The U.S. grain processing industry has evolved to efficiently co-mingle and move vast quantities of grain from rural America to population centers so consumers can have safe, healthful products with consistent quality.
Each year the U.S. wheat milling industry consumes more than 900 million bushels of an approximately two billion bushel crop. U.S. wheat mills of average size produce about one million pounds of flour daily, and the largest produce between 2.0 and 3.2 million pounds per day
Wheat is not just wheat. Six classes and several hundred varieties of wheat make possible the hundreds of wheat foods made worldwide. For example, hard wheat flours provide for a variety of bread products; durum semolina and flour are used in pasta. Soft wheat flours produce an array of crackers, cookies, cereals, cakes, pancakes, breading and pastries. Many mills specialize in the type of wheat they process and this specialization can be based, in part, on mill location.
Grain is delivered to mills by covered trucks and hopper railcars. The distance the grain has traveled varies greatly. In some cases it has traveled hundreds of miles in a 110-car unit train. In other instances it is being delivered from a local farm in the same county. Grain deliveries will frequently have gone through a number of aggregation steps prior to arriving at the mill (farmer, country elevator, terminal elevator etc.). The number of conveyances making deliveries of grain can vary depending on the time of year with more deliveries at harvest time.
Before wheat is unloaded at a facility, samples are taken to ensure it passes inspection. Grain is tested for moisture content, test weight, unsound kernels, and foreign material. Grain is graded according to the US Grain Standards and is also subject to commercial specifications set by the miller. Typically wheat used in milling is #2 grade or better. At unloading, product control chemists begin their tests to classify wheat and determine end-use qualities. The results from these tests determine how the wheat will be handled and stored.
The U.S. Grain Standards Act facilitates trading in grain. Administered by the USDA’s Grain Inspection, Packers and Stockyards Administration (GIPSA) it provides criteria for determining the kind, class, and condition of grains and oilseeds. Standards define quality and condition factors and set grade limits based on those factor determinations. Mills, however, must use commercial specifications that are even more rigorous than the U.S. Grain Standards when testing for natural toxins, evidence of pest exposure, stress cracks, etc. Sampling, grading and testing of grade and quality factors continues throughout the storage, handling and milling processes.
Once the grain has passed inspection it is unloaded directly from the delivery vehicle into pits and moved via conveyors and bucket elevators into large bins or silos. Storing grain is a science. The right moisture, heat and air must be maintained or the wheat may mildew, sprout, or ferment. During storage the grain may go through a fumigation process to eliminate insect pests. Wheat is stored according to protein level and other quality considerations. Storage times vary. Many mills will clean the wheat at this time to obtain better storage results. Millers frequently draw from different silos to blend different types of wheat with distinct performance properties to achieve the desired end product.
Using their written Hazard Analysis and Critical Control Point (HACCP) plan, grain millers identify potential food safety hazards so that key actions can be taken to reduce or eliminate the risk of those hazards. Millers are working to eliminate critical control points before they become problems. This increases the reach and impact of precious food safety resources.
Milling pre-requisite programs consist of inspection and testing of raw grain as it arrives at the mill, sanitation programs, pest control programs, current Good Manufacturing Practices (GMPs), traceability and recall programs, shipping and receiving procedures that cover truck and railcar inspections and sealing, chemical control programs, allergen control programs, customer complaint responses, and lab testing procedures as well as many others. These programs also cover preventative maintenance programs, machinery and equipment programs, supplier programs, grounds and facilities programs.
A number of steps in the milling process enhance product integrity
When it is time to mill the grain it moves from the bottom of the silo/bin through conveyors to the top floor of the mill where the cleaning process begins.
Cleaning the wheat – The first milling steps involve equipment that separates grain from seeds and other grains, removes foreign materials that might have originated during the farmer’s harvest such as metal, sticks, stones and straw; and scours the kernels of wheat. It can take as many as six steps. The machines that clean the grain are collectively called the cleaning house.
Magnetic separator – The grain first passes by a magnet that removes ferrous metal particles. It will pass through other metal detectors after milling to ensure that no metal pieces are in the finished product. Magnets are also positioned throughout the milling process and at the last step prior to load-out.
Separator – Vibrating or rotating drum separators remove bits of wood, straw and almost anything else too big or too small to be the desired grain.
Aspirator – Air currents act as a 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 the desired grain.
Disc separator – The grain 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, dirt in the kernel crease and any smaller impurities with an intense scouring action. Currents of air pull all the loosened material away.
Impact Entoleter – Centrifugal force breaks apart any unsound kernels or insect eggs and aspiration rejects them from the mill flow. From the entoleter, the sound wheat flows to grinding bins, large hoppers that control the feeding of the wheat to the actual milling process.
Color Separator – Newer mills may also utilize electronic color separators to simplify the cleaning process.
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. The length of soaking time can range from 6-24 hours. The time and temperature depend on the type of wheat and its moisture level. Temper water may be treated with ozone or chlorine to maintain sanitation in this wet environment during the tempering process.
The wheat kernels are now ready to be milled into flour. The modern milling process is a gradual reduction of the wheat kernels through a process of grinding and sifting. The millers’ skill is analyzing the wheat and then blending it to meet the requirements of the end use. This science of analysis, blending, grinding, sifting and blending again 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 rotate inward against each other, moving at different speeds. Passing through the corrugated “first break” rolls begins the separation of bran, endosperm (starch) and germ.
There are about five roller mills or breaks in the system. Again, the goal is to remove the endosperm from the bran and the germ. Each break roll must be set to get as much pure endosperm as possible. The “break” rolls, each have successively finer corrugations. After each trip through the break rolls, the grist is sent back upstairs to drop through sifters. The system reworks the coarse stocks from the sifters and reduces the wheat particles to granular “middlings” that are as free from bran as possible.
In some mills double high roller mills eliminate elevating and sifting the product between two successive passages in the milling process, thus increasing efficiency.
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 farther down they go. Up to six different sizes of particles may come from a single sifter. Larger particles are shaken off from the top, or “scalped,” leaving the finer flour to sift to the bottom.
The “scalped” fractions of endosperm called middlings are reduced in a smooth roller system to the particle size of flour. In hard wheat mills, the product is then subjected to a purifying process. 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.
The process is repeated over and over again, sifters to purifiers to reducing rolls, moving up and down and across the mill in a series until the maximum amount of flour is separated, about 75 percent of the wheat kernel.
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 that releases a bleaching-maturing agent in measured amounts. This duplicates the natural oxidation that occurs when flour is allowed to naturally age as in the old days when flour was stored for a few months. 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. The bleaching agents react and do not leave harmful residues or destroy nutrients. In soft wheat products chlorine gas is also used to control cookie diameter and cake height.
Enrichment, malt and leavening
The flour stream passes through a device that measures out and releases specified quantities of enrichment. Malt may be added to bread flours at this point to add loaf height as well for flavor.
The enrichment of flour with iron and B vitamins – thiamin, niacin and riboflavin – began in the 1930s. This enrichment helped eradicate significant diet-based diseases in the US such as pellagra and beri beri. Enrichment has included folic acid (folate) since 1998. This has reduced the rate of neural tube birth defects like spina bifida in newborns by about 30 percent, meaning about 1,000 babies are born healthy each year that otherwise would have suffered debilitating or fatal birth defects. In fact, in 2011 the US Centers for Disease Control and Prevention named folic acid fortification of enriched grains as one of top ten public health achievements of the first decade of the 21st century for its role in birth defects prevention.
Finished product testing
After milling, lab tests are run to ensure that the flour meets specifications. Millers also conduct routine monitoring of indicator natural organisms. Although dry flour does not provide an environment that is 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 reduce the potential risk of food borne illness.
The North American Millers’ Association is the trade association representing the wheat, corn, oat and rye milling industry. NAMA’s 46 member companies operate 170 mills in 38 states and Canada. Their aggregate production of more than 175 million pounds per day is approximately 95 percent of the total industry capacity.
Last updated September 2011