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Wheat

Elieser S. PosnerConsultant, Savyon, Israel

IOrigin

Historic documents confirm that wheat is the earliest field crop used for human foodprocessing [1]. It also became the leading grain used for human consumption due to itsnutritive profile and relatively easy harvesting, storing, transportation, and processing, ascompared to other grains. The earliest varieties, grown 12,00017,000 years ego in the Near

East, were Triticum monococcum (einkorn) and Triticum dicoccum (emmer). Continuedbreeding resulted in the development of new varieties around the world that often becameadapted to areas previously unsuited for the cultivation of wheat. The main wheat varietiesgrown today are Triticum aestivum, subspecies vulgare, which is a hexaploid with six groupsof seven chromosomes in each group. This species includes hard red winter, hard red spring,soft red winter, and white wheats. Another wheat durum is a tetraploid, containing four groupsof seven chromosomes totaling 28 chromosomes. The botanical name of durum wheat isTriticum durum. A limited area is planted with the soft white wheat variety of Triticumaestivum, subspecies compactum, commonly known as club wheat. Currently about 4000different wheat varieties are grown around the world.

II

Morphology of the Wheat Kernel

Data related to the morphology of the wheat kernel and proximate analyses vary in differentresearch reports. This variability is likely due to the different types and growing conditions ofwheats analyzed. In general, there are about 30,000 cells in a wheat kernel, and their contentvaries significantly depending on their location in the kernel [2]. Extensive studies have beenconducted on the botanical outer layers of wheat kernels. Their metabolic significance, size,and thickness changes from fertilization of the ovary by the pollen of the same plant werereported [3]. A longitudinal and cross section of a wheat kernel along with an identification ofits components is shown in Figure 1. Table 1 shows the typical values of wheat kernel partsand their proximate analysis. The morphology of the wheat kernel is unique and as suchcreates technical (milling) challenges in separating the endosperm and the germ from the outerfibrous layers, commonly named the ''bran." The presence of the crease (about 25% of thekernel surface), which extends almost to the center of the wheat kernel [7], requires specialconsideration in grinding. The wheat germ (about 24% of the kernel weight) is located on thedorsal side. The wheat germ parts are the embryo, with rudimentary roots and shoots, and thescutulum, which is a transport organ of nutrition to the embryo during sprouting. At theopposite end of the kernel germ, there is a cluster of short fine hairs about 1015 Pm indiameter and 0.5 mm long [8]. The wheat kernel outer botanical coats (about 78% of thekernel weight) consist of several distinct cellulose-rich layers. The outermost layer, the

pericarp (fruit coat), is made up of the outer pericarp, which includes the outer epidermis,hypodermis, thin-walled cells, and the inner pericarp, which includes intermediate-size cells,cross layers, and tube cells (inner epidermis). The inner layers are the seed coat (testa) andnucellar epidermis (hyaline layer) [8]. The thickness of the bran coat of hard red winter wheat


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Figure 1View of a wheat kernel in (a) longitudinal section and (b) cross section.

(From the Wheat Flour Institute, Washington, DC.)

varieties ranges from 67 to 70 Pm [9]. Between the nucellar epidermis and the starchy endosperm we find the

aleurone layer, having high soluble protein and mineral contents. The aleurone layer constitutes about 58% ofthe wheat kernel. This layer is botanically similar to the endosperm, but it is difficult to separate from the branby conventional milling techniques. Depending on the kind of wheat, the thickness of the aleurone layer varies.

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Bradbury et al. [10] reported its thickness to be about 46.9 Pm; Crew and Jones [11] found the average thicknessof the aleurone layer to be 3060 Pm. Mechanical damage or hydrolysis with cellulase of the aleurone thick cellwall allows access to proteins within the aleurone layer [12]. Although nutritious, incorporation of a fractionwith a large percentage of aleurone layer adversely affects the baking quality of flour [13]. The endosperm ofthe kernel was also shown to follow a gradient [14] in ash, protein content, gluten characteristics, and bakingquality.

IIIBreeding, Growing Conditions, and Their Effect on Quality

Among wheats grown all over the world there are three major distinctions. The first one, which affects theirfunc-


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TABLE 1 Average Values of Wheat Constituents and Their Proximate AnalysisaKernel

(%)Embryo

(%)Scutellum

(%)Pericarp

(%)Aleurone

(%)Endosperm

(%)

WheatsCommon 100 1.2 1.54 7.9 6.77.0 8184Durum 100 1.6 12 86.4

Proximateanalysis

Protein 12.6 35.0 26.0 57 18 7.413.9Ash 1.9 5.45 5.08.0 34 1417 0.280.40Fat 1.6 16.3 32.0 35 10 0.81.5Starch 59.2 68Pentosans 6.7 6.6 34.9 39.0 1.4

Cellulose 2.32.0

38.4 3.5 0.3

Calories 314 354 177 247 354

a14% moisture basis.Source: Adapted from Refs. 46.

tional and concomitantly end-use characteristics, is whether the wheat is a winter or springtype. Winter wheats are fall-planted and require a period of low temperature (vernalization).Winter wheats are harvested during June or July. Spring wheats do not require vernalization,are spring-planted, and are harvested during August or September in the Northern Hemisphere.Spring wheats can be fall-planted in regions with mild winter temperatures [15]. The seconddistinction is the kernel color. The majority of wheats are red or white as a result of the

presence or absence of red-brown pigments in the seed coat. Some wheats of uncommon

colors can be found in some parts of the world. The third distinction between wheats is due todifferences in varieties. Endosperm hardness differs, affecting the wheat millingcharacteristics. Soft, hard, or durum wheats differ in endosperm structure, hardness, and

protein characteristics. Consequently, they are milled differently and the resulting flours aresuitable for different end uses. Figure 2 shows a schematic diagram of the relationship

between protein percentage, wheat type, hardness, and end-product utilization. Soft wheatendosperm includes air spaces in the protein matrix, and, as indicated by light scattering inthese spaces, the endosperm is chalky. In the endosperm protein matrix of hard vitreouswheats, air spaces are absent and its appearance is dense and glossy. The soft wheat varietieswith low protein are also evaluated in terms of suitability for soft wheat milling and in the

production of cakes and cookies. Hard wheat qualities are defined in terms of their millingcharacteristics and the quality of the breads produced.

Quality within the same kind of wheat is influenced by local climate, soil conditions, andvariety. Rain and sun at appropriate periods are important to the yield of wheat per acre and itsquality. Usually there are up to five kernels on the same spikelet of the plant. In the center arethe largest and heaviest kernels. In general, the length of the wheat kernel is attributed to thevariety and the width to the growing conditions. The differences in test weight, ash, protein,sugar, and starch in wheats of the same variety grown in different environments are muchlarger than those in wheats of different varieties grown under comparable conditions [16].Shrunken wheat kernels are a direct result of growing conditions and affect flour extractionand quality. Sprouted wheat kernels are a result of excessive moisture during harvest time.

Breeding programs of new wheat varieties consider production yield per acre, hardness, flourextraction, protein level, as well as other parameters related to processing. One of the mainobjectives of breeding programs to select varieties resistant to diseases that attack the plantduring the growing period. Resistance to rust, smut, and other sicknesses is genetically

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selected during development of new varieties. Wheat grown in adverse weather conditionsmight also be affected by fungi and disease during the development period of the kernel. Fungiaffecting wheat during early development stages might introduce vomitoxin (DON) into thekernels, which will result in shrunken kernels of lower quality [17]. Some of the problemswith diseases in wheat in the last few years can be attributed to the change in tillage practices.Tillage of wheat fields is necessary for the development of a good seedbed, for the control of

weeds, and for the destruction of insects and diseases harbored in the debris left after harvest.

Breeders require meaningful information for predicting the intrinsic value of genotypes inorder to screen the potential lines in early generations. Selection of attributes

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Figure 2

Schematic diagram of relationship between percentages of protein, wheat type, hardness, andend-product utilization.

during the breeding program concentrates also on properties that contribute to the processingquality of the wheat. Usually the final decisions about commercialization of new varieties are

based on comparison to known varieties as standards. While microtests are performed on earlygenerations, comprehensive tests evaluate larger quantities from later generations by large-scale milling and bread baking or other end uses. Analytical as well as milling and bakingmethods were designed to evaluate the small quantities of wheat from early generations of

breading programs [1821]. Analyses of research data generate regression lines to indicatepotential performance of new varieties in baking. The regression lines for different wheatvarieties (Fig. 3) form a fan-shaped family of lines indicating loaf volume increases with

increased protein content within a variety [22]. However, other results shows that in somekinds of wheat, above a certain extraction level there is a decrease in loaf volume [23].

Parameters that could be determined by image analyses and test weight values were used inselecting seeds for breeding [24]. Sixty-six percent of the variation in flour yield wasidentified by those physical parameters. Statistical analysis systems were used as an aid in thehandling of data for quality evaluation of breeder's selections of hard red spring and durumwheat [25]. Personal computerbased software systems were used for the management of wheatquality data of experimental breeding lines and commercial check sample cultivars [26]. Partof the software is a grading system, which facilitates a rapid and unbiased evaluation ofnumerous discrete wheat and flour analytical values. New breeding lines are compared toknown qualities and historically derived limits of statistical differences of the check samples.

New tools for improving wheat processing and end-use qualities are being developed usingbiotechnology. This approach broadens the available germplasm beyond the current

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collections of native germplasm and enables one to modify components such as oils, starches,and proteins. The introduction of high molecular weight glutenin genes into a wheat varietyresults in more of the gluten protein subunit glutenin [27], which provides the extensibilityneeded for good bread.

IV

Wheat Trade and Consumption

The stability of wheat in storage under appropriate conditions made it the first-ranked tradingfood commodity for human consumption. The production and price of wheat


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fluctuate from year to year as a result of supply and demand in different parts of the world.Climatic conditions and diseases affect wheat availability. Figure 4 shows the global economicdata, growing areas, production, leading exporters, and consumption of wheat [28]. Wheat

consumption around the world for food, feed, seed, and other uses is estimated to be 73.8,16.1, 5.6, and 4.5%, respectively [28]. During the 1995/96 crop year, the estimated wheatusage in the United States for food, feed, and seed was 77.4, 13.5, and 9.0%, respectively. Theannual worldwide increase of wheat consumption is between 0.5 and 1.5%. China is thelargest wheat-producing and wheat-consuming country in the world, with a total consumptionduring the 1995/96 crop year of 119 million metric tons [28]. China's consumer demand forfood wheat is growing, while its production capacity is leveling off. Increases in Chineseimport demand usually affect the world markets.

VClassification and Grading of Wheat

Many wheat kinds and classes, available around the world, vary in quality as a result ofclimate, irrigation, specific va-

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Figure 3Loaf volume-protein content regression lines for eight hard spring and two hard winter

wheat varieties.

(From Ref. 22.)

riety characteristics, growing conditions, harvesting, and handling. Figure 2 summarizes theutilization requirements of different wheat kinds, hardness, and protein content for majorwheat-based products. Presently, wheats are graded differently in exporting and importingcountries [29]. In some countries the government is involved in setting limits for contaminantsin imported wheats. In others, mainly exporting countries like United States, governmentofficers inspect, according to official standards, all exported wheat; domestically traded wheatis inspected upon request only.

Table 2 shows the U.S. grading system for wheat [30]. In the United States the gradingstandards for hard red winter wheat, soft red winter wheat, common white wheat, and club

wheat went into effect on July 1, 1916. Standards for all other wheats became effective onAugust 1, 1917 [31]. The current grading system covers eight classes of wheat: durum, hardred spring, hard red winter, soft red winter, hard white, soft white, unclassed, and mixed

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wheat. Durum, hard red spring, and white wheat are further divided into subclasses. Accordingto the U.S. standards for wheat, the definitions for the classes and subclasses are as follows:

1. Durum wheat: all varieties of white (amber) durum wheat. This class is divided into threesubclasses: (1) hard amber durum wheatthis subclass designates durum wheat with 75% ormore of hard and vitreous kernels of amber color; (2) amber durum wheatthis subclass is

durum wheat with 60% or more but less than 75% hard and vitreous kernels of amber color;(3) durum wheatdurum wheat with less than 60% hard vitreous kernels with amber color.

2. Hard red spring wheat: all varieties of hard red spring wheat. This class is divided into thefollowing three subclasses: (1) dark northern spring wheathard red spring wheat with 75% ormore dark, hard, and vitreous kernels; (2) northern spring wheathard red spring wheat with25% or more but less than 75% dark, hard, and vitreous kernels; (3) red spring wheathard redspring wheat with less than 25% dark, hard, and vitreous kernels.

3. Hard red winter wheat: all varieties of hard red winter wheat. There are no subclasses in thiswheat class.

4. Soft red winter wheat: all varieties of soft red winter wheat. There are no subclasses in this

wheat class.

5. Hard white wheat: all hard endosperm white wheat varieties. There are no subclasses in thisclass.

6. Soft white wheat: all soft endosperm white wheat varieties. This class is divided into thefollowing three subclasses: (1) soft white wheatsoft endosperm white wheat varieties thatcontain not more than 10% of white club wheat; (2) white club wheatsoft en-


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Figure 4

Wheat economics, 1995/96, depicted by percentage of world totals. Former Soviet Union.(From Ref. 28.)

dosperm white club wheat containing not more than 10% of other soft white wheats; (3) western whitewheatsoft white wheat containing more than 10% white club wheat and more than 10% other soft whitewheats.

7. Unclassed wheat: any variety of wheat that is not classified under other criteria provided in the wheatstandards. There are no subclasses in this class. This class includes any wheat that is other than red orwhite in color.

8. Mixed wheat: any mixture of wheat that consists of less than 90% of one class and more than 10% ofone other class or a combination of classes that meet the definition of wheat.

Each class and subclass is divided into five numerical grades and a U.S. Sample Grade. The gradedesignation affects the trading value of the wheat.

In Canada the Board of Grain Commissioners enforces the standards for wheat exports. The Boardestablishes ''export standard samples" for a number of grades. The export standard for each grade,established each year, is a mixture of three parts of wheat equal to the average quality of the grade forthe respective crop year and one part of wheat equal to the minimum quality permitted by the basicgrade. There are seven classes of wheat divided into spring and winter classes. The five spring wheatsare Canadian western, hard red spring, Canadian western amber durum, Canadian western utility,Canadian prairie spring, and Canadian western soft white spring. The two winter wheats are Canadianwestern red winter and Canadian eastern white winter. Only the registered varieties are equal tostandard varieties, which are eligible for classification under the seven classes. There are also wheat ofother classes for nonregistered varieties.

The Australian Wheat Board annually issues receiving standards and dockage schedules that list gradespecifications and tolerances for Australian standard white, Australian general purpose, and Australian

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feed wheats. The Australian wheat is classified into classes that fall into two categoriesmilling andnonmilling wheats. The milling wheat group includes Australian prime hard, Australian

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hard, Australian standard white, Australian soft, and Australian durum wheats. They are furtherclassified into grades based on the state of origin, protein content, grain hardness, millingquality, and dough properties. There are two additional classes, Australian general purpose and

Australian feed, which do not conform to the standards of milling wheats in terms of testweight, weather damage, and levels of unmillable material or foreign matter.

The International Association of Cereal Chemistry (ICC) evaluates wheat on the basis of itsBesatz (extraneous matter) content [32]. According to ICC methods, which have been acceptedas the European Economic Community [33] official methods, Total Besatz (Gesamtbesatz) ismade up of two parts: Kornbesatz and Schwartzbesatz. Kornbesatz consists of material withsome milling value such as shrunken and broken kernels. Schwartzbesatz is foreign materialthat has no millin value.

Wheat milling technology is becoming technically similar in different parts of the world as aresult of a reduction in the number of equipment suppliers and easy access to information. Onthe other hand, wheat is still graded differently in countries around the world using differentmethods and

TABLE 2 U.S. Wheat Grades and Grade RequirementsGrades U.S. nos.

Grading factors 1 2 3 4 5Minimum pound limits of:

Test weightHard Red Spring wheat or WhiteClub wheat lbs/bu 58.0 57.0 55.0 53.0 50.0All other classes and subclasses lbs/bu 60.0 58.0 56.0 54.0 51.0

Maximum percent limits of:

DefectsDamaged kernelsHeat (part of total) 0.2 0.2 0.5 1.0 3.0

Total 2.0 4.0 7.0 10.0 15.0

Foreign material 0.4 0.7 1.3 3.0 5.0

Shrunken & broken kernels 3.0 5.0 8.0 12.0 20.0

Total1 3.0 5.0 8.0 12.0 20.0Wheat of other classes2

Contrastin classes 1.0 2.0 3.0 10.0 10.0Total3 3.0 5.0 10.0 10.0 10.0

Stones 0.1 0.1 0.1 0.1 0.1

Maximum count limits of:Other material

Animal filth 1 1 1 1 1Castor beans 1 1 1 1 1Crotalaria seeds 2 2 2 2 2Glass 0 0 0 0 0Stones 3 3 3 3 3Unknown foreign substance 3 3 3 3 3Total4 4 4 4 4 4

Insect-damaged kernels in 100 grams 31 31 31 31 31

U.S. Sample gradeWheat that:

(a) Does not meet the requirements for U.S. Nos. 1, 2, 3, 4, or 5; or

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1Includes damaged kernels (total), foreign material, and shrunken and broken kernels.2Unclassed wheat of any grade may contain not more than 10.0 percent of wheat of otherclasses.

3Includes contrasting classes.4Includes any combination of animal filth, castor beans, crotalaria seeds, glass, stones, orunknown foreign substance.

(b) Has a musty, sour, or commercially objectionable foreign odor (except smut or garlicodor) or(c) Is heating or of distinctly low quality.


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factors. Current classifications and methods used in different parts of the world weredeveloped when processing methods were different and international trade was not at its

present volume. There is a need to develop a comprehensive worldwide universal wheat-

grading system that will identify qualities and values important to farmers, traders, millers, andbakers for domestic and export markets.

VIEvaluation of Wheat

The value of wheat depends upon its milling and flour end use quality. This can be accuratelydetermined through actual milling and baking tests. The miller has to assess wheat quality andevaluate its suitability to produce, individually or in a blend, final flour specifications. Inaddition, the miller has to determine the expected wheat-processing performance in the mill,the resulting flour extraction, and other qualities such as color, particle size, and starchdamage. Flour extraction is the proportion of the wheat recovered as flour during milling. The

following are tests of importance to the miller for evaluating wheats and flours: experimentalmilling, physical, chemical, physical-chemical, dough rheology, and the baking test. Wheatand flour testing can follow different official methods such as those of the AmericanAssociation of Cereal Chemists (AACC), the International Association of Cereal Chemists(ICC), or the Association of Official Analytical Chemists (AOAC).

Physical Wheat Tests

The following tests are used:

1. Test weight: quality test which is basically a rough measure of density of grain in terms ofweight per volume, i.e., the weight (lb.) per volume bushel (Winchester bushel in U.S.;

Imperial in Canada). The hectoliter weight (hL), indicating the weight in kg/hL (100 L), isused in the metric system countries. No uniform conversion factors between test weight andhL weight values are possible due to differences in kernel shape, size, and procedures fordetermination of these values.

2. Thousand kernel weight (TKW): a quality test to determine the potential milling value ofwheat. Weight of 1000 kernels gives an indication of kernel density and its consequent flouryield. The advantage of TKW is that the weight can be expressed on a desired-moisture basis.

3. Kernel size distribution: the size distribution of kernels in a wheat sample can be determinedusing a stack of sieves. The ''theoretical flour yield" can be determined by the total value ofmultiplying the percentage above each sieve by a factor [34]. The factors can be calculated

using multiple regression analysis for a mill, based on a database in which percentages ofwheat sizes are the independent variables and the actual flour yields are the dependentvariables [35].

4. Kernel hardness: a relative term, which is related to the disintegration of the endospermduring its separation from bran and germ. Currently, hardness values are determined by near-infrared refraction (NIR) or mechanical crushing instruments such as the single kernelcharacterization system (SKCS). They are used to identify variation of wheat characteristics inthe trading system as well as indicate processing characteristics [36].

5. Assessment of the milling quality of wheat is performed using an experimental unit using asample of about 10001500 g. Experimental milling can give a preliminary indication whether awheat alone or in a mix of wheats complies with a required quality. An experimental mill

should be differentiated from a laboratory mill that is a milling unit with a fixed setting, whereall wheat samples are treated in the same manner during milling. Flour samples produced with

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laboratory mills in a relatively short time can be used for further testing but do not provideinformation on the wheat-milling properties. Official methods explain the procedures for usingexperimental mills and should be followed rigidly, preferably by the same operator [37].Improved experimental mills are fitted with technical parameters of the commercial mill wherethe wheat is expected to be processed. Accurate sampling, tempering, and controlledenvironment in the facility and uniform practices ensure reproducibility and confidence in the

results. Flours from experimental milling procedures could be used for further rheological andbaking tests.

6. Other physical and chemical evaluation tests performed in the mill laboratory include thosefor moisture, protein, ash, fatty acids, amylase activity, Falling Number, and gluten quantityand quality.

BRheological Tests; Baking Tests

The more sophisticated and automated the bakery plant of the mill's customers, the more effortshould be devoted to achieving a uniformity of the end product.

The data from physical or rheological dough testing and baking tests simulate criticalparameters required by the process in the bakery [38]. Details of these test procedures arediscussed in Chapter 16 of this handbook.


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VIIWheat Processin

Storage

It is important to preserve the quality and economic value of wheat as it moves from the fieldinto storage at the processing mill. If not properly stored, insects, moisture damage, or otherconditions may cause losses. Moisture and temperature are two main factors that influence thedevelopment of grain molds and insects in stored wheat.

In some areas of the world, where wheat is harvested at a high moisture content, wheat shouldbe carefully dried to a moisture below 12.5%, a level regarded as safe for storage. Wheatexposed to different equilibria of temperature and relative humidity will show increases ordecreases in its moisture content [39,40]. The hygroscopic moisture does not increase at a

uniform rate when in equilibrium with an increasing atmospheric humidity. A much greaterchange in hygroscopic moisture is recorded with change in relative humidity from 75 to 90%or from 90 to 100% than from 45 to 60% or 60 to 75% relative humidity. The hygroscopicmoisture of all classes of wheat is similar.

The rates of insect development and spoilage are related to the moisture content andtemperature of the stored wheat. Measures should be taken to control the moisture andtemperature of the wheat by aerating the bins with about 0.10.2 m3/min/1000 kg (0.10.2ft3/min/bu) of air of the appropriate temperature and relative humidity characteristics. Anothermeasure involves using hermetic conditions in the storage bins. It has been established [41]that insects are killed by depletion of oxygen but not by the build-up of carbon dioxide.

Established procedures of plant inspection, good housekeeping, fumigation, and other

measures such as heat treatment of the facility can control infestation in the flour mill. Well-designed and well-manufactured equipment that will not harbor material in ''dead corners"where insects could propagate is an important factor.

After December 31, 2000, usage of methyl bromide will be outlawed in the USA. Thisphasing-out of chemical fumigation will require using alternative insect-control methods. Oneof them would be the use of heat in a prearranged facility for a long enough period to kill allinsects. A temperature range of 48.951.7C (120125F) in all parts of the mill for a duration of1012 hours is sufficient to destroy all insect life [42]. Others recommend 48.857.2C (120135F) for 16 hours [43]. Insect control in the mill is related not only to spoilage of the rawmaterials but also to the production of flour within insect fragment count specifications.

Wheat arriving at the mill is usually precleaned before storage in the mill elevator (silo) bins(Fig. 5). Magnets, large-capacity sieve cleaners, and strong aspiration remove large chaff anddust from the wheat. Precleaning removes contaminants from wheat to allow its longerstorage, more efficient usage of storage space, and subsequently better and uninterrupted flowfrom the bins. Frequently the conveying equipment to transport wheat from the unloading

point to precleaning and then to the storage bins is also used for turning and blending wheat inthe elevator. When moved, wheat dust is produced by abrasion of the kernels. Consequently,all handling equipment and empty spaces in the elevator should be under low negative

pressure. The exhaust system consists of ducts, suction fans, and air filters or dust collectors.An efficient exhaust system to handle dust from all points in the facility prevents loss ofmaterial and dust explosions. Depending on the mill's location, its elevator should have astorage capacity of up to 23 months of production.

In some cases wheat arrives at the mill elevator with an 89% moisture content and water isadded to the precleaned wheat to raise its moisture to a maximum of 12.5%. By adding

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moisture to the wheat before storage, the miller can subsequently reduce the tempering time ofdry wheat in the mill. Wheat will absorb water more readily after it has been tempered.

BBlending

Usually a mill is designed for milling wheat of a certain class and physical characteristics.However, a mill designed for one class of wheat (e.g., hard or soft) does not ensure uniformityof end-product quality.

Wheat arriving at the mill usually varies in quality and requires blending to deliver a "wheatmix" of uniform qualities. Wheat blending is the initial step in providing bakers with a uniformflour. Accordingly, mills prepare "wheat mixes" of certain protein levels or other qualitycharacteristics.

There are different methods of blending. Some millers blend wheats directly in storage bins,others before grinding [44]. Wheat blending just before the milling process is mainly appliedwhen the components of the "wheat mix" differ in endosperm hardness and requireadjustments of moisture levels and tempering times prior to milling.

CCleaning

Intensive cleaning of the wheat before milling ensures that bacteria, mold, undesired seeds,infested kernels, shrunken and broken kernels, and other foreign materials do not contaminatethe mill products or damage the equipment (Fig. 6). Separation in the mill cleaninghouse is

based on the following differences between whole sound wheat kernels


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Figure 5

Schematic diagram of a mill elevator.

and unmillable materials: size and dimension, shape, specific gravity, different behavior in aircurrents, different surface friction, elasticity and texture, magnetic properties, friability underimpact, differences in color, and electrostatic properties. Shrunken kernels in which the ratioof bran to endosperm is higher than in sound kernels cause a reduction in flour extraction [45].Exposed endosperm of broken kernels would affect significantly the tempering waterdistribution in the wheat and cause a deterioration of milling quality [46,47].

Magnets or metal-removing equipment separate foreign materials that could damageequipment or generate a spark in today's fast-turning and precisely designed equipment.Sparks in a confined space, within a machine or in a facility in which dust of optimalgranulation and concentration is generated, could cause a dust explosion.

Initially, the foreign material is removed by a series of screens of selected apertures thatremove matter either smaller or larger in size than the wheat kernels. Sieves in the millingseparator, similar to those used in the receiving section but with about one third to one half ofthe load, are finer and more carefully adjusted to the size of the wheat kernels.

Gravity separators separate out impurities similar to wheat in size but different in specificgravity. Adjusted air currents are drawn through a layer of wheat moving on a tilted screen.Stones or other materials heavier than wheat are segregated and remain closer to the screen.The lighter wheat floats down the screen, while the heavier stones ''climb" the vibrating screento the outlet.

Following the gravity separators, machines such as the disc separator remove impurities thatare similar in diameter but different in shape from the whole wheat kernels. The disc separatorutilizes a series of rotating discs with indentations or pockets on both sides to effect separation.

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The discs rotate within the machine and raise those kernels that fit into the pockets. Thepicked-up particles are dropped into channels between the discs. Pocket configurations dependon the size and shape of the seeds and grain to be separated. The bulk of the wheat in themachine is forwarded to the outlet with angled pallets at the center part of each disc. The levelof wheat is controlled with a gate at the outlet end of the machine. The efficiency of separationis also controlled by the option to divert picked-up particles into a screw conveyor that can

return them to the head end of the machine.


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Another machine using the principle of shape differences is the indentation cylinder. Thisdevice has a lower capacity and is less efficient than the disc separator. Particles are picked up

by indentations in a rotating metal cylinder and dropped into a collecting trough. The cylinder

is rotated at a speed just below that at which centrifugal force would prevent the lifted particlesfrom dropping out. The disc separator or the indentation cylinder pockets sizes can be selectedfrom manufacturer catalogs to separate shorter particles from the bulk of wheat or the wheatkernels from longer kernels, such as those of barley and oats.

Dry scouring of the wheat removes any dirt adhering to the kernel. In the scorer a rotorbounces the wheat against the wall of the machine, which may be of a perforated sheet metal, asteel wire woven screen, or an emery surface. Machines are available with vertical orhorizontal design and different rotor configurations.

Throughout the wheat-cleaning process all machines and handling equipment are undernegative pressure to remove fine dust and light materials. The negative air pressure systemsuse controlled velocities and pressures to secure separation of particles with differentresistance to air flow due to size, density, shape, or other physical characteristics. New wheat-cleaning machine designs include air-circulating units as an attachment. Only about 10% ofoutside air is used during circulation. The concept of circulating air in machines saves energy,air tunnels to air filters, space, and environmental problems. New designs include machinesthat combine different principles in one unit. This advantage is claimed, for example, formachines that combine sieve separator, gravity table, destoner, and light material and dustremoval by air. Another design includes a coarse sieve, disc separator, and indent cylindercombined in a single machine.

DConditioning

Conditioning, a process that adjusts the moisture level of wheat before milling, achieves amellow endosperm and tough bran. Bran that absorbs proper amounts of moisture becomeselastic and will not splinter during grinding to contaminate the flour with fine particles.Mellow endosperm breaks off the bran during grinding, and less power is required to reducelarge pure particles to flour. On the

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Figure 6Schematic diagram of a wheat-cleaning flow.


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other hand, an excessive moisture level softens the wheat endosperm to a degree where it doesnot have the resistance to break down to sharp particles that is important for efficient sievingand separation from the bran. Another objective of wheat conditioning is to equalize the

hardness of the different kernels in the wheat mix before processing. If the moisture contentand hardness of wheat lots in a mix are significantly different, they might be treated separatelyduring the conditioning process. Different methods could be used to condition the wheat

before milling. Heating the wheat, application of warm water, application of live steam, or justintensive mixing of wheat and water are some of the methods used to increase the amount andrate of water penetration into the kernel. Moisture pick-up by wheat capillary action increasesslightly and linearly with increasing water temperature [48]. The increase from the initialtemperature of 26.7C is approximately 2% at 30C and 4% at 90C for each variety of wheat.Excessive heat (above 65C) results in gelatinization of starch and protein denaturation.

The current method most frequently used is termed ''tempering." According to this procedure,a calculated amount of water is added to the wheat, which is then intensively mixed in a

continuous mixer in order to maximize a uniform dispersion of the water on all wheat kernels.Different mixing rotor configurations or vibration during application of the tempering waterare used. The tempered wheat is given a certain rest period in bins to allow the water todistribute optimally within the different parts of the kernel and to equalize or reduce thehydration differences among kernels. Initially the water penetrates at a rapid rate through thegerm, while the surface water is prevented from moving through the seedcoat layers. The

penetration rate of the water entering through the germ side is affected by the protein contentand vitreousness of the endosperm.

Optimally conditioned wheat will ensure breakage of the kernel to the required distribution ofintermediate materials throughout the process, their quality, and the appropriate load to each ofthe machines. Water penetration and optimal distribution in the wheat kernel is also a functionof wheat size and shape. It was shown that water penetrates at different rates into small,

medium, and large kernels of hard red winter wheat [49]. Moisture permeability, surfacetension, and differences in cell structure are also parameters to be studied regarding wheatconditioning for millin [50].

Three factors affect the rate and level of water penetration into the kernel: temperature, amountof water, and time. The ideal water and wheat temperature for general tempering conditions isabout 25C (77F). Higher temperatures will increase the rate of water penetration into thekernel. Temperatures above 50C will change the endosperm starch and proteincharacteristics. The wheat delivered to the grinding stages should have the right moisturecontent and preferably a temperature of about 25C. The bran of cold wheat below 15C willfracture excessively in the breaks and result in higher ash in the flour [51]. At optimummoisture and temperature, a significant increase in flour extraction and quality can be

achieved. Maximum wheat and grinding equipment temperature should not be above 38Cbecause of difficulties that could develop in separating the bran from the endosperm [51].

During the milling process 22.5% of the total moisture in the mill materials evaporates.Accordingly, the amount of water added to the wheat should be adjusted based on the rawwheat moisture, environmental conditions in the mill, evaporation of moisture while productsare treated by air, heat generated during grinding, and the desired moisture content in the finalflour. Typical moisture contents of tempered wheats are: for hard spring wheat, 16.017.0%;hard red winter wheat, 15.516.5%; soft wheat, 14.515.5%; and durum wheat, ~16.017.5%.Tempering time variesthe average times are 36, 24, 10, and 6 hours for hard spring, hardwinter, soft, and durum wheats, respectively. Especially hard vitreous kernels would havelimited water absorption. Without using special means, hard wheat could absorb only about33.5% of water at one time. Recently, mechanical means such as high-frequency vibration and

various modes of rotors in mixers have been applied. According to different engineeringcompanies, with proper equipment up to 78% moisture could be added in a single tempering

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step. The final decision as to the optimum moisture content for milling and tempering time is asubjective decision the miller makes using a trial-and-error approach. To toughen the bran,2030 minutes before the milling process the miller adds 0.51.0% water to the wheat; toachieve good results, hard wheats should be tempered twice as described above. Very hardwheats could be tempered three times before milling or follow the method of initial temperingin the elevator (see Section VII.A). In the past different additives such as 0.1% sodium dioctyl

sulfosuccinate [52] and others [53] were added to the water to increase the rate of penetrationand optimal distribution within the kernels.

Scouring and intensive aspiration also take place after the wheat-tempering stage. During thetempering process some of the outer pericarp is loosened (beeswing), and with the scouringaction it is removed. The intensive scouring of wheats before and after tempering reducessignificantly bacteria, mold, and yeast counts per gram of the finished flours [54]. Someauthors [55] claim a reduction of mold, yeast, and bacteria of infected wheat by 9095%. Thislevel of reduction was achieved by the application of high electromagnetic frequency waves of2325 MHz for 1.52 minutes to a 20 mm wheat layer on a endless belt.


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EThe Wheat- illing Process

Wheat flour milling is a process that consists of controlled breaking, reduction, and separation.The objective during milling is to separate the branny cover and germ of the wheat kernel fromthe endosperm. Breaking of the wheat kernel is affected by corrugated cast steel rolls thatgradually separate the endosperm, bran, and germ. Reduction of relatively pure endosperm to

particles smaller than 180 Pm is achieved by using smooth rolls. Segregation between thekernel parts occurs in sifters and purifiers. In sifters, sieves separate particles of different size.In purifiers with sieves and air, differences in size, specific gravity, and shape of particles areused to separate particles of pure endosperm and those which include different ratios of branand endosperm. None of the kernel fractions coming out of the mill are completely pure, andeach contains some parts of the others. The level of purity of each product at the end of themill is one of the measures of mill efficiency.

Flour extraction in the mill is measured as percentage of flour produced based on a quantity ofwheat that is either dirty, dry, clean, or cleaned and tempered. The basis used for calculation ofthe extraction rate should be stated with the results. Another measure is the gain/loss or thedifference between the wheat arriving in the mill and the total weight of products shipped out.There should be a gain of total product weight after the milling process as a result of thedifference between the moisture content of the wheat arriving at the mill and the cumulativemoisture content of all final products.

The flour-milling process consists of numerous stages that can be divided into the followingsubprocesses: breaking, grading, purification, sizings, reduction, millfeed handling, germrecovery, and flour dressing. The milling stages of the process are shown on the millflowsheet, which is a ''map" of the process. The intermediate materials of the process flowingto each of the grinding stages are named accordingly by the miller, such as sizing or middling

materials. Figure 7 shows an example of a relatively simplified mill flowsheet. This flowsheetdemonstrates the links between the different stages in a milling process as well as the specific

parameters of the machines.

Materials at different stages of the milling process differ in quality or in the ratio of bran toendosperm and particle size. The efficiency of gradual separation between the endosperm,

bran, and germ is directly related to the length and the number of stages in the process.Segregation of the intermediate materials to different grinding stages is based on their size andthe amount of undesirable bran and germ particles. In an optimal system each of the materialswould be treated individually. However, grinding rolls, sifters, and purifiers are manufacturedto standard sizes, and this causes mill designers to compromise on the number of separationsin respect to quality and quantity of the intermediate materials. Accordingly, the extent to

which intermediate materials are subdivided in the mill is a function of the mill capacity. If themill capacity is too small, different stages would be underloaded with standard size equipment,and in this case products that are only slightly different should be combined.

Grinding of the wheat occurs between two cast rolls that are positioned in a machine structureand rotate against each other. The machine, called a "rollstand," includes usually two pairs ofcast rolls, parts that function as the engaging and disengaging mechanism, a system of materialfeeding to the nip of the rolls, and various automation systems. Modern rollstands include

pairs of rolls with diameters of 250 mm and lengths of 6001250 mm. The rolls are held byprelubricated roller bearings and positioned horizontally to each other. Some new rollstanddesigns come with four pairs of rolls, where two subsequent grinding steps are performed oneach side before the material is conveyed to a sifting machine. The rolls rotate at differentspeeds. The ratio of the speeds is called the differential. Fast rolls of the initial grinding stages

(breaks) rotate at about 650 rpm, while those at later stages rotate at about 500 rpm.Differentials range from 2.5:1 to 1.5:1 in the break and reduction rolls, respectively. With

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higher differential, there is a larger shear effect between the rolls, while with lowerdifferential, compression is more significant.

The initial grinding stages in the milling process are named "breaks." The breaks are used inthe grinding steps of the milling process to separate the bran, germ, and endosperm from eachother. The success or failure is measured in the level of achieving, as efficiently as possible,

complete separation between the kernel parts. Between corrugated rolls there always exists asmall gap, which is absent in smooth reduction rolls. In the conventional milling of hard anddurum wheats, the objective is to produce minimal amounts of flour in the breaks but amaximum of clean endosperm chunks. However, with soft wheat, because of the softer, lessdense endosperm, the percentage of flour extracted from the breaks in conventional milling ishigher than that from hard and durum wheats. One study [34] reports that hard, soft, anddurum wheats produced on the first three breaks are 49.8, 44.7, and 77.4 and 5.7, 10.5, and2.0% of sizings and flour, respectively.

The corrugations on the roll surface are grooves with front and back angles (Fig. 8). Thesteeper front angle is 2535 and the back angle could be between 60 and 75. In general,steeper angles would create more granular fractions, while flatter corrugations would generatefiner fractions. The corrugations are cut in a spiral with relation to


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Figure 7Flowsheet of Kansas State University 200 cwt flour mill.

the roll axis ranging in the order of 416%. The inclination would be expressed in inches per foot in the United States or in percent per rolllength in European countries. The number of corrugations on the first break rolls would be about four per centimeter; in later stages there isa gradual increase in the number of corrugations per inch (smaller corrugations) on the roll surface. Between corrugations there should be a''land," which is the width of unmachined roll surface. The land strengthens the corrugations and reduces the bran cuttings to fine particles.The effect of the speed differential between the rolls is also responsible for what is called the "action." The action of the front angle of oneroll against that of the other is named "sharp to sharp" (S:S). In the case that the back angle of the two corrugations act against each other,the action is "dull to dull" (D:D). Millers could subjectively arrange roll action as S:D or D:S based on variables related to the wheatcondition and mill flowsheet.

Starting with the first break, the objective is to open the kernel. The shape and depth of the first break roll corrugations should be selected tofit the size of the kernels. Optimum results in the first break are achieved if the kernels are fed to the gap between the rolls horizontally, held

by the corrugation of the slow-moving roll, and opened exactly at the crease by the fast-moving roll. Optimum for the second break rollsand the subsequent breaks is feeding the material (endosperm attached to a flake of bran) directly to a precisely adjusted gap where with theright pressure the


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Figure 8Roll corrugations: (a) the cutting effect

between rolls as a result of corrugations spiral(b) Action between corrugated rolls (S:S)

(c) Roll cross section showing the shape of corrugation.

fast-moving roll scrapes the endosperm from the bran. As the bran flakes get smaller towardthe final breaking stages and the endosperm layer attached to it becomes thinner, graduallysmaller corrugations are used (or a larger number of corrugations per inch of roll surface).Optimally conditioned wheat and the right corrugations, pressure, and differential minimize

splitting of the bran to particles of a size that can be sieved through with the flour. Goodresults in conventional milling are obtained when most of the endosperm free bran consists oflarge flakes.

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The commercial flow should be designed to meet the required capacity, wheat quality, and endproducts, and it is based on specific machine surface values as shown in Table 3 [56]. Forexample, the roll unit in the United States allocates 0.3 inch of roll length per 100 pounds(cwt) of flour milled per 24 hours. Mills that use the metric system would express the sameroll units as 12.58 mm/100 kg wheat/24 h. Conventionally with a longer break system, up to

six stages in hard wheat and seven in durum wheat mills, it is possible to grind the material fedto the rolls in a less severe manner. Roll surfaces should be maintained in good condition toensure good flour extraction and quality. Depending on the quality of the steel and the type ofmilling technology used, corrugated rolls should be refurbished every 36 months of milling.Other factors that influence the need for refurbishing are roll surface allocation, feed rate perunit, severity of grinding, wheat hardness, and presence of stones or other impurities in wheat.Recent advances in metallurgy that allow casting of harder outer surfaces for corrugated rollsextend the time between refurbishing up to 8 months.

Even when the mix in the mill is changed drastically in wheat size and kernels are smaller orlarger than normal, usually mills will continue using the existing corrugations, keeping manyexiting variables unaltered. Generally, the gap between the rolls will be adjusted intuitively bythe miller based on his or her experience. A few studies were conducted to evaluate the firstroll action and the different

TABLE 3 Mill Technical Specifications for Major Equipment forDifferent Kinds of Wheatsa

WheatHard Soft Durum

Roll unit (mm) 1015 1013 1620Sifter surface (m2) 0.0550.081 0.0830.088 0.0860.093Purifier width (mm) 37 03 812aPer 100 kg processed wheat in 24 hours.Source: Ref. 56.


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parameters that could effect conventional milling of different kinds of wheat. Grinding of softand hard wheats on a set of rolls at different rotating speeds indicated that better separation

between bran and endosperm occurred on the first break with a lower speed and smaller

diameter [57]. Wheat moisture is another important factor that affects the grinding process forcommon and durum wheat [58]. The best semolina production with first break rolls fromdurum wheat was achieved by sharp to dull action, angle profile of 25/65, and a differentialof 1.5:1 [59].

The severity of grinding between the rolls and the particle size distribution of the ground mealis controlled by adjusting the break release, which is defined as the percentage of material

passing through the first group of overtailing sieves in a break sifter, based on the amount fedto the sifter. The miller adjusts the release of the different grinding stages using a laboratorysifter on which a representative sample taken from under the rolls is sifted. With a given millflow the miller sets the appropriate break releases for each wheat mix. Normally thecumulative release of all the breaks should be about 23% higher than the expected total flour

extraction from the mill. Following each grinding stage, the material is conveyed to a siftersection.

1Sieving

In the sifter, particles of the grounded material are separated according to size. Sifters areavailable in two, four, six, and eight sections. Modern sifters are more sanitary than those usedin the past, which often were a source of infestation. Each section contains 2630 framescovered with tightly stretched sieves of appropriate apertures. Properly tensioned sieves on theframes are critical for a sifting efficiency. The optimum degree of tension (~11 N/cm) isrelated to the cloth material used. Excessively slack sieves reduce the mill throughput up to4%. In the past, sieves were stretched by hand over the frame and stapled. Today, special

stretching devices are used to uniformly stretch the sieves, which are glued to the frames.Sifter sieve areas in mills are specified in m2/100 kg wheat/24 h (Table 3).

The sieves in a sifter section are divided into groups. At the top of the section, there areusually coarser sieves separating the larger material that flows out of the sifter through a sidechannel. The material passing through the sieve is either transferred out of the machine ordirected down to finer-aperture sieves for a further separation. Below each sieve, a backwire isattached to the frame on which hard rubber balls, plastic elements, or cotton pads bounce tokeep the sieve clean. ''Throughs," a stream passing through the upper sieves in a break stagesifter, is a mixture of flour and chunks of endosperm to which often some bran is alsoattached. While the "overs" of the top sieves are transferred to the next break for additionalscraping of endosperm, the mixture of the throughs is segregated, based on particle size

differences on lower sieve groups in the section. This is evident from a schematic view of afirst break sifter section where six materials that differ in quality and size flow out (Fig. 9).

2Grading or Redustin

Graders are sifter sections used to handle mainly materials directed from the breaks. A blendof medium-sized and fine sizings as well as middlings is directed to the graders. Materialsfrom primary breaks are directed to the first grader. Materials from secondary breaks (e.g., thethird or fourth) are directed to second or third graders. The main objective of the grader is toremove the remaining flour from the middlings and to separate the granular material to narrow

particle size ranges for better efficiency in the purifiers.

3Purification

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At the head end of the milling system granular intermediate materials of the same size rangeare directed to machines called purifiers. The different size groups differ also in the amount of

pure endosperm, bran, and such particles of endosperm to which bran is still attached. Themore similar the particles are in size, the more effective is the purifier performance. The

purifier's main purpose is to separate particles into fractions of pure endosperm, a mixture of

particles to which bran is attached, and bran particles. This is achieved by using sieves and aircurrents. The purifiers classify the material into several fractions according to size, shape, andspecific gravity. The endosperm particles, essentially free from bran and germ, are spouted tosmooth rolls, where they are ground into flour. Other particles to which bran and other outerlayers of kernel adhere are delivered to different pairs of rolls ("sizings") for careful reductionand separation of the bran.

The purifier includes two set of sieve "beds" with one to three layers of graded sievespositioned on top of each other (Fig. 10). Each layer in the bed consists of four sieves that arefiner in the head than in the tail end. The upper sieves in each bed are coarser than the lowerones. Vibrating motors apply a reciprocating motion to the sieves that hang in an inclined

position. In older models, sieve hangers could be adjusted to vary the sieve inclinations andstrokes that move the material. In today's modern machines the vibrating motors and theircounterweights are adjusted to control the sieves motion. This permits the miller to adjust themachine to have more pitch for fibrous material than pure endosperm particles. Brushesmoving back and forth or rubber balls bouncing on a backwire attached to the sieve framekeep the sieves clean. Air cur-


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Figure 9Schematic view of a first break sifter section.

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Figure 10(a) Schematic view of a purifier.

(b) Schematic view of a purifier sieve bed.


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rents drawn through the sieves fluidize and stratify the material based on the particle size,specific weight, and shape. The vibrating motion of the purifier sieves also stratifies thematerial on the sieve layer. The heavier endosperm particles move closer to the sieve surface

while the more branny material floats on top. At the head end of the purifier the purest andmost dense endosperm particles pass through the sieves. Materials with more bran attached

pass to sizing rolls through the coarser sieves. Tailings over the sieves are materials that aredirected to the last fine break stages.

The purifier air hood is divided into sections and is positioned above the enclosed airtightsieve bed, allowing air to move only through the sieves. The amount of air drawn through thelayer of material moving on is controlled by valves in each of the sections. The miller can alsoregulate the amount of material to the purifier to keep the sieves covered and prevent bareareas on the sieves. Bare areas allow the air to flow through because of the reduced resistance,causing ineffectiveness in stratifying material on the sieves. The number of purifiers in a millis specified based on the total sieve width per 100 kg of wheat processed in 24 hours (mm/100

kg wheat/24 h) (Table 3). In some cases where space is limited, two machines are stacked ontop of each other.

4Sizings

The material at each of the sizing stages is a mixture of particles close in size range, some pureendosperm, and others still with attached bran. The objective of the sizing stages is to reducethe particle size and, during reduction, to separate the still attached bran from the endosperm.

Material from the sizing stages can be diverted to purifiers, to middlings for final reduction, orto flour as a final product. However, the miller tries to refrain from severe grinding in thesizing stage to avoid production of flour that may be contaminated by the presence of bran.

Some millers use corrugated rolls on sizing stages, while others use smooth rolls. Smooth rollswill have a more delicate effect and produce lower-ash flour than corrugated ones. Whencorrugated rolls are used in sizings stages, the corrugation features are adjusted to the particlesize and the bran adhering to them.

5Middlings or Reductions

Coarse and fine pure endosperm particles from breaks, purifiers, sizings, and reductions in themill are reduced to flour on smooth rolls. The outer layer of smooth rolls is of ''softer" steelthan that of corrugated rolls. The "softer" steel, which includes more carbon molecules in thecast, "loses" them with time, thus keeping a rough surface. Table 4 [60] shows the differenteffects of rough, polished, and finely corrugated reduction rolls on the middlings' groundmaterial, particle size, and flour quality. Smooth roll surfaces should be refurbished about oncea year depending on the steel quality. The speed differential between smooth rolls is1.15:11.8:1, i.e., much lower than in breaks or other corrugated rolls (2.5:1). The lowdifferential causes higher pressure and lower shear forces between the rolls.

Between smooth rolls that practically touch each other, high pressure is exerted on thematerial. However, that pressure should be optimized for each reduction stage. Testsconducted with a third middling material showed that maximum flour was extracted through a11XX (124 Pm aperture) bolting cloth following the use of 64.5 pounds per linear inch

pressure between a pair of smooth rolls [61]. Higher pressures flaked part of the endospermmaterial, resulting in a lowering of the amount of flour passing through the bolting cloth. The

pressure causes a rise in the temperature of the smooth rolls, which can reach 50C (122F) or

higher. To decrease the rise in tempera-

TABLE 4 Effect of Matte, Polished, and Fine Corrugated Rolls on Second

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Middlings MaterialsSmooth rolls,

matteSmooth rolls,

polishedFine,

corrugated

Flour throu h 136 m % 65.6 62.1 64.7

Flour Ash (%) d.m. 0.52 0.54 0.56

>107 m % 29 29 41>95 Pm (%) 9.5 11.5 14

>73 m % 18 18 15

Throu h 73 m % 43.5 41.5 30

Dough resistance (Dw) 665 640 540

Dough elasticity (Dl, cm) 11 11 13Bread volume: cm3 (ml/100 gflour)

561 576 571

Dw and Dl = Extensigraph values.Source: Adapted from Ref. 60.


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ture, certain rollstand models include a water cooling system. The material is acted uponbetween the rolls for about 1/390 of a second. In the nip between the rolls the materialtemperature can reach 60C (140F) for a short time. However, the temperature of the material

usually rises about 7C (12.6F), as indicated by measurements taken above and under therolls. In addition to the action of the rolls on the material, it was also recognized that reductionof particles occurs among the particles themselves. This depends on the layer of material fed tothe rolls [60]. The pressure exerted by the rolls on the endosperm particles is responsible forthe physical reduction in size but also causes other physical and chemical changes, includingdamage to the starch and some modification of the proteins [60].

In general, the reduction system substantially affects the quality of the end product through thecompression and shear applied on the endosperm matrix of protein in which starch granulesare embedded. In hard wheat the adhesion between the starch granules and the protein matrixof the endosperm cells is stronger than in soft wheat. Therefore, flours from soft wheatdisintegrate easier in milling and produce finer flours than those of hard wheats. Millers adjust

the flowsheet and mill equipment to produce flours of coarser granulation from weaker wheatsand finer granulation from stronger wheats to achieve optimum results in baking.

Starch damaged by millin absorbs five times more water during the dough process and issusceptible to diastatic activity by enzymes that decompose starch to dextrin, oligosaccharides,and simple sugars during the dough preparation. When present at an excessive level, damagedstarch has an adverse effect on dough and bread quality. Because of its harder cell structure,hard wheat endosperm generates flour with more damaged starch by the action of high roll

pressure or high impact forces during the reduction stages of the mill. A matte surface willgenerate more starch damage than polished surface. The amount of starch damage is alsoaffected by the velocity differential between the rolls [62]. On the other hand, if thisdifferential is unchanged but the roll speed is increased, the starch damage would increase

because of the difference between the peripheral speed of the rolls.

Some flaking of endosperm occurs during reduction with smooth rolls. To disintegrate theflakes, different types of flake disruption or impact machines are used. Disruption of the flakescan be achieved by impaction with a fast rotating rotor on which an arrangement of blades,

pins, hammers, or stripes hits the endosperm particles at an appropriate tip speed. Impactmachines are used in some cases instead of rolls to reduce the size of clean endosperm

particles of flour. If the position of the impactor and speed are set correctly to grind endospermof appropriate hardness, impact milling could be more effective than rolls in reducing it toflour. A rotor tip speed of 110 m/s applied to granular endosperm produced 87% flour [63].Flour produced with impact milling is finer and has a lower level of starch damage ascompared to that from roll stand grinding. Protein levels in these flours were higher afterimpact grinding than in flours produced by rolls. The investment costs in an impact mill are

lower, but the energy expense per quantity of material reduced is higher than that of a rollstand[64].

6Air As a Means of Processin

Machine location and product transfer in the mill are optimized by maximizing the use ofgravity flow for intermediate materials. For vertical transfer of materials positive or negative

pneumatic systems are used. Negative pneumatic systems are usually used for the transfer ofall intermediate materials in the grinding unit. Properly designed and efficient air-handlingsystems for pneumatic conveying or suction in various locations in the mill reducesignificantly the energy consumption of the operation. In a modern mill about 10 times moreair weight than wheat weight is moved through the system. Accordingly, it is essential to

maintain the relative humidity at about 65% and temperature at about 25C (77F) in the millto control moisture evaporation in intermediate and final products. In locations where extreme

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humidity levels or temperatures exist, air control units should be installed in the mill. Ifintermediate stocks are too dry or too wet this affects the sieving efficiency, the breaking up ofthe bran, and accordingly the final quality of the flours.

VIIIMill Control

Control of mill performance is a continuous chore of the miller who sets methods andprocedures to achieve optimal performance. As an example, when changing wheat mixes inthe mill, the flours are directed to a set-off bin until the mill is adjusted for the new wheat mix.The mill flours are directed to the set-off bins also upon starting and shutting down the mill.The reason for such measures is to prevent production of off-grade flours while the mill isunderloaded. The flour in the set-off bins is reblended to the main stream at a very low rate.Scales to weigh wheat at receiving point, before and after cleaning, tempered wheat, and final

products could indicate changes in loads, extraction levels, and any other problems in eachsection of the mill. On-line instrumentation to determine moisture, protein, ash, and colorensures uniformity of raw materials and final products.

Evaluation of the mill technological performance is measured by using the ash content ofwheat, intermediate


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Figure 11A cumulative ash curve of a flour mill.

materials, individual flour streams, and final products. The significant difference in ash contentamong the three main parts of the wheat kernel endosperm, bran, and germ is used as ameasure to determine the level of the separation efficiency from each other. However, in the

past, because no other accurate tools were available, ash was used as a criterion of flourquality. Flour ash was an inconclusive parameter and in the past created significant economiclosses to millers and bakers. The reason is that ash values of flours are not directly related tothe flour end user's specifications. Millers compromised on flour extraction to supply flourwithin specifications from good baking quality wheats that inheritably had higher endospermash. Today, fast and accurate instrumentation to determine flour qualities such as color, starchdamage, rheological characteristics, and baking qualities is widening the parameters for flourspecifications.

The objective in milling is to achieve as high as possible flour extraction with the lowestcontamination of bran and germ that increase ash content. The ash curve is a mean to expresscumulative ash of the flour streams in the mill. To construct the ash curve the streams are

arranged in increasing order of ash content, and they are weighted based on the extraction ofeach into a function that is a relationship between the cumulative ash content of a number ofstreams and the related total flour extraction (Fig. 11). The miller's objective is to reach an ashcurve that is flat and start to turn upward at the highest possible flour extraction.

While the ash values and curve are an indication of the mill separation efficiency between theendosperm and bran, the granulation curve is a function of mill adjustment and screenselection. The granulation curve (Fig. 12) expresses the disintegration of the wheat kernel atdifferent stages of the milling process. The curve is drawn as a graph where the horizontal axisshows the various sieve apertures in micrometers, and the vertical axis shows the cumulative

percentage tailovers of the respective sieves. The granulation curve shows the particle sizedistribution of the ground material. By drawing granulation curves for each of the grindingstages, the miller can monitor variability in kernel disintegration and make the necessaryadjustments in the system. The data to construct the granulation curve can be generated withan experimental sifter. The miller sieves the stock from under the rollstand on a stack of sieves

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and then calculates the percentages of all the quantities remaining on the sieves and thematerial in the bottom pan from the total weight.

If a different set of sieves is used for the separation of a grounded stock, different points willbe allocated on the same graph to determine a change in the amount overtailing from eachsieve. The shape of the curve does not depend on the sieve aperture, but on the sample

granulation distribution. The miller draws the granulation curves of the mill for each wheatmix at the time when mill performance is optimum. Granulation curve analysis can generatethe following information: (1) corrugation condition, (2) mill balance, (3) roll adjustment, and(4) sieve area, aperture, division, and efficiency of the sieving stages.

IXThe Mill End Products

Flours

Flour quality is a subjective concept that relates to final product usage. For different types of

bread around the world specific wheat characteristics and flour qualities are required. Qualityparameters such as color, protein, granulation distribution, gluten quantity and quality, andstarch damage play a role in the suitability of flour for the baker. Another important factor

besides the determination of

Figure 12A mill granulation curve.


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quality is the concept of flour uniformity. For the commercial baker uniformity of flour supplied ismore important than variations in characteristics such as premium protein or reduced starch damage.

Flours from the different stages in the mill are not identical in physical appearance, chemical analysis,

or baking properties. These flour streams are composed of varying amounts of different parts of thewheat kernel. In the case that all the flour streams are blended to one composite, the result is a''straight-grade flour." The quality of the straight-grade flour is directly related to the quality of theprocessed wheat. It is possible to combine these flour streams in different ratios to producesimultaneously two or more final flours that differ in color, ash content, protein content, dough-handling properties, and bread baking characteristics. This method of producing more than one finalflour from one wheat mix is called "split milling" or "divide milling." In wheat-importing countries themethod of split milling is used to accommodate the requirements for flour qualities of different enduses. In wheat-growing countries such as the United States split milling is not frequently used since thewide variety of wheat types accommodate different end uses.

In the United States the common types of flours produced in a mill are patent, first clear, and secondclear. Figure 13 shows an example of products from a flour mill and their proximate analysis. Amounts

and types of final products vary among mills are a result of differences in flow-sheet, adjustments, andkinds of wheat milled. Flour streams from the head end middlings, primary sizings, and in some casesthat of second and third breaks originate from the center of the wheat kernel. The blend of these

Figure 13Flour grades for a typical milling system. 14% moisture basis. KJ = KentJones Color Grader.

(Adapted from Ref. 65.)

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flour streams is called ''patent flour." Patent flour is about 77% of the total flour, is the whitest,and contains the lowest relative amount of ash (0.380.42%, corrected to 14% moisture

basism.b.). Other flour streams of the process that contain a higher percentage of the

endosperm parts adjacent to the bran and germ are distinguished from the former by higher ashand protein contents, darker color, and inferior baking qualities. These flour streams can becombined to make up "first-clear flour." First-clear flour is about 20% of the total flour andcontains about 0.75% ash. "Second-clear flour," made up of the rest of the streams, is 3% oftotal flour and contains up to 1.2% ash (14% m.b.). The ratio between patent, first clear, andsecond clear could vary substantially in percentages in other instances and, accordingly, in ashand quality. Blending part or all of the first clear into the patent comprises the "baker's patent."

The miller subjectively blends the flour streams from different stages in the mill to make upthe final products. Each of the final flours are collected under the sifters in conveyors. As aresult, characteristics of the final flours do not follow a regression line of quality.

Optimum flour granulation distribution is an important parameter for the baking process.Drastic change in granulation effects water absorption, water retention during fermentation,

proofing, and quality of finished breads. The mill adjusts product granulation to the kind ofadditives added during dough preparation and to the types of breads baked. Control of flour

particle size distribution is a parameter the miller controls by wheat selection, tempering, millflow, and mill adjustment.

The ash content does not affect the baking quality of the flour; it relates basically to the levelof bran in the flour. Ash content of flour is a very valuable test for mill control. However, inmany cases flour ash is used in flour quality specifications disproportionately to its value andsignificance in baking. This creates a situation where millers are constrained to lower flourextraction when using good baking quality wheat of inherently high endosperm ash.

Flour color depends on wheat cleanliness, tempering level, finesse of flour, and the amount ofbran particles it contains. Too much fine bran effects flour shade, producing a darker shade.Frequently during the mill operation the miller slicks a flour sample and wets it. This method,called the Pekar test, is used by the miller to evaluate the color and amount of bran particles inthe flour. Change in mill ambient conditions could also affect flour color. In addition, flourcarries a yellow cast due to the presence of carotene. Natural aging during storage of the flourfor up to 2 weeks or usage of different bleaching agents, where permitted, could overcome this

problem.

In some countries improvers and enrichments are fed into the flour in the mill or in theblending facilities before load-out. The powders are added to the flour with great accuracy anduniformity by special feeders. Modern systems use programmable logic controller (PLC)-controlled feeding systems. At the end of the milling process the microingredients areconveyed by air and introduced and mixed into the flour by special agitators. In mills wheremicroingredients are added to flour according to customers' specifications, they are introducedinto large-capacity, high-speed batch mixers during final blendin and before load-out.

B

Bran

Commercial bran differs from the botanical outer layers of the wheat kernel. The bran that isremoved during the various stages of the milling process is made up of fractions that differ insize and endosperm content. Bran is described using factors such as minimum protein,minimum fat, maximum fiber, and maximum moisture. In the United States "wheat mill run"would be a product that includes all offal fractions from a typical mill. According to the

American Feed Control Officials [66], wheat mill run consists of the following: minimumprotein, 13.0%; minimum fat, 4.0%; maximum fiber, 9.5%; and maximum moisture, 14.0%.The American Feed Control Officials [66] define proximate analysis for all other by-products

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from the milling process. Specifications will vary from country to country based on millingtechnology, feed regulations, kind of wheat used, and climatic conditions.

C

Wheat Germ

The germ constitutes about 2.53% by weight of the wheat kernel depending on the size of thewhole kernel. The two main parts of the wheat germ are the embryo and the scutellum. Theloosely held embryo part of the germ can be extracted relatively easily, but the soft scutellum,high in fat and protein, is difficult to separate from the endosperm and the bran [67]. Theembryo and the whole germ differ in size, shape, and the level at which they are embeddedinto the kernel among the different kinds of wheat.

The mill flow is designed to separate whole embryos during the breaking stages. The moist,soft, and easily flattened embryos are directed in the mill flow, usually from a purifier, to a

pair of smooth rolls with low differential, where they are flaked [68]. The small flakes areextracted in the sifters over a 14 US mesh sieve (1410 Pm). According to definitions of theAssociation American Feed Control Officials [66], pure wheat germ that is used primarily forhuman food should contain a minimum of 30% protein. In some mills the germ is separatedwith an impact machine ahead of the first break roll. After impaction the material is sifted on asifter, where it is separated into differ-


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ent fractions. The coarse material is diverted to the first-break coarse, the intermediate materialto first-break fine, and the fines containing the embryos to a smooth pair of rolls, where it isflaked for separation.

XChemical Composition of Wheat and Mill Products

Protein

Various classes of wheat are intentionally bred and selected for a specific composition, usuallyto meet end-use requirements for a product. For example, commercial soft wheats aremaintained at low protein levels, although certain soft wheats are associated with genes forhigh protein and are used as germplasm in breeding programs to develop high-protein hardwheats [69]. Protein content in a single variety of wheat can vary from 7 to 20% depending

upon growing environment and fertilizer use.

Typical protein ranges for selected world wheats are given in Table 5. Protein content isnegatively correlated with grain yield, so that spring wheats are generally higher in proteincontent than winter-grown types.

Constituents of hard and soft wheats are given in Table 6. The high-protein hard wheat ishigher in protein in all constituents except the germ. Constituents of wheat grains are notdistributed uniformly. Composition of anatomical

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