Wheat Flour Milling, Second Edition

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Wheat: The Raw Material The flour miller is the first wheat user who is affected by the quality of wheat. It is estimated that 25% of the flour quality is determined by the milling technology, mill adjustment, and environmental conditions in the mill, and 75% by the quality of the wheat. The miller evaluates incoming raw material for its price and quality. Price is dependent on factors such as supply, demand, and transportation costs. In the trade, quality is mainly based on wheat grading and factors such as protein level and any damage to the wheat. Following the wheat purchase, the miller has the power to evaluate, select, segregate, prepare, and blend wheat mixes for milling. The miller has two ultimate aims: first, to supply the customer with the specified product quality and, second, to efficiently separate the three main parts of the wheat kernel (bran, germ, and endosperm), the economic values of which are related to their purity.

One of the major contributors to variance in quality is wheat variety.

Wheat is cultivated on all continents except Antarctica, and about 30,000 wheat varieties of 14 species are grown throughout the world. However, only about 1,000 varieties are of commercial significance. Breads and other products made from flour reflect the characteristics typical of the wheat grown in various parts of the world. Emigration, changes in demographics, changing living styles, and, in many cases, supplying wheat-deficient areas are the main reasons for the transport of wheat from one side of the globe to the other.

Wheat is harvested globally in unbroken continuity throughout the year.

Table 1-1 shows the approximate harvest months for some of the major wheat- producing areas.

More than 500 wheat varieties are available in the United States, where a wheat breeder can certify a new wheat variety and sell it on the market.

Wheat classification presently is based on a system established in 1916 (U.S. Congress, 1916) and put into effect in 1917. Descriptors such as brush size, germ angle, cheek angle, kernel shape, seed coat texture, kernel color, 1

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and kernel vitreousness are used for classification. However, none of these characteristics is directly related to milling and baking quality. This has led to criticism, and currently the wheat-classification system in the United States is under revision.

In some countries, the number of wheat varieties on the market is fixed.

For instance, in Canada, the Wheat Board certifies the varieties that farmers can grow. The different varieties grown by farmers are brought together from large areas and blended by local elevators before the bulk of the wheat reaches the mill elevator for storage. The miller evaluates the incoming wheat and segregates it to different storage bins. However, the wheat handling system now also provides identity-preserved (IP) wheat, i.e., wheat that is segregated during growing and harvesting, kept in separate bins, and transported separately from harvest to milling because of specific qualities or milling objectives. The extra efforts required to segregate IP wheat usually result in a premium market price.

Universally, a wheat buyer’s first concerns are the cost and sanitation of the raw material. In addition, the buyer must consider the following end-use quality factors: moisture level, percent flour extraction, test weight (TW), kernel size, presence of impurities, percent of damaged kernels, protein content, mycotoxin level, pesticide residue, and end-product functionality.

Not all of these factors are considered in the various wheat-classification TABLE 1-1 Harvest Months in Some of the Major Wheat-Producing Areas Month January February March April May June

July

August

September October November December

Area Argentina, Uruguay, Chile, and New Zealand Upper Egypt and Southern India Egypt, Libya, and India India, Lower Egypt, Iran, Iraq, Syria, Southern Morocco, and Mexico Algeria, Tunis, Morocco, Central and Southern Asia, and, in the United States, South Carolina, Georgia, Alabama, and Louisiana Italy, Spain, Portugal, Greece, Turkey, Asia Minor, Central China, Southern France, and, in the United States, North Carolina, Georgia, Arkansas, Texas, Virginia, Indiana, Illinois, Kentucky, Tennessee, Oklahoma, Missouri and Kansas France, Austria, Hungary, Romania, Bulgaria, Yugoslavia, Switzerland, Southern Russia, North China, Japan, Southern Germany, and, in the United States, New York, Pennsylvania, Ohio, Indiana, Illinois, Michigan, Missouri, Nebraska, Kansas, Colorado, and Oregon Southern Canada, Central Russia, Great Britain, Germany, Belgium, Holland, Denmark, Poland, Manchuria, and, in the United States, Minnesota, North Dakota, South Dakota, Montana, Oregon, and Washington Sweden, Norway, Finland, Northern Russia, Canada, Siberia, and, in the United States, North Dakota, Montana Northern Scandinavia, Northern Russia, Northern Canada, and Alaska Peru, Brazil, and Northern Argentina Argentina, Australia, and South Africa

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systems used for trading throughout the world. While wheat milling technology is becoming similar in different parts of the world as a result of knowledge transfer, grading and evaluation of raw material is still inadequate. The lack of an internationally acceptable grading system causes lack of uniformity in shipments, confusion between suppliers, and dissatisfaction among customers. Uniformity among shipments will become more important as processing technologies become more sophisticated and additional quality factors are considered in wheat grading.

In wheat-growing countries, the miller’s selection of wheat depends upon the market and upon the location of the mill relative to the wheat supply. A mill located in a soft wheat-growing region usually processes soft wheat.

However, Tembo et al (1999) used a decision-making model that confirmed the traditional conclusion that mills should be located near flour users rather than in wheat-production areas. Wheat-importing countries tend to have mills that process many different classes of wheat. Milling several classes requires a more sophisticated milling operation to efficiently process the wheat.

The Wheat Kernel A kernel of wheat is a dry, one-seeded fruit. Its color is one of the most constant variety characteristics; length and endosperm texture are the other two. Wheats are classed as white or red, with the exception of some Abyssinian and durum varieties. The dark color of the red wheat is primarily from pigments in the seed coat, but it is influenced also by the texture and vitreousness of the endosperm and the level of pericarp transparency.

The endosperm length is related to variety and to the location of the kernel in the spikelet during development. Levi and Anderson (1950) studied the protein content of individual wheat kernels on a wheat head. The protein content of kernels within a wheat head showed a variation of 2.7% and had a standard deviation of 0.6%. The standard deviation of protein content for spikelets within heads was about 1.1% (each spikelet contains one to three kernels, Fig. 1-1). Protein contents for spikelets tended to decrease toward

Fig. 1-1. Development of a wheat head. (Adapted from Smith, 1945)

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the top in about the top third of the head; the top two spikelets of each head generally had decidedly lower protein content than the remaining spikelets.

The unique morphology of the wheat kernel presents a technical challenge in the process of grinding it to flour. This is because the kernel has a surface crease that, in commonly grown varieties, extends inward nearly to or beyond the center of the kernel. Evers (1970) studied the creation of the crease in a developing wheat kernel from the second day of endosperm development until maturity. He suggested that thick-walled cells on the developing wheat kernel’s ventral side are less active meristematically than the other peripheral cells. Hence, in the central region, all or most of the starchy endosperm cells originate by division on the dorsal side, whereas, in the lateral regions, divisions occur from all areas of the peripheral layer.

Mabille and Abecassis (2003) suggested a method for modeling the morphology of the wheat kernel from which milling yield can be predicted better than from the hectoliter weight. The model is based on five parameters: grain length, thickness, width, crease depth, and a parameter describing the furrow shape.

Endosperm The wheat endosperm contains, on average, about 30,000 cells that vary in size, shape, and composition of starch granules and protein depending on their location in the kernel (Ziegler, 1969). Table 1-2 shows the various constituents of the wheat kernel and their specific gravity. Starch, protein, and bran content are all important in determining the potential flour yield from the wheat. The amount of flour that can be extracted from the kernel depends mostly on the percentage of endosperm. Heavier kernels with large endosperm contain more starch and protein and have the potential to yield more flour. The protein and mineral contents of the endosperm follow a pattern. For protein, distribution in the endosperm is the lowest in the center, with a gradient of increase in protein content through the endosperm to the bran coat (Morris et al, 1945). The gradient in mineral content (analytically named “ash� because it is what is left

TABLE 1-2 Specific Gravity of the Wheat Kernel Constituents Substance Starch Sugar Cellulose Water Fats Gluten Mineral content Air

Specific Gravity 1.53 1.60 1.53 1.00 0.94 1.297 2.50 0.001293

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after incineration of the endosperm or other parts of the wheat kernel) also increases from the center to the outer layers of endosperm, but it is not identical in all wheats (Hinton, 1959).

Bran The pericarp and the outermost tissues of the wheat kernel, including large portions of the aleurone layer, compose what is known commercially as “bran.” The pericarp (fruit coat) consists of two layers. The outer pericarp is made up of the epidermis (epicarp), hypodermis, and remnants of thin-walled cells. The inner pericarp is made up of intermediate-cell, cross-cell, and tubecell layers. The pericarp envelops the seed and is fused with the seed coat, which consists of the testa (or episperm), the pigment strand, and the hyaline layers (MacMaster et al, 1971). Together, they form two protective layers around the kernel’s interior components, the endosperm and the germ. When tissues beneath the seed coat are exposed, moisture, mold, etc. gain access to them more readily than when the seed coat and fruit coat are intact.

There is no natural line of cleavage between the pericarp and the seed coat layers that envelop the germ and starchy endosperm. This fact accounts for some of the difficulties encountered in separating the two during flour milling.

The pericarp and the seed coat layers form the “bran,” which is separated during the milling process. The adjacent layer, the aleurone, which is actually part of the endosperm, normally remains attached to the bran during conventional milling. The mean thickness of bran at ordinary moisture content (13–18%) was found to be 67 µm regardless of the type of wheat; that of the aleurone layer was 30–36 µm (Crewe and Jones, 1951). The total bran is about 14.5% of the whole wheat; a more detailed breakdown is epidermis 3.9%, cross-cell layers 0.9%, testa 0.6%, and hyaline and aleurone 9.0%. The ash content of bran is known to be 10–20 times that of the endosperm.

In the classical milling process, using rolls to separate the endosperm from the bran, the miller tries to achieve minimal abrasion or damage to the bran layers. The goal is to keep the bran as whole as possible and in its original thickness, so that certain spots are not weakened and likely to split during milling.

Germ The germ is structurally a separate entity of the kernel; therefore, the separation of germ from endosperm should require no breaking of the endosperm cell walls. The wheat germ contains the embryo and the scutellum, which are separated from the endosperm by the epithelial layer. The embryo draws materials for initial germination and growth from the endosperm, through the epithelial layer. Germination is initiated by the activation of the

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germ enzymes via heat and moisture. These enzymes are of two main types: proteolytic, or protein-liquefying, and amylolytic, or sugar-producing. Due to the action of the latter type, some of the starch in the proximity of the germ is changed to sugar and is used to feed the germ and permit growth (Scott, 1951). Fleming and Johnson (1964) recognized a “gibberellin-like” hormone secreted from the embryo that effects the formation of α-amylase in the endosperm during the first three or four days of germination.

The germ, usually about 2–3% of the kernel by weight, is partly embedded in the endosperm at the base of the kernel. It is rich in oil and protein.

The germ is composed of two major parts, the embryonic axis, which at germination develops into the seedling, and the scutellum, which nourishes it. The embryonic axis is composed of the shoot (plumule), which points toward the brush end of the grain, and the primary root, which points toward the base. Protective sheaths cover these delicate parts; the coleoptile sheathes the plumule, and the coleorhiza covers the primary root. This root, the projecting lower tip of the germ, is especially vulnerable to mechanical injury during harvesting and handling and is often broken, exposing germ tissue. There is a direct relationship between the length of the embryo projection and the amount of mechanical damage done to the embryo. The embryo projection and shape of the area around the germ also affect the ease of separating the germ from the rest of the kernel.

The “germ” separated in the commercial mill is actually the embryonic axis of the wheat kernel; the softer and less-rigid scutellum is left attached to the bran.

Brush At the kernel end opposite the germ, there is a “brush” or cluster of hairs. Wheat varieties differ materially in the size of the brush. The kernel hairs, which are extensions of the pericarp, are about 10–15 µm in diameter and 0.5 mm long (MacMasters et al, 1971). Undesirable materials are sometimes entangled in them. Depending on the milling practices used, the hairs might end up in the flour. Intensive scouring of wheat during cleaning stages usually removes the kernel hairs. A study by Keenan (1923) indicated that flours made from purified middlings material showed a low hair count, while flours originating in the breaks showed a higher hair count.

Wheat Grading Advances are being made in research and regulations to grade wheat on parameters that would exhibit its trading value and processing qualities.

Grading terms and methods are still not defined by an international standard, although processing equipment and systems are similar, and end usage

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qualities are expressed with the same terms. Many trade and processing problems would be eliminated if the same parameters and methods were used for wheat grading and evaluation internationally. It is just a matter of time before open markets will force wheat producers, traders, millers, and wheat flour end users to create a global method of wheat grading.

Professionals in producing, trading, grading, and processing should identify the factors of importance for different wheats. For example, durum wheat for the production of bright, speck-free semolina for pasta is processed differently from common wheat and should also be graded differently on global standards that ensure quality. In the United States and other major wheat-producing countries, the grain grading system is under constant revision. Proposals for change are made public in the Federal Register, allowing 60 days after publication for comments before the final decision is made.

Wheat is bought in the cash market based on a sample shown to the buyer. In the United States, there are eight classes for wheat: durum, hard red spring, hard red winter, soft red winter, hard white, soft white, unclassed, and mixed. Unclassed wheat is any variety of wheat that is not classifiable under other criteria provided in the wheat standards. This class, which has no subclasses, includes any wheat other than red or white in color. Mixed wheat is any mixture of wheat that consists of less than 90% of one class and more than 10% of another class or combination of classes that meet the definition of wheat (GIPSA, 1995).

In the grain exchange or in an export transaction, wheat is evaluated according to official grades. Table 1-3 shows the combined factors that determine the grade of wheat in commercial channels in the United States.

The wheat grade is determined in the United States according to various factors, on a sample free from dockage. Wheat specifications change continuously as a result of new variety development and trade and quality requirements. Websites listed at the end of the chapter and other information sources should be used to follow the changes in grading procedures.

Wheat Sampling Very large quantities of wheat can be transferred today with the equipment available in ships and elevators. One of the greatest challenges of modern wheat handling and milling operations is the rapid evaluation of incoming wheat, so that the wheat can be directed to the appropriate bin. A representative sample must be collected, weighed, and tested in a matter of minutes. The wheat can be evaluated objectively when a representative sample of at least 2,000 g from the entire lot is available. Sampling can be a constant source of error in all methods of wheat evaluation; therefore, procedures should be adopted according to official standards. In the United

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States, and in some other countries, wheat-grading agencies use the same sampling devices and procedures as the U.S. Grain Inspection, Packers, and Stockyards Administration (GIPSA) for official inspection.

The probe is the best tool for obtaining a sample from a truck or rail car awaiting unloading. To achieve accurate wheat mixing, the elevator operator TABLE 1-3 U. S. Grades and Grade Requirements for Wheata Grades U.S. Nos. Grading Factors

1

2

Minimum Pound Limits of Test weight Hard red spring wheat or white club wheat, lb/bu 58.0 57.0 All other classes and subclasses, lb/bu 60.0 58.0

3

4

5

55.0 56.0

53.0 54.0

50.0 51.0

Maximum Percent Limits of Defects Damaged kernels Heat (part of total) Total Foreign material Shrunken and broken kernels Totalb Wheat of other classesc Contrasting classes Totald Stones

0.2 2.0 0.4 3.0 3.0

0.2 4.0 0.7 5.0 5.0

0.5 7.0 1.3 8.0 8.0

1.0 10.0 3.0 12.0 12.0

3.0 15.0 5.0 20.0 20.0

1.0 3.0 0.1

2.0 5.0 0.1

3.0 10.0 0.1

10.0 10.0 0.1

10.0 10.0 0.1

1 1 2 0 3 3 4 31

1 1 2 0 3 3 4 31

1 1 2 0 3 3 4 31

Maximum Count Limits of Other material Animal filth Castor beans Crotalaria seeds Glass Stones Unknown foreign substance Totale Insect-damaged kernels in 100 g

1 1 2 0 3 3 4 31

1 1 2 0 3 3 4 31

U.S. Sample grade Wheat that: (a) Does not meet the requirements for U.S. Nos. 1, 2, 3, 4, or 5; or (b) Has a musty, sour, or commercially objectionable foreign odor (except smut or garlic odor) or (c) Is heating or of distinctly low quality. a

Source: Grain Inspection, Packers and Stockyard Administration (1995).

Includes damaged kernels (total), foreign material, and shrunken and broken kernels. c Unclassed wheat of any grade may contain not more than 10.0% of wheat of other classes. d Includes contrasting classes. e Includes any combination of animal filth, castor beans, crotalaria seeds, glass, stones, or unknown foreign substance. b

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or the miller should install a sampler at the end of the mixing process, before the wheat reaches the ship hold or the mill cleaning house.

Systems have been suggested that evaluate samples automatically using a video camera to record the view of a spread-out sample and compare the different materials observed to a given standard. The standard is based on recorded views of foreign materials and dockage, as well as the data calculated from that material. The system, which can run a 50-g sample in 2–3 min, operates without human intervention (Conrads, 1995).

Dockage Dockage, or nonwheat material, is separated from the sample using the Carter-Day Dockage Tester, a machine (Fig. 1-2) that is set differently for each kind of wheat. Dockage has never been an official part of the grade in the United States. Dockage is traditionally deducted from wheat sale, not by law, but rather as a long-standing practice in the trade. Usually, wheat buyers would specify in the tender or contract the maximum dockage that would be acceptable.

Fig. 1-2. Carter-Day Dockage Tester officially used by the Federal Grain Inspection Service to separate the dockage before grading. (Courtesy of Carter Day International, Inc.)

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Grading Factors Below is a discussion of the grading factors for wheat and their relationship to milling and the resultant flour quality. Additional parameters related to wheat quality are described in Chapter 2.

Test Weight Test weight (TW) is a factor that has served the grain-processing industry for a century. It began about 1890 with the early trading of wheat from the Mississippi Valley to East Coast mills. To the miller in the past, the weight of a specific volume of grain, the bushel weight, was a rough guide to the amount of flour that the wheat might be expected to produce. The weight of a specific volume has its practical use today to estimate the weight or content of a ship’s load, a rail car, or a storage bin. For this purpose, it will be used for many years to come. However, for processing purposes, it is not accurate enough to accommodate the needs of the milling industry, which uses sophisticated equipment to improve efficiencies and profit margins.

In the United States, TW is expressed in terms of pounds per Winchester bushel (2,150.42 in.3 capacity) as determined on a dockage-free test portion of the original wheat sample using an approved device in accordance with instructions in the GIPSA manual. A test weight determination procedure is also described in Approved Method 55-10 (AACC, 2000). In metric measurements, it is the weight, in kilograms, of a hectoliter of wheat. The procedures used to determine the hectoliter weight of wheat are different from those used to determine the bushel weight (Fig. 1-3). They differ with regard to quantity of sample used, the dimensions of the measurement kettle, the grain-drop procedure, the “pack factor,” the means of striking excess grain from the kettle and, consequently, the measured results. Therefore, formulas were developed, instead of the previously used factor, to predict the hectoliter weight from the TW (Orr, 1997). For durum wheat, the following formula could be used: MBD = [1.292 × (TW)] + 0.630

To predict the hectoliter weight of all other wheat except durum, the following formula could be used: MBD = [1.292 × (TW)] + 1.419

where MBD = metric bulk densities, expressed in units of kg/hL, and TW = customary test weights, expressed in units of lb/bu. Prediction is based on the use of a 1-L chrondrometer apparatus.

The TW of wheat is not always an indication of the amount of flour that should be extracted from a certain quantity of wheat. When wheat varieties

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and classes drawn from the same locations are used in a mill blend, the TW may be considered as one of the factors in determining the potential yield.

This is not true when widely varying varieties and classes of wheats are used and the percentage of types is changed. TW determination is affected

Fig. 1-3. Instruments to measure test weight (A) and hectoliter weight (B).Test weight scale and filling hopper. (A, Courtesy of Seedburo Equipment Co.; B, courtesy of Buhler Corp.)