- Wheat flour is the product obtained by grinding whole wheat kernels, which consist of bran, germ, and endosperm components
- During the milling process, kernel components are separated and recombined to make different types of flour
- Flour can be enriched, fortified, bleached, or unbleached
- Wheat flour contains dietary fiber, B vitamins, calcium, iron, magnesium, phosphorus, potassium, and zinc
- Flour is considered a raw agricultural product and wheat can become contaminated from soil, animal feces, insects, diseased plants, and other agents during production, harvest, storage, or transport
- Flour has low water activity and does not support microbial growth under normal storage conditions, but bacteria such as Salmonella and Escherichia coli can survive in dry flour for extended periods of time
- Wheat can be debranned or treated with water and tempered with ozone or chlorine to reduce microbial load
- Flour can be heat treated or irradiated with gamma rays to reduce microbial load
- Wheat infected with a Fusarium species of fungi can be contaminated with the mycotoxin deoxynivalenol, a highly stable vomitoxin that can be carried through the process of milling and contaminate flour
- Between 1998 and 2018, at least 5 flour-associated outbreaks were reported to CDC’s National Outbreak Reporting System (NORS), causing 102 illnesses, 21 hospitalizations, and no deaths.
Figure 1. High resolution of Triticum aestivum (Steve Hurst, hosted by the USDA-NRCS PLANTS Database)
Figure 2. Longitudinal section of a wheat kernel (https://col.st/Vmdki)
Wheat (Figure 1A) flour is the product obtained by grinding whole wheat kernels, sometimes called berries (Figure 1B). A wheat kernel consists of three parts – the bran, germ, and endosperm (Figure 2). During the milling process, these three parts are separated and recombined to make different types of flour. For example, white flour is composed of the finely ground endosperm, while whole wheat flour contains all three parts of the kernel. Other common types of flour include all-purpose, bread, cake, self-rising, pastry, semolina, durum, and gluten flours.
There are six classes of wheat grown in the U.S., including: Hard Red Winter, Hard Red Spring, Soft Red Winter, Soft White, Hard White, and Durum. These classes have unique characteristics, particularly protein and gluten content, and are used to make different types of foods. Flours made from hard wheats have higher protein content and are typically used for making breads. Soft wheat flours are used for cakes, pastries, cookies, crackers, and Asian noodles. Durum flour is used in pastas. All-purpose flour is milled from a blend of hard and soft wheats and is therefore suitable for creating a wide range of products.
Flour can be enriched, meaning processed; flour is supplemented with an amount of nutrients equal to or greater than levels in the unprocessed flour, including iron and B-vitamins (thiamin, niacin, riboflavin, and folic acid). Flour can also be (chemically) bleached to whiten or enhance baking qualities or unbleached (naturally aged and bleached by oxygen present in the air).
When flour is mixed with water, the flour proteins; gliadin and glutenin, combine to form the protein known as gluten. Gluten gives dough elasticity, strength, and structure. Individuals with celiac disease, certain neurological diseases, certain skin conditions, or gluten sensitivities of other etiologies often benefit from a gluten-free diet.
Although usually derived from wheat, flour can also be milled from almond, amaranth, barley, buckwheat, chickpea, coconut, corn, millet, oats, quinoa, rice, rye, sorghum, soy, tapioca, and teff.
By-products of wheat milling, such as bran, “shorts” (inward layers of the wheat seed coat that contain starchy or floury components), and “middlings” (a combination of bran and shorts), are used in the production of animal feeds.
Foodborne outbreaks involving flour have implicated Salmonella, Shiga-toxin producing Escherichia coli (STEC), and preformed toxins or other short-acting agents. Flour has not been a food of major concern until recently as illnesses have been associated with the consumption of raw flour or products containing raw flour, such as raw cookie dough or baking mixtures. Between 1998 and 2018, at least 5 flour-associated outbreaks were reported to CDC’s National Outbreak Reporting System (NORS), causing 102 illnesses, 21 hospitalizations, and no deaths.
During 1961 through 1985 and in 1987, outbreaks associated with contaminated grain products occurred in China and India, respectively. Approximately 35 outbreaks caused 7,818 illnesses in China and a single outbreak caused 97 illnesses in India. Testing of corn and wheat samples from China and implicated wheat products from India revealed elevated levels of the mycotoxin deoxynivalenol (DON).
From 1997 through 1998, there were 16 outbreaks associated with burritos served primarily in schools across seven states. Due to the symptoms and short-incubation periods, a preformed toxin, such as DON, or other short-acting agent were suspected as the cause of illness. There were approximately 1,900 illnesses, of which 1,700 were children. Because burrito fillings differed by outbreak location, the wheat flour tortillas were suspected to contain the etiologic agent. Testing of burrito samples detected DON within the acceptable FDA advisory level of 1 ppm for finished wheat products. Burritos produced by two manufacturers were implicated and all implicated products were recalled or withheld from distribution. A similar phenomenon occurred from 2003 through 2004, during which 10 outbreaks associated with flour tortillas from a single manufacturer occurred in schools throughout Massachusetts. An FDA inspection of the plant revealed improper storage, use, and labeling of chemicals, improper storage of food ingredients, additives in unlabeled containers, unprotected food contact surfaces, and lack of backflow protection from a piping system discharging wastewater. Testing of product revealed elevated levels of calcium propionate and potassium bromate, which can cause gastrointestinal illness when consumed in large amounts, particularly in children.
In 2005, there was an outbreak of Salmonella enterica serotype Typhimurium involving cake-batter ice cream served at Cold Stone Creamery locations in the United States. There were 26 illnesses, 5 hospitalizations and 0 deaths associated with this outbreak. The ice cream contained raw cake mix, which was found to be contaminated with the outbreak strain. Cold Stone Creamery voluntarily removed all cake batter products from its stores. Ingredients of the cake mix included egg whites and flour, which can both become contaminated with Salmonella, as well as additional low risk ingredients.
In 2008, an outbreak of Salmonella enterica serotype Typhimurium associated with raw flour caused 67 illnesses, 12 hospitalizations and 0 deaths in New Zealand. In this outbreak, raw flour was typically consumed in the form of uncooked, homemade baking mixtures, such as cake or pancake batter. The outbreak strain was isolated from both unopened and open product from ill persons’ homes. All implicated batches of flour were voluntary recalled by the mill and an advisory statement against consuming uncooked flour was added to product labeling. In alignment with good manufacturing practices, a dry clean scourer-aspirator was also installed at the mill to better remove soil from incoming grain. Two weeks prior to this outbreak, there was an outbreak associated with poultry feed. Traceback efforts discovered that the implicated ingredient of the raw poultry feed was produced at the same grain mill that produced the contaminated flour for human consumption.
In 2009, flour was the suspected vehicle in an outbreak of STEC O157:H7 associated with the consumption of prepackaged Nestlè Toll House refrigerated raw cookie dough. There were 77 illnesses associated with this outbreak, with 35 hospitalizations and no deaths across 30 states. Ten cases developed hemolytic-uremic syndrome (HUS). FDA testing of finished product yielded the outbreak strain. Production was temporarily halted, and all Nestlè Toll House cookie dough products were voluntarily recalled. Following this outbreak, Nestlè converted to using heat-treated flour in cookie dough production and changed its product labeling to more prominently display warnings against eating raw dough. Although flour was not definitively identified as the contaminated ingredient of the cookie dough, it was considered the prime suspect for the source of this outbreak.
In 2016, an outbreak associated with General Mills flour sickened 63 people across 24 states with STEC O121 and O26. There were 17 hospitalizations, no deaths, and 1 case of HUS. The outbreak strain was isolated from open product from ill persons’ homes. General Mills voluntarily recalled flour sold under the brand names Gold Medal, Gold Medal Wondra, and Signature Kitchens. Other products containing General Mills flour, such as prepackaged baking and breading mixes that are sold under various brand names were also recalled.
In 2019, an ongoing outbreak of STEC O26 associated with ADM Milling Co. flour has thus far caused 17 illnesses across 8 states, with 3 hospitalizations and no deaths. The outbreak strain has been isolated from unopened product. Flour produced by ADM Milling Co. is packaged by other companies, including ALDI’s Baker’s Corner, King Arthur, and Hometown Food Company’s Pillsbury Best Bread brands of flour. Implicated products have been voluntarily recalled.
These outbreaks suggest flour can be a source of contamination and cause illness when consumed raw.
In the U.S., wheat is grown in 42 states and is the third largest crop in terms of acreage, production, and value, behind corn and soybeans. During the 2018/2019 growing season, U.S. farmers produced an estimated 1.884 billion bushels of wheat on 47.8 million acres of cropland. Wheat planting production peaked in the U.S. in 1981 but has since fallen by more than 30 million acres and 900 million bushels. Foreign competition, changes in U.S. farm legislation, and changing consumer preferences have contributed to this decline. In 2008, the top wheat-producing states (in order of production) in the U.S. were Kansas, North Dakota, Washington, Montana, and Oklahoma. Almost half of the U.S. wheat crop is exported.
The U.S. classifies wheat varieties as “winter” or “spring,” depending on the season they are planted in. Winter wheat is planted in the fall and becomes established before going into dormancy in the winter. During spring of the following year, winter wheat resumes growth until being harvested in the summer. Winter wheat represents 70-80% of total U.S. wheat production and has a higher yield potential than spring wheat because of its longer growing season. Spring wheat is planted in the spring and harvested in late summer or fall of the same year. Spring wheat is primarily grown in the Northern Great Plains region, where cold winter temperatures can kill winter wheat during dormancy. Yield potential in this region is low due to suboptimal moisture levels and higher growing-season temperatures, which cause wheat plants to mature faster. Irrigation and application of fertilizer increases crop yield potential, while drought conditions and freezing temperatures during spring can reduce yield potential.
Wheat & Flour Quality Measurements
- Test Weight: the weight of a specific volume of grain; provides an indicator of grain quality and an estimate of flour yield
- Grain Protein: the concentrations of proteins in the grain; flours with higher protein concentrations have higher water absorption and stronger, more extensible dough properties
- Falling Number: a measure of the enzymatic activity of the grain; enzymatic activity in dough is needed to convert starches to sugars for the yeast
- Deoxynivalenol (DON) Content: the level of DON toxin present in the grain; acceptable levels in finished products for human consumption are <1ppm
- Moisture Content: the measure of moisture content in the flour; higher moisture contents are conducive to the growth of mold, bacteria, and insects
- Ash Content: the measure of mineral (ash) content in the flour or wheat; provides an indication of the yield that can be expected during milling, because ash is primarily concentrated in the bran
Upon arrival at the mill, wheat proceeds through a cleaning process to remove coarse impurities and is stored according to its quality.
Wheat is then further cleaned via screening, which removes coarse and fine materials and separates grains by size, shape, and weight. Pure whole wheat is moved into conditioning bins.
Conditioning occurs prior to milling to create a uniform moisture content throughout the grain. This helps prevent breakup of the bran during milling and improves separation from the endosperm.
After conditioning, different batches of wheat are mixed (gristed) together to create a blend capable of producing a desired quality of flour.
Milling is the separation of the bran and germ from the endosperm, and the reduction of the endosperm to uniform particle size (flour). Milling is performed through a sequence of breaking, grinding, and separating operations.
Quality of the wheat determines the type(s) of flour to be produced. Different types of flour produced at a mill can be combined to create further variations. Wholemeal flour contains all parts of the kernel, including the bran, germ, and endosperm. Brown flour also contains all parts, but with some germ and bran removed. White flour is composed of only the endosperm part of the kernel.
Types of Flour
Consists of the finely ground endosperm of the wheat kernel.
Consists of white flour milled from hard wheats or a blend of hard and soft wheats. It is usually enriched and can be either bleached or unbleached.
Whole Wheat Flour
Consists of ground whole wheat kernels, but can be created through combining white flour, germ, and bran that have been separating during milling. It contains higher insoluble fiber levels than white flours.
Consists of white flour milled from a blend of hard wheats and has greater gluten strength than other types of flour. It is sometimes conditioned with ascorbic acid.
Consists of white flour from a blend of soft wheats. It has lower protein and higher starch contents than other flour types.
Consists of all-purpose flour with added salt and leavening agents. It is also referred to as phosphate flour.
Consists of white flour from a blend of soft wheats and has properties between that of all-purpose and cake flours.
Consists of ground spring wheat. It has higher protein and lower starch contents than other types of flour.
Consists of the coarsely ground endosperm of durum wheat and is usually enriched.
Consists of finely ground semolina flour.
Until recently, low-moisture foods such as flour have not generated food safety concerns. However, flour is typically considered a raw agricultural product that is not ready-to-eat or pasteurized. There are also multiple opportunities for contamination along the farm-to-fork continuum as grain is processed into flour. While kill steps during food preparation and processing (such as boiling, baking, roasting, microwaving, or frying) eliminate pathogens that cause foodborne illness and may be present in raw flour, consumption of products containing raw flour that have not undergone a kill step have been implicated in outbreaks. Implicated products include prepackaged raw cookie dough, prepackaged baking mixes, raw homemade doughs and batters, cake batter ice cream, and homemade play dough. Potential also exists for cross-contamination when utensils or containers that come in contact with raw flour are used with cooked or ready-to-eat products.
In outbreaks of Salmonella associated with raw dough, raw or undercooked eggs may be considered the most likely source of contamination, but flour has recently emerged as a vehicle for infection and should be investigated. A 2007 study reported Salmonella in 0.14% of wheat flour samples in the U.S., but earlier studies have reported the incidence of Salmonella as 1.34% and E. coli as 12.5%. Additionally, grain is not usually treated to kill microbial pathogens before being milled into flour, so any contamination that occurs in the field can potentially be passed into the final product.
One study demonstrated the ability of E. coli O157:H7 to survive on flowering wheat heads for at least 15 days and internalize in wheat seedlings via contaminated seed, soil, and irrigation water, at internalization rates of 2, 5, and 10%, respectively. Wheat fields in close proximity to cattle operations present potential opportunities for contamination, via abundant rainfall or contaminated irrigation water. Various studies have demonstrated that E. coli is able to survive for 2 months, 6 months, or as long as 500 days in soil, depending on temperature.
Flour should be stored in a cool, dry place in airtight containers. All-purpose, bread, and cake flours will keep for 6 months to 1 year if stored at 70°F and for 2 years if stored at 40°F. Wheat flour should be kept refrigerated or frozen, if possible. Naturally occurring oils in flour, particularly whole wheat flour, oxidize when exposed to air, especially at room temperature, and cause flour to turn rancid.
Food safety programs in the grain milling industry designed to reduce and prevent the risk of microbial and mycotoxin contamination include hazard analysis & critical control point programs (HACCP), good manufacturing practices (GMP), food defense programs, and audits. Ready-to-eat, heat-treated flour is also available to consumers.
Deoxynivalenol (DON) is a mycotoxin produced by the Fusarium species of fungi that frequently infect grains, such as corn, wheat, oats, and barley, in the field or during storage. DON is also known as a vomitoxin due to its strong emetic effects after consumption. DON is associated with Fusarium Head Blight, also called scab, a disease of wheat and barley that causes yield loss, low weight, low seed germination, and mycotoxin contamination of grain. There is a 1ppm limit set by the U.S. Food and Drug Administration for DON in all finished wheat products intended for human consumption. Mycotoxins, including DON, are highly stable, able to withstand high temperatures, and its levels in grain will remain unchanged even after years in storage. DON has been detected in buckwheat, popcorn, sorgum, triticale, flour, bread, breakfast cereals, noodles, infant foods, pancakes, malt, and beer.
The best method for addressing DON contamination in flour is to prevent contaminated grain from entering the milling process. Screening of incoming wheat, using quick tests such as ELISA test kits, allows millers to reject supply sources exceeding safe levels of DON. It is also possible to remove DON contaminated kernels during the cleaning process, using gravity tables and optical sorters, but this process concentrates DON in wheat screenings, preventing its use in animal feeds.
Flour is a low moisture food with a water activity (Aw) level of 0.87 or lower. Generally, an Aw of 0.95 or higher is required to support microbial growth. The main concern with flour is that microbial contamination can be carried through the milling process, survive in processed flour, and pose a risk for foodborne illness when flour is incorporated into a food product.
One method that has proven effective in removing microbial contamination is aggressive scouring or abrasion, also referred to as debranning or peeling, of wheat to remove the outer bran layer during the conditioning process. This process requires a high amount of energy and the rate of bran removal is difficult to control. Additionally, it is not possible to entirely remove the bran from the kernel. Debranning also results in the concentration of microbial contamination in the removed bran.
Another method for controlling microbial contamination in wheat is tempering water with ozone or chlorine. Chlorination is cheaper and more effective than ozone but may leave residue.
Treatment of the final product, flour, is more expensive than treating grain. Effective methods include gamma ray irradiation and heat treatment. However, gamma ray irradiation has been shown to negatively impact dough quality. Likewise, if performed improperly, heat treatment can also negatively impact dough quality and cause browning.
U.S. per capita use of wheat flour trended upward from the 1970s to the late 1990s, but sharply dropped in 2000 due to changing consumer preferences; most notably, an increased interest in low-carbohydrate diets. In 2014, the estimated per capita use of wheat flour was 135 pounds per person.
U.S. consumption of wheat products (such as breads, pastas, and pizza) has dropped sharply since 2000, reversing a three-decade trend of growth in per-capita consumption. Wheat consumption fell from an estimated 146.3 pounds per person in 2000 to a low of 133.4 pounds in the mid-2000s, recovered slightly, then dropped back to 132.5 pounds per person for 2011. The drop from 2000 may reflect public interest in lowering carbohydrate consumption. The rise in wheat consumption that started approximately 30 years ago was also triggered by health concerns. In the 1970s, Americans began shifting from animal products to grain-based foods, including wheat products, because of concerns about cholesterol and heart disease.
This shift was later upturned when gluten became increasingly vilified by the public in the 2000s, where some evidence suggested that gluten was associated with schizophrenia, autism, chronic inflammatory conditions, diabetes, and other ailments. These assertions were coupled with a rise in gluten-free diets among individuals without Celiac disease or non-celiac gluten sensitivity and a concurrent boom in the gluten-free food and beverage industry in the early 2010s. Consequently, there are currently a wide variety of gluten-free products available to replace wheat and gluten-containing products, including breads, cereal, pizza, noodles, and snack products.
In spite of the reduction in consumer consumption of products containing wheat flour, outbreaks have become more common in recent years. To lower the risk of foodborne illness, it is recommended that products containing raw flour, such as raw doughs or batters, not be consumed. Consumers are also reminded to wash their hands, work surfaces, and utensils thoroughly after contact with raw dough products or flour.
Information on storing flour for safe consumption please visit FoodKeeper App.
All-purpose, enriched, bleached wheat flour
(kcal per 1 cup or 125g)*
|*Calories by source: 2% fat, 86% carbs, 11% protein|
Wheat flour contains dietary fiber, B vitamins (thiamin, riboflavin, niacin, and folate), calcium, iron, magnesium, phosphorus, potassium, and zinc and is also a good source of complex carbohydrates. The bran component of a wheat kernel contains fiber, the endosperm contains protein, carbohydrates, and small amounts of B vitamins, and the germ contains trace minerals, unsaturated fats, B vitamins, antioxidants, and phytonutrients. Dietary fiber helps reduce blood cholesterol, lowers the risk of heart disease and type 2 diabetes, and aids in bowel function. B vitamins are important for metabolism and a healthy nervous system. Folate (folic acid) helps with the formation of red blood cells. It is also important for women of child-bearing age who may become pregnant to consume adequate amounts of folic acid to reduce the risk of neural tube defects such as spina bifida and anencephaly, during fetal development. Iron is required to carry oxygen in the blood and inadequate amounts of iron can result in iron-deficiency anemia.
Food products made from wheat grain can be classified as whole grain or refined grain. Whole grain products contain the entire wheat kernel, including the bran, germ, and endosperm, while refined grain products only contain the endosperm. Refined grain products generally have a finer texture and longer shelf life, but less dietary fiber, iron, and B vitamins than whole grain products. Many refined grain products, such as white flour, are enriched with B vitamins (thiamin, riboflavin, niacin, and folic acid) and iron, meaning nutrients are added back to the processed product in equal or greater amounts than those found in the unprocessed product. Likewise, grain products may also be fortified with folic acid or calcium, meaning the nutritional status of the product is higher than that of the unprocessed product. Bleaching does not affect the nutritional value of flour.
For a 2,000-calorie diet, the 2015-2020 Dietary Guidelines for Americans, Healthy U.S.-Style Eating Pattern recommends consuming 6 ounce-equivalents of grain per day, of which 3 or more ounce-equivalents should be whole grain.
Figure 4. The Whole Grain Stamp (https://col.st/xqa8J)
The Whole Grain Stamp (Figure 4), created by the Whole Grains Council, is a graphic that appears on food labels to help consumers identify products containing a half or a full serving of whole grains per serving. A “Good Source” contains half a serving of whole grains with at least 8g of whole grain per serving. An “Excellent Source” contains a full serving of whole grains with at least 16g of whole grain per serving. A “100% Excellent Source” contains a full serving of whole grains with at least 16g of whole grain per serving and no refined grains.
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