steeping, or soaking, is the most important step in wet milling. It loosens the bond that holds the starch granules together in the kernel; it removes solubles, mainly mineral matter, from the germ so that it is lightened and can be floated off in the germ separators; and it softens the kernel for grinding. Steeping does not harm or modify the starch very much.

Sulfurous acid water is used for the purpose. Steeping is done in a series of tanks in which lactic acid fermentation is controlled by a counter-current flow of steep Water and the addition of sulfur dioxide. Each tank holds 2,000 to 3,500 bushels of grain.

The sulfurous acid is made by burning sulfur and absorbing the sulfur dioxide in gluten settler water. This acidic steep water, amounting to 7 to 14 gallons per bushel and containing 0.07 to 0.25 percent sulfur dioxide, is heated to 116 to 133 F. and put on the oldest corn in the steeping system. The acid water is transferred counter-currently through the steeps and at times is circulated over each steep. The oldest corn, therefore, is soaked or steeped with water containing the smallest amount of solubles and the newest corn with water containing the largest amount of solubles.

The corn is completely covered during the steeping. The steeping time varies from 28 to 48 hours. The temperature of the contents of each steep is maintained at 116 to 133 F. by means of tube and shell heat exchangers or by sparging (injecting) the steam directly into the water. The water drained off the newest corn is called light steep water. It amounts to 4 to 10 gallons to the bushel and varies greatly in its composition. Soluble or colloidal matter amounting to 1.5 to 9.5 percent of the dry substance of the original grain is removed in steeping. The steeped corn, when withdrawn, has a moisture content of 40 to 55 percent, depending upon the condition of the grain used. More sulfur dioxide is required for steeping moldy and heat-damaged corn.

The starch is separated from the steeped corn in four steps. In the first, the grain is ground by degerminating mills to free the germ, which is then floated off in large tanks known as germ separators. Then, the underfloor from the separators is screened and the resultant hull and grits are ground by means of buhrstone mills or vertical hammer mills. The third step is screening to separate coarse and fine fiber from a starch and gluten slurry. The germ, coarse fiber, and fine fiber are washed free of starch and gluten by a counter-current flow of water over three or four sets of screens. The starch is separated from the gluten in the fourth step by tabling or centrifuging.

The trend today is to replace the tables with centrifuges because there is less chance of any contamination, processing time is shorter, less floor space is required, and results are better.

The starch from the tables or centrifuges is filtered and washed on the filters to remove soluble matter.

Starch from the filters is dried to 5 to 18 percent moisture by kiln, belt, rotary, vacuum, spray, or shelf driers.

The starch that leaves the driers with 10 to 14 percent moisture is known as pearl starch. It can be packed in 140-pound jute, burlap, or cotton bags, or in 100-pound paper bags. This starch will have a protein content of 0.26 to 0.38 percent on a dry basis and contain 0.5 to 1.2 percent total impurities, which include protein, ash, solubles, and oil.

Pearl starch is pulverized to produce food and powdered starch. The main difference between them is that food starch must not contain more than 0.005 percent of sulfur dioxide. Pearl starch may be dried further either before or after it is pulverized. To avoid explosions the moisture content has to be kept above 7.5 percent while the starch is being pulverized. Therefore, to reduce the moisture below 7.5 percent the starch is dried after pulverizing. The powdered starch is sifted through 200-mesh screens and bagged. Pulverized starch contains 5 to 14 percent moisture, depending upon its intended use.

BY PRODUCTS are recovered by drying the wet material from the milling system in flash driers, rotary fire driers, or rotary steam-tube driers. Two passes are generally used to maintain a more uniform moisture content in the product. Flash driers are used to dry gluten feed, gluten meal, and zein. In other installations, rotary fire driers are generally used for the first pass and steam-tube driers for the second pass. The rotary driers are used to dry gluten feed, gluten meal, and germ.

The washed germ from the screens is pressed with a screw- or disk-type water extractor and then dried with two passes through rotary driers. The dry germ will contain 42 to 50 percent oil and 14 to 18 percent protein. The oil is recovered by screw-type oil presses or by a combination of screw presses and solvent extractors. The resulting cake or meal, which contains 0.5 to 15 percent oil, 19 to 22 percent protein, and 3 to 6 percent moisture, is called oil cake or corn-germ meal. It may be sold as such, but most of it is added to the gluten feed. The oil is shipped out as crude corn oil or is refined in the plant.

The gluten liquor or overflow from the tables or first-pass centrifugals is concentrated by centrifuges or settlers. Most of the gluten settler water or overflow from the settlers is used to make the steep acid. The rest is mixed with the other process waters and used to wash the germ and fiber.

The settled heavy gluten is mixed with some fine fiber and filtered with recessed-plate presses or a string-discharge drum filter. The filtrate is mixed with the other process waters to wash the germ and fiber. The filter cake is dried by two passes through rotary driers. The tailings from screening the dried material pass into a hammer mill and can be added to the gluten meal or gluten feed. The material through the screen is called gluten meal and will contain 40 to 45 percent protein and 8 to 14 percent moisture. About 75 percent of the gluten can go into gluten meal and the remaining 25 percent into gluten feed.

The light steep water is evaporated from 2 to 5.5 Baume (average about 4 Baume) to 19 to 26 Baume. Small plants commonly use single-effect and larger plants triple-effect evaporators. The heavy steep water, which contains 35 to 48 percent dry substance protein and 2 to 16 percent dry reducing substances, is loaded into barrels or tank cars or is mixed in the gluten-feed system.