How Wheat Flour is Milled
Product Control
Wheat arrives at the mill
by truck, ship, barge, or rail car. Before the wheat
is even unloaded, samples are taken to ensure it
passes inspection. X-rays may be used to detect any
signs of insect infestation. Meanwhile, product
control chemists begin their tests to classify the
grain by milling and baking a small amount to
determine end-use qualities.
The results from these tests determine how the wheat will be handled and stored. Millers may blend different wheats to achieve the desired end product. The wheat will then be stored at the mill in large bins.
Storing wheat is an exact science. The right moisture, heat and air must be maintained or the wheat may mildew, sprout, or ferment.
Cleaning the Wheat
The first milling steps involve equipment that separates wheat from seeds and other grains, eliminates foreign materials such as metal, sticks, stones and straw; and scours each kernel of wheat. It can take as many as six steps.
Magnetic Separator
The wheat first passes by a magnet that removes iron and steel particles.
Separator
Vibrating screens remove bits of wood and straw and almost anything too big and too small to be wheat.
Aspirator
Air currents act as a kind of vacuum to remove dust and lighter impurities.
De-Stoner
Using gravity, the machine separates the heavy material from the light to remove stones that may be the same size as wheat kernels.
Disc Separator
The wheat passes through a separator that identifies the size of the kernels even more closely. It rejects anything longer, shorter, more round, more angular or in any way a different shape.
Scourer
The scourer removes outer husks, crease dirt and any smaller impurities with an intense scouring action. Currents of air pull all the loosened material away.
Conditioning the Wheat
Tempering
Now the wheat is ready to be conditioned for milling. This is called tempering. Moisture is added in precise amounts to toughen the bran and mellow the inner endosperm. This makes the parts of the kernel separate more easily and cleanly.
Tempered wheat is stored in bins from eight to 24 hours, depending on the type of wheat - soft, medium or hard. Blending of wheats may be done at this time to achieve the best flour for a specific end-use.
Impact Scourer
Centrifugal force breaks apart any unsound kernels and rejects them from the mill flow.
From the entoleter, the wheat flows to grinding bins, large hoppers that will measure or feed wheat to the actual milling process.
Grinding the Wheat
The wheat kernels, or berries, are now in far better condition than when they arrived at the mill and are ready to be milled into flour. Wheat kernels are measured or fed from the bins to the "rolls," or corrugated rollers made from chilled cast iron.
This modern milling process is a gradual reduction of the wheat kernels. The goal is to produce middlings, or coarse particles of endosperm. The middlings are then graded and separated from the bran by sieves and purifiers. Each size returns to corresponding rollers and the same process is repeated until the desired flour is obtained.
The rolls are paired and rotate inward against each other, moving at different speeds. Just one pass through the corrugated "first break" rolls begins the separation of bran, endosperm and germ.
The Miller's Skill
The miller's skill is demonstrated by the ability to adjust all of the rolls to the proper settings that will produce the maximum amount of high-quality flour. Grinding too hard or close results in bran powder in the flour. Grinding too open allows good endosperm to be lost in the mill's feed system.
The miller must select the exact milling surface, or corrugation, on the break rolls, as well as the relation and the speed of the rollers to each other to match the type of wheat and its condition. Each break roll must be set to get as much pure endosperm as possible to the middlings rolls. The middlings rolls are set to produce as much flour as possible.
From the rolls, the grist is sent way upstairs to drop through sifters. The grist is moved via pneumatic systems that mix air with the particles so they flow, almost like water, through tubes. This is a great advance in health and safety from earlier methods of moving the grist with buckets.
Sifters
The broken particles of wheat are introduced into huge, rotating, box-like sifters where they are shaken through a series of bolting cloths or screens to separate the larger from the smaller particles.
Inside the sifter, there may be as many as 27 frames, each covered with either a nylon or stainless steel screen, with square openings that get smaller and smaller the farther down they go.
Up to six different sizes of particles may come from a single sifter, including some flour with each sifting. Larger particles are shaken off from the top, or "scalped," leaving the finer flour to sift to the bottom.
The "scaled" fractions are sent to other roll passages and particles of endosperm are graded by size and carried to separate purifiers.
Purifiers
In a purifier, a controlled flow of air lifts off bran particles while at the same time a bolting cloth separates and grades coarser fractions by size and quality.
Four or five additional "break" rolls, each with successively finer corrugations and each followed by a sifter, are usually used to rework the coarse stocks from the sifters and reduce the wheat particles to granular "middlings" that are as free from bran as possible. Germ particles will be flattened by later passage through the smooth reduction rolls and can be easily separated. The reduction rolls reduce the purified, granular middlings, or farina, to flour.
The process is repeated over and over again, sifters to purifiers to reducing rolls, until the maximum amount of flour is separated, consisting of close to 75 percent of the wheat.
There are various grades of flour produced in the milling process.The remaining percentage of the wheat kernel or berry is classified as millfeed - shorts, bran and germ.
Bakers buy a wide variety of flour types, based on the products they produce. The flour the consumer buys at the grocery store, called "family flour" by the milling industry, is usually a long-patent all-purpose or bread flour. Occasionally short patent flour is available in retail stores.
"Reconstituting," or blending back together, all the parts of the wheat in the proper proportions yields whole wheat flour. This process produces a higher quality whole wheat flour than is achieved by grinding the whole wheat berry. Reconstitution assures that the wheat germ oil is not spread throughout the flour so it does not go rancid so readily.
Bleaching the Flour
Toward the end of the line in the millstream, if the flour is to be "bleached," the finished flour flows through a device, which releases a bleaching-maturing agent in measured amounts.
It has been known for centuries that freshly milled flour makes a lesser quality baked product. In the old days, flour was stored for a few months to mature, or naturally oxidize. This whitened the flour and improved its baking characteristics. The modern bleaching process simply duplicates this natural oxidation process, but does so more quickly.
In the bleaching process, flour is exposed to chlorine gas or benzoyl peroxide to whiten and brighten flour color. Chlorine also affects baking quality by "maturing" or oxidizing the flour, which is beneficial for cake and cookie baking. The bleaching agents react and do not leave harmful residues or destroy nutrients.
Enrichment
The flour stream passes through a device that measures out specified quantities of enrichment. The enrichment of flour with four B vitamins (thiamin, niacin and riboflavin) and iron, begun in the 1930s. In 1998 folate, or folic acid, was added to the mix of vitamin B. If the flour is self-rising, a leavening agent, salt and calcium are also added in exact amounts.
Before the flour leaves the mill, additional lab tests are run to ensure that the customers get what they ordered.
Finally, the flour millstream flows through pneumatic tubes to the packing room or into hoppers for bulk storage.
Family flour for retail sale may be packed in 5-, 10-, 25-pound bags. Bakery flour may be packed in 50- to 100-pound bags or sent directly to bulk trucks or rail cars.
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