Flour is a finely ground powder prepared from grain or other starchy plant foods and used in baking. Although flour can be made from a wide variety of plants, the vast majority is made from wheat. Dough made from wheat flour is particularly well suited to baking bread because it contains a large amount of gluten, a substance composed of strong, elastic proteins. The gluten forms a network throughout the dough, trapping the gases which are formed by yeast, baking powder, or other leavening agents. This causes the dough to rise, resulting in light, soft bread.
Flour has been made since prehistoric times. The earliest methods used for producing flour all involved grinding grain between stones. These methods included the mortar and pestle (a stone club striking grain held in a stone bowl), the saddlestone (a cylindrical stone rolling against grain held in a stone bowl), and the quern (a horizontal, disk-shaped stone spinning on top of grain held on another horizontal stone). These devices were all operated by hand.
The millstone, a later development, consisted of one vertical, disk-shaped stone rolling on grain sitting on a horizontal, disk-shaped stone. Millstones were first operated by human or animal power. The ancient Romans used waterwheels to power millstones. Windmills were also used to power millstones in Europe by the twelfth century.
The first mill in the North American colonies appeared in Boston in 1632 and was powered by wind. Most later mills in the region used water. The availability of water power and water transportation made Philadelphia, Pennsylvania, the center of milling in the newly independent United States. The first fully automatic mill was built near Philadelphia by Oliver Evans in 1784. During the next century, the center of milling moved as railroads developed, eventually settling in Minneapolis, Minnesota. During the nineteenth century numerous improvements were made in mill technology. In 1865, Edmund La Croix introduced the first middlings purifier in Hastings, Minnesota. This device consisted of a vibrating screen through which air was blown to remove bran from ground wheat. The resulting product, known as middlings or farina, could be further ground into high-quality flour. In 1878, the first important roller mill was used in Minneapolis, Minnesota. This new type of mill used metal rollers, rather than millstones, to grind wheat. Roller mills were less expensive, more efficient, more uniform, and cleaner than millstones. Modern versions of middlings purifiers and roller mills are still used to make flour today.
Although most flour is made from wheat, it can also be made from other starchy plant foods. These include barley, buckwheat, corn, lima beans, oats, peanuts, potatoes, soybeans, rice, and rye. Many varieties of wheat exist for use in making flour. In general, wheat is either hard (containing 11-18% protein) or soft (containing 8-11% protein). Flour intended to be used to bake bread is made from hard wheat. The high percentage of protein in hard wheat means the dough will have more gluten, allowing it to rise more than soft wheat flour. Flour intended to be used to bake cakes and pastry is made from soft wheat. All-purpose flour is made from a blend of soft and hard wheat. Durum wheat is a special variety of hard wheat, which is used to make a kind of flour called semolina. Semolina is most often used to make pasta.
Flour usually contains a small amount of additives. Bleaching agents such as benzoyl peroxide are added to make the flour more white. Oxidizing agents (also known as improvers) such as potassium bromate, chlorine dioxide, and azodicarbonamide are added to enhance the baking quality of the flour. These agents are added in a few parts per million. Self-rising flour contains salt and a leavening agent such as calcium phosphate. It is used to make baked goods without the need to add yeast or baking powder. Most states require flour to contain added vitamins and minerals to replace those lost during milling. The most important of these are iron and the B vitamins, especially thiamin, riboflavin, and niacin.
Grading the wheat
- 1 Wheat is received at the flour mill and inspected. Samples of wheat are taken for physical and chemical analysis. The wheat is graded based on several factors, the most important of which is the protein content. The wheat is stored in silos with wheat of the same grade until needed for milling.
Purifying the wheat
In 1795, an American engineer published a book called The Young Millwright and Miller's Guide. In the book, simple theories are transformed into a set of mechanical devices that form a flour mill. At the back of the book is a drawing, illustrating how these devices make a continuous production line in which the human hand is eliminated from the beginning of the process to the end of production. The author of this book was Oliver Evans, himself the son of a miller. He and his brothers ran their own mill, developed the systems, and perfected the operations that led to the automated grain mill.
Today, Evans is considered one of America's most ambitious mechanical innovators. He used his understanding of the way in which water turned a mill wheel and developed it into a viable grain-milling system.
Most important was the fact that his system contained the idea of the integrated and automated factory. When a machine substitutes human intervention, the problems of the fully automated assembly line are solved. This concept was not fully applied until the 1920s by Henry Ford, who was able to develop a successful, operational assembly line. Ford had the advantage of living at the end of the machine age, but Oliver Evans was the first to present the concept of automation before it was even possible.
- 2 Before wheat can be ground into flour it must be free of foreign matter. This requires several different cleaning processes. At each step of purification the wheat is inspected and purified again if necessary.
- 3 The first device used to purify wheat is known as a separator. This machine passes the wheat over a series of metal screens. The wheat and other small particles pass through the screen while large objects such as sticks and rocks are removed.
- 4 The wheat next passes through an aspirator. This device works like a vacuum cleaner. The aspirator sucks up foreign matter which is lighter than the wheat and removes it.
5 Other foreign objects are removed in various ways. One device, known as a disk separator, moves the wheat over a series of disks with indentations that collect objects the size of a grain of wheat. Smaller or larger objects pass over the disks and are removed.
- 6 Another device, known as a spiral seed separator, makes use of the fact that wheat grains are oval while most other plant seeds are round. The wheat moves down a rapidly spinning cylinder. The oval wheat grains tend to move toward the center of the cylinder while the round seeds tend to move to the sides of the cylinder, where they are removed.
- 7 Other methods used to purify wheat include magnets to remove small pieces of metal, scourers to scrape off dirt and hair, and electronic color sorting machines to remove material which is not the same color as wheat.
Preparing the wheat for grinding
- 8 The purified wheat is washed in warm water and placed in a centrifuge to be spun dry. During this process any remaining foreign matter is washed away.
9 The moisture content of the wheat must now be controlled to allow the
- 10 Cold conditioning involves soaking the wheat in cold water for one to three days. Warm conditioning involves soaking the wheat in water at a temperature of 115°F (46°C) for 60-90 minutes and letting it rest for one day. Hot conditioning involves soaking the wheat in water at a temperature of 140°F (60°C) for a short period of time. This method is difficult to control and is rarely used. Instead of water, wheat may also be conditioned with steam at various temperatures and pressures for various amounts of time. If conditioning results in too much moisture, or if the wheat happens to be too moist after purification, water can be removed by vacuum dryers.
Grinding the wheat
- 11 Wheat of different grades and moistures is blended together to obtain a batch of wheat with the characteristics necessary to make the kind of flour being manufactured. At this point, the wheat may be processed in an Entoleter, a trade name for a device with rapidly spinning disks which hurl the grains of wheat against small metal pins. Those grains which crack are considered to be unsuitable for grinding and are removed.
- 12 The wheat moves between two large metal rollers known as breaker rolls. These rollers are of two different sizes and move at different speeds. They also contain spiral grooves which crack open the grains of wheat and begin to separate the interior of the wheat from the outer layer of bran. The product of the breaker rolls passes through metal sieves to separate it into three categories. The finest material resembles a coarse flour and is known as middlings or farina. Larger pieces of the interior are known as semolina. The third category consists of pieces of the interior which are still attached to the bran. The middlings move to the middlings purifier and the other materials move to another pair of breaker rolls. About four or five pairs of breaker rolls are needed to produce the necessary amount of middlings.
- 13 The middlings purifier moves the middlings over a vibrating screen. Air is blown up through the screen to remove the lighter pieces of bran which are mixed with the middlings. The middlings pass through the screen to be more finely ground.
- 14 Middlings are ground into flour by pairs of large, smooth metal rollers. Each time the flour is ground it passes through sieves to separate it into flours of different fineness. These sieves are made of metal wire when the flour is coarse, but are made of nylon or silk when the flour is fine. By sifting, separating, and regrinding the flour, several different grades of flour are produced at the same time. These are combined as needed to produce the desired final products.
Processing the flour
- 15 Small amounts of bleaching agents and oxidizing agents are usually added to the flour after milling. Vitamins and minerals are added as required by law to produce enriched flour. Leavening agents and salt are added to produce self-rising flour. The flour is matured for one or two months.
- 16 The flour is packed into cloth bags which hold 2, 5, 10, 25, 50, or 100 lb (About 0.9, 2.3, 4.5, 11.3, 22.7, or 45.4 kg). For large-scale consumers, it may be packed in metal tote bins which hold 3000 lb (1361 kg), truck bins which hold 45,000 lb (20,412 kg), or railroad bins which hold 100,000 lb (45,360 kg).
The quality control of flour begins when the wheat is received at the flour mill. The wheat is tested for its protein content and for its ash content. The ash content is the portion which remains after burning and consists of various minerals.
During each step of the purification process, several samples are taken to ensure that no foreign matter ends up in the flour. Since flour is intended for human consumption, all the equipment used in milling is thoroughly cleaned and sterilized by hot steam and ultraviolet light. The equipment is also treated with antibacterial agents and antifungal agents to kill any microscopic organisms which might contaminate it. Hot water is used to remove any remaining traces of these agents.
The final product of milling is tested for baking in test kitchens to ensure that it is suitable for the uses for which it is intended. The vitamin and mineral content is measured in order to comply with government standards. The exact amount of additives present is measured to ensure accurate labeling.
A kernel of wheat consists of three parts, two of which can be considered byproducts of the milling process. The bran is the outer covering of the kernel and is high in fiber. The germ is the innermost portion of the kernel and is high in fat. The endosperm makes up the bulk of the kernel and is high in proteins and carbohydrates. Whole wheat flour uses all parts of the kernel, but white flour uses only the endosperm.
Bran removed during milling is often added to breakfast cereals and baked goods as a source of fiber. It is also widely used in animal feeds. Wheat germ removed during milling is often used as a food supplement or as a source of edible vegetable oil. Like bran, it is also used in animal feeds.
Where to Learn More
Besant, Lloyd. Grains: Production, Processing, Marketing. Chicago Board of Trade, 1982.
Kent, N. L. Technology of Cereals: With Special Reference to Wheat. Pergamon Press, 1975.
Sokolov, Raymond. "Through a Mill, Coarsely." Natural History, February 1994, pp. 72-74.
Wrigley, Colin W. "Giant Proteins With Flour Power." Nature, June 27, 1996, pp. 738-739.
"How Flour is Made." The Story of Wheat. University of Saskatchewan College of Agricultural Sciences. December 7, 1996. http://pine.usask.ca/cofa/displays/college/story/wheat.html .
— Rose Secrest