Flour

Key Facts

Flour is the product obtained from grinding the endosperm of uncooked cereal grains, usually wheat kernels.

Although various grains, pseudo-grains, and even nuts and tubers can be used to make flour, wheat flour is the predominant consumer choice because of its superior baking properties.

A food staple for hundreds of years, flour has emerged recently as a potential carrier of pathogens like E. coli and Salmonella.

When grain is milled, the fatty acids deteriorate and oxidize, creating the potential for rancidity to develop. The rate of deterioration varies, depending on temperature, fat content, and grain quality.

Flour is classified as a minimally processed agricultural ingredient and is not a ready-to-eat product. Baking temperatures will generally inactivate any pathogens in flour, but raw products containing flour, such as cookie dough, have been the source of several foodborne illness outbreaks.

Deoxynivalenol (DON) is one of several mycotoxins produced by certain Fusarium species that frequently infect grains in the field or during storage.

Introduction

Wheat (Triticum spp.) flour is the product obtained by grinding whole wheat kernels, sometimes called berries (Figure 1). The three parts of the kernel—the bran, germ, and endosperm—are separated during milling then recombined to create different types of flours. Although any grain can be used to make flour, wheat flour is the predominant consumer choice because it provides the protein structure needed for baked goods. Gluten proteins, found in wheat, barley, rye, and triticale (a wheat hybrid), make dough elastic and stretchy, trapping gas released during yeast fermentation and forming the light, airy structure desired in baked goods.

Although most flours are derived from grains such as wheat, barley, or rye, they can be milled from corn, rice, potatoes, beans, nuts, roots, or crops known as pseudo-grains, such as quinoa, amaranth, or millet. The type of flour selected affects the quality of the product, and many products are made with a combination of different flours.

Foodborne Outbreaks

Pathogens that have been associated with foodborne outbreaks involving flour are Salmonella spp. and Escherichia coli (STEC 0121 and O157H7).Due to flour’s low moisture content and because it is typically exposed to heat during cooking, microbial and bacterial contamination of wheat flour has not been a major food safety concern. Wheat flour generally has a low water activity (aw) level < 0.87; however, if bacteria are present, growth can occur when the flour is further processed into a food product with a higher moisture content.

Although flour has not been a food of major concern, illnesses have been associated with the consumption of raw flour. In 2016, 63 people across 24 states were infected with E. coli O121. All of the cases reported consuming specific flour between December 2015 and May 2016. While tracking the cause of the illnesses, investigators determined approximately half of the individuals reported making something homemade with flour at some point prior to becoming ill, and some reported using a General Mills brand of flour (US FDA, 2016). There were 17 people hospitalized as a result of the outbreak, including one who developed hemolytic uremic syndrome, but no deaths were reported.

In 2009, flour was the suspected contamination source in an outbreak involving the consumption of raw cookie dough. There were 77 illnesses associated with the outbreak. Approximately 3.5 million packages of cookie dough and reformulated products were recalled (Neil, et al., 2009). Over 10% of cases had consumed the cookie dough raw. This was the first outbreak in which flour was suspected as the vehicle of transmission of E. coli O157:H7.

In New Zealand, in 2008, 66 cases of Salmonella were associated with the consumption of raw baking mix containing flour (Eglezos, 2010). Another outbreak occurred in 2005 involving cake-batter ice cream served at Cold Stone Creameries in the United States. Twenty-six cases were directly linked to the cake-batter ice cream, which was subsequently found to be contaminated with Salmonella (Zhang, et al., 2007). The outbreak caused 5 hospitalizations but no deaths. These outbreaks suggested flour could be a route of contamination for humans when it is consumed raw. Salmonella was found in 1.3% of collected flour samples in Australia and North America (Gilbert, et al., 2010).

Production

During the 2016/2017 growing season, farmers in the United States produced a total of .820 billion bushels of wheat on 50.2 million acres of cropland. Due to decreased demand for wheat and declining per capita wheat consumption, the 2017/2018 season is projected to experience a record-low reduction to 46 million acres of producing cropland. The top wheat-producing states (in order of production) in 2018 were Kansas, North Dakota, Washington, Montana, and Oklahoma (Dvorak, 2009). Almost half of all U.S. wheat crop is exported.

Wheat grains grow at the top of the plant and are closely packed together in clusters called ears. Each ear of wheat consists of 45-50 grains; however, this can vary depending on the type of wheat. Each grain of wheat has three distinct parts: the coarse outer layer (bran), the embryo (wheat germ), and the starchy store of food the germ feeds on while it grows (endosperm). The endosperm is used in making white flour. The majority of wheat grown in the U.S. is soft, or common, wheat. Soft wheat is well-suited for cakes, cookies, crackers, and pastries and is lower in protein content.

Milling is the process which turns wheat into flour. Different parts of the wheat grain are used to make different types of flour. After milling, the wheat germ and bran can be added back to white flour in different amounts to create either brown or wholemeal flour.

Wheat Specifications

Variety: different varieties produce different flour qualities

Moisture content: this determines the length of time that a grain can be stored

Specific weight: a high weight is considered more desirable

Enzyme activity: high enzyme levels can result in sticky bread, making slicing difficult

Protein quality and quantity: critical for baking purposes

Processing

During processing, powerful magnets, metal detectors, and specially designed equipment extract metal objects, stones, and other plant materials from the wheat grain. Throughout the cleaning process, air currents lift off dust and chaff (grain husks). Conditioning with water softens the outer pericarp (bran) layer of the wheat and makes it easier to remove the endosperm during milling.

Stage 1: The grist (grain that has been separated from the chaff) is passed through a series of fluted ‘break’ rolls rotating at different speeds. These rolls are set so they do not crush the wheat but shear it open, separating the white, inner portion from the outer skins.

Stage 2: The fragments of wheat grain are separated by a complex arrangement of sieves. White endosperm particles are channeled to a series of smooth ‘reduction’ rolls for final milling into white flour. The cleaned and conditioned wheat is blended with other types of wheat, in a process called gristing, to make different kinds of flour. The bran, wheat germ, and endosperm are separated to facilitate blending to make different types of flour. Wholemeal flour uses all parts of the grain. Brown flour contains about 85% of the original grain, with some bran and germ removed. White flour is made from the endosperm only. For some types of products, wheat gluten is added to increase the protein content of milled flours. Wheat germ and bran may be used for certain breads and cereals or sold as health foods. The remainder is blended into wheat feed for livestock food.

Flour must be kept cool and dry. All flours, even white flour, have a limited shelf life. Millers recommend storing flours for no more than 6 months. Over time, oils in flour are oxidized, resulting in rancid off-flavors. The process is more rapid if flour is exposed to air or warm temperatures. To maximize quality, flour should be stored and covered in a cool, dry area to prevent absorption of moisture and odors and to prevent exposure to insects or rodents (Dvorak, 2009).

Types of Flour

White Flour
White flour is the finely ground endosperm of the wheat kernel made from hard, high-protein wheat. It has more gluten strength and protein content than all-purpose flour and can be unbleached, meaning it is naturally aged to enhance baking qualities, or bleached, meaning chemicals are used to speed up the natural aging process. Sometimes it is conditioned with ascorbic acid, which increases volume and creates a better texture.

Whole Wheat Flour
Whole wheat flour is made from either grinding whole wheat kernels or recombining the white flour, germ, and bran that have been separated during the milling process. White whole wheat flour is ground to a finer particle size.

Bread Flour
Bread flour is a white flour blend of hard, high-protein wheats with 12% to 14% protein (gluten), which leads to a better rise and better gas formation in yeast breads.

Gluten-Free FlourGluten-free flours include flour blends made from grains other than wheat, barley, or rye. They are typically used by people with gluten sensitivities, and they often require additional ingredients to bind baked goods.

Food Safety

Flour requires extensive handling during harvesting, milling, packing, and storage, creating multiple opportunities for contamination. Wheat flour is an agricultural product usually sold without pasteurization treatment and, consequently, could contain pathogenic bacteria (Martinez, 2015). Wheat production in the U.S. takes place largely in Kansas, North Dakota, Montana, Texas, and Washington (Vidovic, et al., 2007), states which also have high numbers of cattle operations (USDA, 2014). Wheat fields that are located close to livestock feedlots may increase the risk of E. coli O157:H7 contamination of wheat heads during rainstorms or irrigation. Therefore, the control of pathogenic bacteria in irrigation water is a key strategy in preventing contamination of wheat.

Managing food safety risks

“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 process of baking, frying, boiling, and cooking are adequate to destroy any pathogens that may be present in flour and eliminate any potential risk of foodborne illness.” (North American Milling Association)

The risk of flour contamination is challenging to control for several reasons: grains are grown in an open environment; harvesting equipment is not easily cleaned and sanitized; and treatments, like washing, are not an option with grains, which need to stay dry. In general, contamination of flour is not a problem because pathogens would be destroyed by baking, boiling, or frying. However, if flour is consumed raw, in cookie dough, for example, illness may result. Potential also exists for cross-contamination when utensils or containers that come in contact with flour are used with ready-to-eat products.

Martinez, et al., investigated whether E. coli O157:H7 could be translocated into the internal tissues of wheat (Triticum aestivum) seedlings from contaminated seed, soil, or irrigation water and whether the bacterium could survive on flowering wheat heads. The results showed internalization was possible using contaminated seed, soil, and irrigation water in wheat seedlings, with internalization rates of 2, 5, and 10%, respectively. Even though the rates were low, this appears to be the first study to demonstrate the ability of this strain to reach the phylloplane in wheat. In the head contamination experiment, all samples tested positive, showing the ability of E. coli O157:H7 to survive on the wheat head (Martinez, et al., 2015).

Food safety programs in milling and flour production include Good Manufacturing Practices (GMPs), Hazard Analysis Critical Control Points (HACCP), and audits to ensure best practices are being followed. Several experimental treatments of wheat during the cleaning process are being researched to determine their effectiveness in lowering the microbial load of flour. To reduce the risk of contamination, specialized heat treatments can be used on flour for commercial production; however, treated flour is not generally available to consumers.

Deoxynivalenol (a.k.a. DON/Vomitoxin)

Deoxynivalenol (DON) is one of several mycotoxins produced by certain Fusarium species that frequently infect corn, wheat, oats, barley, rice, and other grains in the field or during storage (Sobrova, et al., 2007). It is a naturally occurring mycotoxin, mainly produced by Fusarium graminearum (Kushiro, 2008), and is also known as vomitoxin due to his strong emetic effects after consumption. It is transported into the brain, where it binds to dopaminergic receptors. DON can be produced in wheat kernels that are infected with Fusarium head blight (FHB) or scab prior to harvest while in the field. It has been detected in buckwheat, popcorn, sorghum, triticale, flour, bread, breakfast cereals, noodles, infant foods, pancakes, malt, and beer (Sobrova, et al., 2010). DON can withstand high temperatures, making it a concern in food products (Hughes et al., 1999).

In the United States, foods or clinical samples linked to gastroenteritis outbreaks are not routinely tested for DON or F. graminearum by state or federal public health labs, as is commonly done for other foodborne pathogens and their enterotoxins. In one notable exception, however, the Centers for Disease Control and Prevention (CDC) reported 16 gastroenteritis outbreaks in 1,900 school children from seven states who consumed burritos from two manufacturing plants (Steinberg et al. 2006).

Preventing contaminated grains from entering the milling process is the best control method. Quick tests for DON utilize kits based on ELISA technology. This technology allows milling companies to screen incoming wheat deliveries. It is possible to remove DON contaminated wheat kernels during the wheat cleaning process using gravity tables and optical sorters. One processing method that has proven effective is aggressive scouring or abrasion of the wheat to remove the outer layers of bran prior to milling. Referred to as debranning or peeling, this processing step can be added during the wheat conditioning process. However, this method cannot completely remove the bran from the crease of the kernel, which may be the part of the wheat berry most susceptible to microbial contamination. Debranning also has the drawback of concentrating the microbial contamination in the bran portion removed, creating the challenge of how to dispose of or treat the byproduct (Fowler, 2013). In addition to debranning, various food safety treatments are used during grain storage. For example, ozone has been used as a fumigant to control insect infestation (Fowler, 2013).

Consumption

U.S. per capita all-wheat flour use has fluctuated significantly since the 1930s but has been steadily declining since the early 2000s. Per capita consumption for 2015 is estimated at 133.0 pounds, well below the most recent peak of 147 pounds in 1997 and an earlier peak of 163 pounds during World War II (USDA, 2016).

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 (USDA, 2016). 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.

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 (FDA, 2016). Consumers are also reminded to wash their hands, work surfaces, and utensils thoroughly after contact with raw dough products or flour.

Nutrition

Bread and flour-based foods are an important part of the diet for millions of people worldwide. Their complex nature provides energy, protein, minerals, and many other macro and micronutrients.

Most all-purpose flour found in grocery stores is enriched, which means nutrients lost during the processing procedure are added back into the flour. Products labeled as enriched must contain no less than 10 percent more nutrients than the same products not labeled as enriched. The FDA requires enriched flour to contain a certain amount of iron, thiamin, riboflavin, folic acid, and niacin. Dietary reference intake (DRI) percentages listed are for ¼ cup of enriched, white, all-purpose flour and apply to adults under age 50. Because iron is added to enrich all-purpose flour, one cup provides 18 percent of the DRI for men. However, even when enriched with iron, one serving only provides 8 percent of the DRI for women, who require more than twice as much iron per day as men. All-purpose flour that is not enriched supplies only 5 percent of the DRI for iron for men and 2 percent for women (Thompson, 2015).

Nutrition summary: wheat flour is low in saturated fat, cholesterol, and sodium and is a source of thiamin, folate, and selenium.

Nutrition Summary

Calories (per 1 cup)*

Fat

Carbs

Protein

455

1.22 g

95.39g

12.91 g

*Calorie breakdown: 2% fat, 86% carbs, 12% protein.

Because flour is not a high-profit commodity, manufacturers often mill from strains of wheat that offer the greatest yield of flour from a given amount of grain, a practice that can be at odds with good baking properties (Cook’s Illustrated, 1999). High-protein flours are generally recommended for yeasted products and other baked goods which require structural support. The higher the protein level in a flour, the greater the potential for the formation of gluten. The sheets that gluten forms in dough are elastic enough to move with the gas released by yeast but also sturdy enough to prevent that gas from escaping. Typically, all-purpose flours are bleached with either benzoyl peroxide or chlorine gas. The latter not only bleaches the flour but also alters the flour proteins, making them less inclined to form strong gluten. Chemically bleached flour is whiter in color and often associated with higher quality.