The process that turns whole grains into flour is the milling of wheat. A consistent product, a range of flours suitable for a variety of functions and flours with a consistent production are the ultimate priorities of the miller.
Wheat Milling Process
There are six categories and a few hundred varieties of wheat making it possible to manufacture hundreds of wheat foods from all over the world. For example, hard wheat flours are used for a variety of bread products; durum semolina and flour are used in pasta. Soft wheat flours produce a variety of crackers, biscuits, cereals, cookies, pancakes, breads and pastries. Many mills specialize on the type of wheat they process and this preference may be partially dependent on the position of the mill.
Below are the processes involved in the milling process of wheat:
Grain Delivery
Grain is supplied to the mill by covered trucks and hopper railcars. The path the grain travelled varies significantly. Grain supplies would also have passed through a variety of aggregation phases before arriving at the factory (farmer, country elevator, terminal elevator etc.).
Grain Standards
Before the wheat is unloaded at the factory, samples are taken to ensure that the inspection is carried out. Grain is tested for moisture content, test weight, sound kernels and foreign material. When unloading, product control chemists begin their tests to identify wheat and decide the end-use consistency. The results of these experiments decide how the wheat will be treated and packed. Sampling, grading and checking of grade and condition variables occurs in the storage, handling and milling process.
Grain Storage
If the grain has passed the test, it is discharged directly from the delivery vehicle to the pits and transferred by conveyors and bucket elevators to wide bins or silos. The right moisture, heat and air must be preserved or the wheat will be mild, sprinkled or fermented. During storage, the grain can undergo a fumigation process to remove insect pests. Wheat is stored on the basis of protein levels and other consistency requirements. Storage times are different. Most mills would be washing up the wheat at this time in order to achieve improved storage results. Millers also draw from various silos to mix different varieties of wheat with distinct performance properties in order to produce the desired final product.
Maintaining Product Integrity During the Milling Process
When it is time for the grain to be milled, it travels from the bottom of the silo/bin into the conveyors to the first floor of the mill where the cleaning process starts.
- Cleaning the Wheat: The first milling process includes machinery that removes grain from seeds and other grains, eliminates foreign materials that may have originated during the farmer’s harvest, such as metal, rocks, stones and straw; and scrubs the kernels of wheat. It could take as much as six steps. The machines that clean the grain are generally referred to as the cleaning house.
- Magnetic Separator: The grain first passes through a magnet that removes ferrous metal particles. After milling, it will pass through other metal detectors to ensure that no metal parts are in the finished product. The magnets are also positioned throughout the milling process and at the last step prior to loading.
- Separator: The vibrating or rotating drum separators remove bits of wood, straw and almost anything else that is too large or too small to be the desired grain.
- Aspirator: Air currents act as vacuums to remove dust and lighter impurities.
- De-stoner: Using gravity, the system divides the heavy material from the light to extract stones that might be of the same size as the desired grain.
- Disc Separator: The grain goes into the separator, which identifies the scale of the kernels even more closely. It excludes anything longer, thinner, rounder, more angular or in some way different in form.
- Scourer– The scourer extracts the outer husks, the soil in the kernel crease and other minor impurities with an extreme scouring operation. Currents of air dragging up all the loose stuff.
- Impact Entoleter: The centrifugal force tears off any unsound kernels or insect larvae, and the aspiration rejects them from the flow of the mill. From the entoleter, the sound of wheat flows into the grinding bins, large hoppers that regulate the feeding of wheat to the actual milling process.
- Color Separator: Newer mills can also use automated color separators to ease the cleaning process.
Tempering Wheat
Now the wheat is prepared to be processed for milling. It’s called tempering. Moisture is applied in precise quantities to reinforce the bran and smooth the inner endosperm. This makes it easier and cleaner to separate the parts of the kernel. The length of the soaking time will range from 6 to 24 hours. Time and temperature depend on the variety of wheat and its moisture content. Temper water can be treated with ozone or chlorine to preserve hygiene during the tempering process in this wet environment.
Grinding Wheat
The kernels of wheat are now able to be milled into flour. The modern milling method is a slow reduction of the kernels of wheat by the grinding and sifting process. The millers’ ability is to evaluate the wheat and then mix it to satisfy the end-use specifications. This science of analysis, blending, grinding, sifting and blending results in consistent end products.
Wheat kernels are measured or fed from the bins to the “roller mills”, corrugated cylinders made from chilled steel. The rolls are paired and turned inward to each other, rotating at varying speeds. Passing through the corrugated “first break” rolls, the separation of the bran, the endosperm and the germ begins.
There’s about five roller mills or breaks in the system. Again, the aim is to remove the endosperm from the bran and the germ. Each break roll must be set in order to get as much pure endosperm as possible. The “break” rolls, each of which has successively finer corrugations. After each trip through the break rolls, the grist is sent back upstairs to drop through sifters. The system reworks large stocks of sifters and reduces wheat particles to granular “middlings” that are as free of bran as possible.
In some mills, double high roller mills eliminate the lifting and sifting of the product between two successive phases of the milling process, thus increasing efficiency.
Sifters
The broken particles of wheat are elevated through pneumatic tubes and then dropped into huge, vibrating, 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 further down they go. Up to six different particle sizes may come from a single sifter. Larger particles are shaken off from the top, or “scalped,” leaving the fine flour to sift to the bottom.
The “scalped” fractions of the endosperm called middlings are reduced to the particle size of flour in a smooth roller system. The product is then subjected to a purifying process in hard wheat mills. The controlled flow of air removes the particules of the bran while, at the same time, the bolting cloth separates and grades larger fractions by size and quality.
The process is repeated over and over again, sifting to purifiers to reduce rolls, moving up and down and across the mill in a series until the maximum amount of flour is separated, about 75% of the wheat kernel.
Bleaching the flour
Towards the end of the line in the millstream, if the flour is to be “bleached,” the finished flour flows into a system that releases a bleaching agent in measured quantities. This duplicates the normal oxidation that happens when flour is allowed to mature spontaneously as in the old days when flour has been preserved for a few months. This whitened the flour and enhanced its baking characteristics. The modern bleaching process simply duplicates this natural oxidation process, but it does so more quickly.
In the bleaching process, flour is subjected to chlorine gas or benzoyl peroxide to whiten and brighten the color of flour. The bleaching agent responds and does not leave toxic traces or kill nutrients. Chlorine gas is also used in soft wheat goods to monitor cookie diameter and cake height.
Enrichment, Malt and Leavening
The flour stream passes through a system that tests and releases specified amounts of enrichment. Malt can be applied to the bread flours at this stage to add the height of the loaf as well as the taste. Grains are fortified with B vitamins riboflavin, niacin, thiamine and folic acid added to the enrichment recipe.
Finished Product Testing
After milling, laboratory tests are carried out to ensure that the flour meets the specifications. Millers also monitors indicators of natural organisms on a routine basis. Although dry flour does not provide an environment conducive to microbial growth, it is important to understand that flour is a minimally processed agricultural ingredient and is not a ready-to-eat product. Flour is not intended to be consumed raw. The heat processes of baking, frying, boiling and cooking are adequate to destroy any pathogens that may be present in flour and to reduce the potential risk of foodborne illness.
