WHEAT FLOUR FUNDAMENTALS
May 6, 2004
What could be more basic to baking than wheat and wheat flour?
Intentional cultivation of cereal grains probably began in the Middle East. Archeologists, working in the West Bank region at Jericho, the oldest existing city in the world, uncovered a grain storage bin dated to about 6000 B.C. Numerous other finds of wheat and barley remains throughout the region range from prior to that date to the beginning of recorded history.
Some of the earliest tomb murals in Egypt depict the planting and harvesting of grain, and converting it into bread. The conversion process includes isolating the seed from the husks (threshing), grinding it (milling), wetting and mixing to make dough, allowing some natural fermentation to occur and, finally, baking. While each step has been refined over the centuries, the process still uses the same starting materials to produce a foodstuff that is widely consumed and enjoyed.
Wheat, like other cereal grains, is a grass belonging to the family Graminae and comprises the genus Triticum. A cross between wild einkorn (T. aegilopoides) and an unknown wild grass produced cultivated einkorn (T. monococcum). Kernels of both varieties, carbon-dated to 6700 B.C., were found in excavations at Jarmo, a settlement located in the fertile crescent around the Tigris and Euphrates rivers. Two other species, wild and cultivated emmer (T. dicoccoides and T. dicoccum, respectively), have been found in numerous Egyptian burial sites.
Fourteen different species of wheat are recognized by plant biologists. Of these only three — common wheat (T. aestivum), durum wheat (T. durum) and club wheat (T. compactum) — account for more than 95% of all wheat grown in the world today. Common wheat represents approximately 90% of all wheat and includes most of the varieties: hard or soft, red or white, winter or spring. Durum wheat is about 4% to 5% of total wheat crop and is used primarily for the production of dry pasta goods. Club wheat, which is very soft, accounts for only 1% to 2% of total production and grows only in limited areas.
The wheat kernel consists of three parts: endosperm, about 83% of the kernel; bran, about 14.5%; and germ, about 2.5%.
Endosperm is the source of white flour. It contains about 90% starch and protein, with the rest being moisture and small amounts of lipid, ash and pentosans.
Bran is high in vitamins, minerals and dietary fiber. It is often considered as two separate components, which occur in about equal parts. The pericarp is the outer layer, consisting mainly of fibrous materials, such as tannins and cellulose. The aleurone is a layer of cells adjacent to the endosperm, containing roughly 16% ash, 19% protein and 9% lipid, with the remainder being moisture and fiber.
Germ is the embryo part of the kernel and contains much of the enzymes and lipids of the whole seed. It contains about 25% protein and 10% lipid.
The difference between hard and soft wheat resides in the endosperm, the starchy interior part of the kernel. In soft wheat varieties, the starch granules are less tightly bound to the protein matrix than in hard wheats. This is apparently due to friabilin, a small protein present in soft wheat.
Colored phenolic compounds in the bran coat create the color differences among wheat varieties. These compounds also give a bitter taste. Whole white wheat flour has a blander flavor than whole red wheat flour. This difference benefits various whole-wheat products such as bread, rolls and muffins, where the milder taste of the white wheat flour is often more acceptable to consumers.
Agronomic differences characterize winter and spring wheat. Winter wheat is sown in the fall and germinates, then goes dormant during the winter. It revives in spring and grows to maturity by mid-summer. Spring wheat is planted in the spring and undergoes the complete growth cycle to maturity during one growing season. In North America, spring wheat predominates in the northern tier of states (Minnesota, the Dakotas and Montana) and in central Canada. Winter wheat is grown in other parts of the United States and eastern Canada. White wheat, both winter and spring types, grows in roughly the same areas as the corresponding red types.
Wheat and wheat products comprise six components: moisture, carbohydrates, protein, lipids, ash and fiber. Each is functional in some fashion in baking. Due to nutritional implications, it is also important to consider the vitamin content of wheat and wheat products.
Moisture. Wheat moisture is both an economic and a handling factor. Trading is done on the basis of 14% moisture in the kernel. In other words, a bushel of wheat weighing 60 lb is reckoned as containing 51.6 lb of dry matter. Excess moisture in the wheat can be a quality detriment and a danger. Wet wheat can begin to respire, generating heat that can harm its quality for milling and baking purposes. In extreme cases, heat generation can cause the grain to begin burning.
By law, wheat flour may not contain more than 14% moisture. The baker and supplier may, of course, reach an agreement that flour deliveries will have a lower limit. In practice, bulk flour generally contains 13.0% to 13.5% moisture, although this may vary according to climatic conditions. Flour reversibly adsorbs and desorbs humidity from the air. A baker in a hot, dry location or a cool, humid region should be aware that pneumatic transportation of flour in the plant, where it is carried with ambient air, may affect actual moisture of the flour when it reaches the weighing bin.
In packaged cake mixes, moisture from the flour may react with the chemical leavening, causing some deterioration (loss of CO2) during storage. For this application, cake flour is often dried to a moisture content of about 8%. The improved storage stability of the mix justifies the expense involved in drying.
Carbohydrates. Starch is the main carbohydrate of wheat and flour. Normal wheat starch is made up of about 25% amylose (the smaller, linear starch molecule) and 75% amylopectin (the much larger, branched molecule). In the presence of excess water, as in an Amylograph, wheat starch gelatinizes at about 65°C (159°F). In limited water systems, including most baking formulations, the gelatinization temperature is 5°C to 15°C (9°F to 27°F) higher. In extreme water-limited situations, such as cookie dough, most starch granules never gelatinize.
Flour contains 1.0% to 1.5% sugars and dextrins. These include glucose, fructose, maltose, sucrose, melibiose, raffinose, gluco-difructose, oligofructans, and oligoglucans. Yeast prefers glucose and fructose for fermentation, but it will synthesize the enzyme maltase to hydrolyze maltose to glucose.
In a lean dough, the natural sugars alone are not sufficient to support the amount of fermentation normally desired. The action of a-amylase on damaged starch (releasing dextrins) and b-amylase on the dextrins (forming maltose) provides the substrate for continued yeast fermentation. The addition of barley malt or fungal amylase to a dough provides the needed a-amylase; flour contains sufficient b-amylase to hydrolyze dextrins for fermentation. The addition of sugar to the dough eliminates the need for added a-amylase, although the enzyme may be added for other purposes such as improving pan flow or extending shelf life.
Protein. The protein content of wheat ranges from 7% to 20%. The actual protein content is governed by several factors — genetics, geography, climate conditions and nitrogen fertilizer application, if any.
Soft wheat generally contains protein in the range of 7% to 11%; hard wheat has 10% to 17% protein; and durum wheat 14% to 20%. Different varieties within a species, grown under identical conditions, may well have different protein contents. The observed range of protein content in any one cultivar may be quite wide due to environmental factors.
Protein is higher in the bran and germ portions of the kernel than the endosperm, which is the source of flour. Thus, wheat containing 12% protein will yield straight-grade flour containing about 11% protein. Increments of 0.8% to 1% are fairly constant across different types of wheat. Of this 11%, about 2% will be non-gluten proteins (mostly enzymes of various sorts), while the other 9% is gliadin and glutenin. These two proteins are the important ones with respect to dough mixing and baking properties.
Bread bakers consider protein quality to be very important. Flour from two different cultivars, at the same protein content, may give quite different results as with loaf volume in the bakery. Genetics accounts for some of the performance differences, and certain genetically determined types of glutenin subunits result in stronger glutens than other types. As part of the screening program applied to new varieties, acceptable results from bake testing are required before the cultivar is released to the farmer.
Lipids. The wheat kernel contains about 3.5% to 4.0% total fat. Approximately half is in the endosperm, about one-quarter in the bran layers and one-quarter in the oil-rich germ. The lipids in the endosperm, which carry through to flour, are of interest to bakers since they play a role in the development of gluten during dough mixing and influence the gluten quality.
Flour contains about 1.0% nonpolar lipids, mainly triglycerides, and 0.8% polar lipids, phospholipids and glycolipids. The phospholipids are essentially the same as those found in soy lecithin. A glycolipid is a diglyceride with an attached sugar, either galactose or digalactose. These polar lipids are emulsifiers and act as such during dough mixing. During mixing, the lipids interact strongly with the gluten. xylose with attached side chains of sugars, mainly arabinose. Tannins are polyphenolic compounds, found mainly in the bran layers.
Flour contains about 2% arabinoxylan and 1% arabinogalactan (which has galactose rather than xylose as its backbone polymer). The terms arabinoxylan and pentosan are synonymous, but in bakery literature, pentosan is more often used. Likewise, enzymes that hydrolyze arabinoxylans are called either xylanases or pentosanases. Pentosans absorb five to 10 times their weight in water, so this rather small fraction contributes greatly to the water absorption of a flour.
Three types of pentosans are defined in a bakery situation by the effect of enzymes. High-molecularweight soluble xylans have a relatively large number of arabinose side chains, adsorb much water and impart a sticky characteristic to dough. Debranching pentosanase removes many of the arabinose side chains, forming high-molecular-weight insoluble xylans that bind somewhat less water and are not sticky. Xylanase, an endo enzyme, cleaves bonds in the backbone polymer, forming soluble xylans of lower molecular weight that adsorb even less water but may make the dough somewhat sticky. In baking, the major effect of differences in total pentosan content in flour comes in absorption changes.
The arabinogalactans in flour are linked to peptides. From the research carried out on them thus far, it appears they have low water absorption properties.
Vitamins. Whole-wheat is a good source of B-complex vitamins — thiamine, riboflavin, niacin, pyridoxine — as well as pantothenic acid, folic acid and vitamin E. Fat-soluble vitamins A, D and K are essentially absent. Most of the vitamins are present in the bran and germ of the wheat kernel. The content of various water-soluble vitamins in flour is only 15% to 40% that in whole-wheat. Vitamin E, a fat-soluble vitamin, is located mainly in the germ. Small amounts of germ oil are expressed during milling, ending up in the flour.
During the 1920s and 1930s, the incidence of vitamin B deficiency diseases, such as beriberi and pellagra, in the US population caused concern among public health officials. The switch in consumption from whole-wheat to white flour was seen as one contributing factor. In 1941, the federal government issued standards for enrichment of white flour and self-rising flour with three B vitamins. Folic acid was added in 1998. Enrichment of corn meal and grits was also approved. As a result of the widespread adoption of enriched flour, cases of
Ash. Whole-wheat contains about 1.7% ash. The three main parts of wheat contain ash at 0.5% in the endosperm, 4.2% in the germ and 8.1% in the bran layers. There is a gradient of ash and protein levels in the endosperm, from low in the center to high near the bran (aleurone) layer. Flour specifications often include a maximum acceptable value for ash. This is not due to any bakery functionality of ash, but rather it indicates how well the miller has separated bran cells from endosperm. At one time, bakers used a benchmark of 0.42% ash for bread flour; today, a specification of 0.50% to 0.54% is more usual. Flours from two different wheats at the same extraction rate may have identical baking properties but different ash contents.
Fiber. Wheat fiber is made up mainly of four components: cellulose, hemicellulose, xylans and tannins. Cellulose is the b-1,4-linked glucose polymer commonly found in plant stems and present to some degree throughout the wheat kernel. Hemicellulose is a complex mixture of polymers of various sugars, linked through glycosidic bonds that are not hydrolyzed by amylase. It is a major component of cell walls and occurs in both the endosperm and the bran layers. Arabinoxylans, also called pentosans, have a backbone of polymerized beriberi and pellagra were rare by 1950. Iron is also a component of flour enrichment, and calcium supplementation is strongly advised but not mandatory.
Milling’s goal is to maximize the yield of flour with minimal contamination by bran or germ. This is called the extraction rate. In the past, 72% extraction was considered a typical rate, but advances in milling technology have raised this to 75%. The difference is significant. Flour sells for $8 to $12 per hundredweight (cwt) while millfeed goes for $2 to $3 per cwt. The miller maximizes flour yields by using a combination of roll stands to reduce particle size, purifiers to separate bran from endosperm chunks and sifter stacks to segregate materials of various particle sizes.
A typical flour mill employs six sets of break rolls (spirally grooved rollers that break wheat and large endosperm chunks into smaller pieces) and six sets of reduction rolls (smooth rollers that grind purified endosperm pieces into flour). From the sifters associated with these roll stands, the miller collects numerous streams of flour. The total flour is called straight grade, but various streams are taken from it to make the different types of flour. For example, the 50% of streams lowest in ash and protein might be blended to make “fancy patent” flour. The remaining 45% of the flour would be sold as “first clear.” “Poor second clear” flour is almost never used for baking; it is sold to makers of wallpaper paste and similar adhesives. Ordinary bread flour, called “bakers patent,” comprises 90% to 95% of the straight grade flour.
The composition of products of wheat milling varies according to the type of wheat and the extraction rate. Flour obtained from soft red winter wheat will have much lower protein content than flour or semolina from durum wheat. Within a given wheat type, the variations mentioned earlier will result in flours with differing protein, ash and lipid contents.
The yield of the fractions also varies, depending on the milling setup and the extraction rate used by the miller; however, typical distributions might be 75% as flour, 1% as purified germ, 12%; as bran and 12% as “shorts.”
“Red dog” is the flour obtained toward the end of the milling process. It is a high-protein, high-ash fraction, made up mostly of aleurone material. It is included in the “poor second clear” part of the milling chart. “Shorts” is a stream derived mainly from the sifters during the break stages of milling. Shorts includes bran flakes with tightly adhering endosperm, such that the two portions cannot be efficiently separated. The shorts stream is usually combined with bran and germ to make millfeed, sold for animal feed. Some mills purify germ; the high vitamin content makes it attractive as a health food component.
Flours with many different characteristics are required to accommodate today’s broad spectrum of bakery products.
Hard red winter (HRW) patent flour is used for pan bread and rolls. Twenty years ago, flour with 10.8% protein would have been considered unsuitable for this purpose. Plant breeders have increased the protein quality in recent cultivars, and this factor, plus advances in baking technology such as the judicious use of dough strengtheners and enzymes, allow the successful use of this flour. HRW flour is also used in laminated doughs and donuts, often with the admixture of some soft wheat flour for better final product characteristics.
Hard red spring (HRS) patent flour is used for hearth bread and rolls and pizza crusts. Its higher gluten level gives the added strength needed for these products. HRS flour is also often used in variety bread, where the presence of non-wheat components requires more gluten strength.
HRS high-gluten flour is made from high-protein wheat. It functions as a strengthening flour, added where the regular patent flour is somewhat weak for the application.
HRS clear is the traditional flour for use in bagels. Some bagel makers use a “stuffed” high-gluten flour, which is HRS high-gluten plus 10% to 20% of clear flour.
Soft red winter (SRW) cookie flour is intended primarily for the production of cookies. How batter spreads during baking is an important factor in cookie production. The baking test for cookie flour measures the width-to-height ratio — the spread factor — as a flour quality factor. The alkaline water retention capacity test for soft wheat flours shows the best correlation with spread factor. Use of hard wheat flour or chlorinated soft wheat (cake) flour decreases the spread factor. If the spread in the bakery is too great, substitution of 10% to 20% cake flour will often provide the necessary correction.
SRW cracker flour must have sufficient strength to give lift to the crackers during baking. It is made either by starting with soft wheat having a higher protein content or by selecting the flour remaining after the very low protein streams (cake flour) have been removed.
SRW cake flour is the low-protein (“fancy short patent”) cut from the soft wheat flour streams. Chlorination lowers the gelatinization temperature of the starch, allowing it to swell and increase batter viscosity at the point during baking where the chemical leavening gases are being released. Two different methods are commonly used to write formulas (recipes) for baked products: “bakers percent” and “formula percent.” Both are based on measuring the weight (not volume) of ingredients. The two methods are mathematically related, and conversions can be made back and forth between them, according to individual plant practice.
This method measures the weight of individual formula ingredients as a percentage of the total flour weight. The total flour is always 100%. Thus, the sum of all ingredient percentages is always more than 100%.
To change measurements in lb and oz (or g) into bakers percent, first determine the total amount of flour in the formula. If more than one type of flour is needed, add all the weights together. This figure becomes the base number for figuring the other percentages. To determine the bakers percent of water, for example, you divide the weight of water by the total weight of flour.
In order for this method to work properly, you must convert all weights and volumes into a common measure. One oz equals 1/16 (0.0625) lb. Thus 20 lb 8 oz becomes 20.50 lb. Volumetric measurements, too, must be converted to the common measure. Honey, for example, weighs 12 oz (340 g) per cup. Thus 3 cups of honey weigh 2 lb 4 oz or 2.25 lb (1,020 g).
This method measures the weight of individual formula ingredients as a percentage of the total formula weight. The sum of all ingredient percentages is always equal to 100%.
Most formulas will indicate their “basis” in their headings. When total flour weight is being used as the formula basis, the terms flour weight basis, per cwt flour and bakers percent are used. When total formula weight is being used as the formula basis, the terms conventional percentage, formula weight basis and formula percentage will be used.
Another way of identifying how a formula has been balanced is to examine the figures given for flour. If the formula contains more than one type of flour, make sure to add all flours together. If the total flour figure sums to 100, then the formula was written in bakers percent.
PARTS PER MILLION (PPM) AND PARTS PER BILLION (PPB)
Some ingredients (oxidants, reductants, vitamins, minerals) are specified in formulas in parts per million (ppm). Mathematically, 1 ppm equals 0.0001% of the basis. Based on 1 cwt of flour, 100 ppm equals 0.16 oz (1/6 oz), or 4.54 g. For example, if ascorbic acid is added at 150 ppm, this amounts to 1 oz per 400 lb of flour. One part per billion (ppb) is only onethousandth as much as a ppm. This is practically never encountered in bakery formulations, but limits on trace contaminants are often expressed in terms of maximum ppb allowable.
To give an idea of the relationship, one drop of vermouth in 13 gal of gin would make a 1-ppm martini. Closer to our industry, 1 ppb is represented by one kernel of soft white wheat in a farm’s 1,200 bushel grain bin filled with hard red wheat. Back