Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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the corrugations, which runs parallel to the axis of the

roller, is referred to as the twist. The cutting and rear

angles, together with the position of the corrugations,

and the absolute speed of the surface of the roller

casing determine the efficiency of the grinding pro-

cess. Where the cutting angle is small, the cutting

effect of the corrugations predominates, favoring the

production of grits. A larger cutting angle causes

the material to be crushed instead, resulting in the

production of flour.

0046 The size of the cutting angle ranges from 15



and that of the rear angle from 60



to 75



,so

that the angle of the corrugation usually lies in the

region of 90–115



. The size of the slope of the corru-

gations, measured at the circumference, may range

from 20 to 300 mm, given a twist of 5–20% with a

roller length of between 400 and 1500 mm, for

example. The flat angles of the corrugations

have a lateral length of 0.05–0.30 mm, measured in

the direction of the circumference of the roller. The

depth of the corrugations (h) can be calculated using

the formula given below, ignoring the fact that the

root of the corrugation is rounded:

h ¼ T  SðÞ= tan a þtan bðÞð7Þ

The efficiency of the roller mills depends on the

amount of material fed on to the rollers, the type

of corrugations, and the differential speed of the

rollers as a function of the circumferential speed.

The amount of material processed is limited by the

air cushion which forms over the rollers owing to

the parameters referred to above. The air cushion is

therefore frequently reduced by means of special feed

mechanisms in order to increase efficiency, either by

compacting the material to be ground, for example,

or by drawing off the air above the gap.

0047 The differential speed can be as high as 1:15,

depending on the product. For this ratio, the driven

roller, which has a fixed position, is reduced to

the circumferential speed of the movable roller.

The upper limit of the circumferential speed is in

the region of 6 m s

1

. For example, corrugated and

smooth rollers with a standard diameter have a cir-

cumferential speed of 3.3–4.4 m s

1

when grinding

wheat and rye at a speed of 250–360 rpm.

Hammer Mills

0048Hammer mills are mainly used for coarse grinding of

brittle and fibrous materials. The material is ground

by beating, during which it may be partly broken

down into its constituents. The disintegration of the

material often serves to prepare it for subsequent

processing stages, such as separating the constituents

by sieving, squeezing, and extraction.

0049The mills consist of a rotor with suspended

hammers which swing freely and which are then

forced upwards and outwards radially by centrifugal

force. The hammers strike a sieve casing fitted with

impact ribs. The material, which is fed into the

machine tangentially from above, is crushed by

impact and rebound forces and expelled through the

sieve casing. The fineness of grinding depends on

the size of the sieve perforations, the circumferential

speed, the number and width of the hammers, and the

throughput.

Beater Mills

0050The structure of beater mills is similar to that of

hammer mills. Their rotors are equipped with rigid

ribs, plates, or arms, arranged in the shape of either a

cross or a star. Such mills are suitable for finely grind-

ing spices, for example. They may also operate as wet-

milling colloidal mills, in which part of the energy

used in grinding is applied by imploding cavitation

bubbles which form on the backs of the rotor parts.

Slicing Mills

0051Slicing mills are mainly used to grind products which

are resistant to compression, impact, and friction,

such as fruits and vegetables. The rotors of slicing

mills are fitted with removable blades with strike

blades attached to the housing and a sieve casing.

The knives slice the material which is fed into the

mill from above, after which it passes through the

slits in the sieve casing. The material is ground as a

result of the shear stress which builds up between the

stationary and rotating blades.

Toothed Disk Mills

0052Toothed disk mills are used for fine dry- or wet-

grinding of soft, nonadhesive materials. They comprise

Flat angle

Rear angle

Cutting angle

fig0005 Figure 5 Corrugation of a grinding roller.

3994 MILLING/Types of Mill and Their Uses

a housing containing a stator and a rotor (Figure 6).

The material to be ground is fed into the mill through

an opening in the centre of the stator and falls on to the

rotor. It is then transported by shear, compressive, and

frictional forces through the teeth arranged in rows on

the disk from the center to the outer rim, being ground

in the process. The ground material passes into a col-

lecting ring, from which it is transported into a radial

outlet. The final fineness of the ground material is

determined by the distance between the axes of the

disks, the profile of the grinding tools, and the speed.

Pinned Disk Mills

0053 Pinned disk mills are used to grind either oily mater-

ials or moderately hard to soft materials which are

dry or contain little moisture. They comprise a housing

containing a rotor and a stator, both of which are

fitted with removable pins arranged in concentric

circles such that they engage the opposite spaces.

The material to be ground is fed into the mill through

the center of the stator, falling on to the rotor. The

radially increasing circumferential speed causes the

rotor pins to press the material at high speed through

the rows of pins, grinding it in the process by impact

and rebound. The final fineness of the ground mater-

ial depends on the distribution and shape of the pins,

the circumferential speed of the rotors, the through-

put, and the strength characteristics of the material.

0054Pinned disk mills may be used as either dry or wet

mills. They are employed as wet mills in starch ex-

traction, for example, where steeped maize is ground

in order to remove the germ by disintegrating the

maize kernels.

Underrunner Disk Huller

0055The underrunner disk huller is a corundum stone disk

mill. It is used above all for dehulling rather than

grinding, serving as the main huller in rice mills to

separate the husk from the rice kernel. Other cereal

grains and leguminous seeds, such as beans, peas, or

lentils, may also be dehulled using this machine. (See

Legumes: Legumes in the Diet; Peas and Lentils.)

0056The huller consists of a stator disk located above a

rotor disk, both of which have abrasive coverings of

corundum stone. The hulling process and its effect-

iveness are the result of the grains being rubbed be-

tween the disks and the distance between them. The

grains describe a curved path between the disks, pass-

ing from the central opening to the outer rims. The

disks are surrounded by a casing which is completely

closed.

Stirrer Bead Mills

0057Stirrer bead mills are employed predominantly in the

chocolate and confectionery industry, although they

may also be used in the delicatessen industry. They are

used for fine grinding (1–50 mm) of raw cocoa and

chocolate, as well as in the production of peanut and

hazelnut fillings and in the manufacture of mustard.

(See Mustard and Condiment Products.)

0058Stirrer bead mills comprise a cylindrical or conical

rotor body which moves in a corresponding stator

body. In the example illustrated (Figure 7), a gap

(2–25 mm) is formed between the conical rotor and

the stator, and in this gap the beads move radially

outwards from the centre. The material is thus

ground uniformly, resulting in a close particle size

distribution. The geometry of the grinding space

insures that every single grain in the suspension passes

fig0006 Figure 6 Toothed disk mill. Courtesy of Dorr-Oliver

Deutschland GmbH, Grevenbroich, Germany.

MILLING/Types of Mill and Their Uses 3995

along the prescribed path through the grinding /elem-

ent. The suspension is fed into the mill through a

central inlet and discharged through an outlet attach-

ment near the shaft, the beads being separated from

the ground material at the same time.

0059 A large number of different settings is possible for

this mill, depending on the fineness of grinding re-

quired, as not only the rotor speed may be freely

selected but also the width of the gap between rotor

and stator, the number of beads, the material of which

they are made (glass, steel, ceramic, etc.), their diam-

eter, and the rate of flow of the suspension. The rotor,

stator, and cover of the mill can be cooled so that it is

possible to maintain the suspension at a moderate

temperature.

Drier-Mills

0060 Among the mills which simultaneously grind and

dry, particular attention is drawn here to one which

is frequently used in the food industry, both as a

grinder and as a drier-mill. It is known as the Ultra-

Rotor mill and is used for fine grinding of materials

such as starch derivatives, guar, and stearates.

Furthermore, it is also employed in the selective

grinding of various cereals which, when combined

with subsequent sieving or sifting, serves to separate

the endosperm fraction from the bran. As a drier-mill,

it is used above all in wheat starch plants to dry wheat

gluten.

0061The mill consists of a specially designed rotor

(Figure 8) mounted vertically in a housing which

acts as a stator and is equipped with a removable

corrugated inner casing. A fan disk is attached to

each end of the rotor shaft. Disk-shaped grinding

tools, arranged in a radial pattern, are fitted to plates

between the fan disks. A second disk, on to which

rod-shaped metal parts are screwed, is located below

the upper fan disk. Between rotor and stator there is a

narrow gap, through which the material to be ground

passes at high speed.

0062The material is fed into the machine through an

inlet in the wall of the housing at the level of the

lowest grinding disk, being drawn into the grinding

zone by the air stream produced there. The rotation

of the rotor causes strong eddies of air to form in the

chamberlike grinding tools which, together with the

impact of the particles on the walls of the grinding

tools and the stator casing, grind the material. Owing

to the high throughput of air, a large part of the heat

produced in the material during grinding is expelled

with the air.

0063After leaving the grinding zone, the ground mater-

ial passes into the space between the disk fitted with

metal rods and the upper fan disk. The impact of

the material on the rods produces a sifting effect.

The rods cause the coarser particles to concentrate

in the proximity of the wall of the housing, from

which they are extracted through an adjustable

opening placed at a tangent, and then recycled. The

finer particles leave the mill as a finished ground

product, being collected separately after passing

through a sieve and subsequently through filters.

The mill can be used to advantage to dry the ground

material owing to the way in which the air flows and

fig0007 Figure 7 Coball mill. Courtesy of FRYMA-Maschinen

AG, Rheinfelden, Switzerland.

Ultra-Rotor mill

fig0008Figure 8 Drier-mill. Courtesy of Altenburger Maschinen,

ckering GmbH, Hamm, Germany.

3996 MILLING/Types of Mill and Their Uses

the type of grinding. Only the air stream requires

heating. This is best carried out by means of an inter-

posed heat exchanger. The formation of eddies of air

and the considerable increase in the surface area

owing to grinding result in an intensive exchange of

material and heat, so that the air outlet temperature

does not need to exceed 70



C even where the air inlet

temperature is as high as 400



C. The temperature of

the product is normally in the range of 40–50



C and

the limiting dew point temperature is in the region

of 38–45



C. The thermal efficiency of the mill is

approximately 90%. The average heat consumption

for simultaneous grinding and drying is about 2800–

3000 kJ kg

1

of evaporated water.

See also: Barley; Cocoa: Production, Products, and Use;

Flour: Roller Milling Operations; Legumes: Legumes in

the Diet; Mustard and Condiment Products; Oats; Peas

and Lentils; Rye; Spices and Flavoring (Flavouring)

Crops: Fruits and Seeds; Tubers and Roots; Sugar:

Refining of Sugarbeet and Sugarcane; Wheat: The Crop;

Grain Structure of Wheat and Wheat-based Products

Further Reading

Bernotat S and Scho

nert K (1988) Size reduction. In: Ger-

hartz W (ed.) Ullmann’s Encyclopaedia of Industrial

Chemistry, Unit Operations I, 5th edn, vol. B 2,

pp. 5/1–5/39. Weinheim: Verlag Chemie.

Earle RL (1983) Unit Operations in Food Processing, 2nd

edn, pp. 159–165. Oxford: Pergamon Press.

Leschonski K (1988) Particle size analysis and characteriza-

tion of a classification process. In: Gerhartz W (ed.)

Ullmann’s Encyclopedia of Industrial Chemistry, Unit

Operations I, 5th edn, vol. B 2, pp. 2/1–2/33. Weinheim:

Verlag Chemie.

Loncin M (1969) Die Grundlagen der Verfahrenstechnik in

der Lebensmittelindustrie, pp. 726–747. Aarau am

Main: Verlag Sauerla

nder.

Meuser F, Reimers H and Fischer G (1977) Physikalische

und chemische Untersuchungen an Mehlen aus unter-

schiedlichen vermahlenen Weizen im Hinblick auf

deren Backqualita

t. Deutsche Mu

ller-Zeitung 75:

145–154.

Pallmann H (1977) Zerkleinerungstechnik im Bereich der

Nahrungs und Genussmittelindustrie. Verfahrenstechnik

11: 270–276.

Rumpf H (1954) Die Zerkleinerung unter besonderer

Beru

cksichtigung lebensmitteltechnologischer Fragen I:

Aufgabenstellungen. Zerkleinerungsaufgaben in der

Lebensmittelindustrie. Fette, Seifen, Anstrichmittel 56:

404–408.

Vauck WRA and Mu

ller HA (1988) Grundoperationen

chemischer Verfahrenstechnik, 7th edn, pp. 254–303.

Weinheim: Verlag Chemie.

Zogg M (1987) Einfu

hrung in die mechanische Verfahren-

stechnik, 2nd edn, pp. 13–76. Stuttgart: BG Teubner.

Characteristics of Milled

Products

E S Posner, ESP International, Savyon, Israel

Background

0001The quality and characteristics of commercial mill

products are a matter of geography and are set in

commercial mills based on end users’ expected

demands. In addition to quality characteristics, speci-

fications may include notes stating that products are

untreated with chemicals during production, free

from foreign matters, rancidity and/or other alimen-

tary substances, suitable for human consumption and

free from dead or alive insects in any developing

stages. Nomenclature of wheat products such as

‘All-Purpose Flour,’‘Type 00,’ or ‘Type 812’ creates

confusion, especially in a global trading environment.

An international effort is required to set guidelines for

wheat mill product specifications. In many cases, the

flours originate from mixes of different kinds of

wheat or blends of flours after processing. Following

the description of the different wheat products, qual-

ity variables of products and milling methods that

control the characteristics are dealt with.

Mill Products Description

0002Figure 1 shows a schematic diagram exemplifying

products from a flour mill and their proximate analy-

sis. Products in this case are referred to in typical USA

nomenclature.

Flour Types from Milling Operations

0003Straight-grade flour This denotes all the flour

streams generated in the mill combined to one final

product. Depending on the wheat kind and mill

design, in general, straight-grade flour would have a

protein content about 1% below that of the wheat.

0004All-purpose flour This is medium-protein wheat

flour made from hard wheat or a combination of soft

and hard wheat. All-purpose flour is used mainly for a

wide range of household baked products. The color

and gluten characteristics are the important variables.

0005Patent flour This is the blend of flour streams from

the front head of the mill. It is low in ash and protein,

with a good color that is considered highest in

economical value. Various blends of flour are named

patent such as ‘top patent’ or ‘baker’s patent,’ etc.

They differ in the amount of clear or tail end streams

that they contain.

MILLING/Characteristics of Milled Products 3997

0006 Clear flour The portion of flour streams remaining

after patent flour has been taken off. Clear flour nor-

mally contains a higher amount of bran particles, ash,

and protein than patent and economically is second-

ary in value. The miller can produce more than one

clear by further dividing the tail end flour streams.

0007 Farina Regulations by the US Food and Drug Ad-

ministration (FDA) define farina as a coarse granular

separation of endosperm material extracted from

wheat other than durum wheat and red durum

wheat. Farina passes though a 841-mm (No. 20 US

Standard) sieve, but no more than 3% passes through

a 150-mm (No. 100 US Standard) sieve. It is separated

from the bran coat, or bran coat and germ, to such an

extent that the percentage of ash therein, calculated

to a moisture free basis, is no more than 0.6%. Its

moisture content is no more than 15%.

0008 Semolina Semolina is defined in a similar way to

farina, except that it is made only of durum wheat,

and its maximum dry ash content is 0.92%. Durum

wheat semolina is used for the production of pasta

products. Coarse durum semolina, preferably be-

tween 550 and 1100 mm is used for the production

of couscous, which is consumed mainly in North

Africa. Semolina extraction can be up to 72% of the

wheat kernel.

0009Durum flour Durum flour particles are such that

98% passes through a 212-mm (No. 70 USA) sieve.

It is separated from the bran coat, or bran coat and

germ, to such an extent that the percentage of ash

therein, calculated on a moisture-free basis, is no more

than 1.5%, and its moisture content is no more than

15%. ‘Durum patent flour’ is the re-ground semolina.

‘Durum first clear flour’ is Kaan, defined by the FDA

as the through of a 150 mm (No. 100 USA) sieve.

‘Durum second clear flour’ is made during the milling

of semolina, has a high amount of ash, is ‘specky,’ and

has a dull color. It is used in the manufacture of pet

foods. ‘Durum granulars’ is a combination of durum

Protein 11.0%

Ash 0.40%

Protein 11.5%

Ash 1.44%

Protein 10.7%

Ash 0.60%

Fat 0.88%

Fat 1.54%

Average particle size 50 µm

Color KJ 2

Protein 12.7%

Ash 0.75%

Fat 1.3%

Protein

13.8%

Protein 13.8%

Ash 6.1%

Ash 1.49%

Color KJ 1.5

Fat 6.6%

100 pounds (45 kg) of dry wheat

Varied flour extractions 74−78%

Farina 2−3% (of total flour)

Patent flour up to 80% (of total flour)

Baker's patent up to 97% (of total flour)

Ash 0.50%

Color KJ-1

Straight-grade flour 100%

Fiber 2.11%

Fat 0.87%

Fiber 10.1%

Starch 68.4%

Moisture 12.5%

Moisture 14%

Starch 56.28%

Starch 18.5%

Protein 13.1%

Ash 1.49%

Fat 2.29%

Fiber 2.9%

Starch 36.96%

Protein 22%

Ash 4.05%

Fat 6.6%

Fiber 3.55%

Starch 21.4%

Color KJ 0

1st clear flour up to 15%

(of total flour)

2nd clear flour 3%, protein 13.5%, ash 1.2%, fat 1.3%, color KJ 3

Total feed

Shorts 8%

Red dog 3%

Germ 1%

Bran

10%

Fat 3%

Fiber

10.1%

Starch

5.46%

Gain 1−2%

fig0001 Figure 1 Products from a typical wheat-milling system. Adapted from Posner ES (2000) Wheat. In: Kulp K and Ponte JG, Jr. (eds)

Handbook of Cereal Science and Technology. New York: Marcel Dekker, with permission.

3998 MILLING/Characteristics of Milled Products

semolina and durum flour containing no more than

15% durum flour. The ash content is normally about

0.72%. It is sometimes used as a substitute for durum

semolina.

0010 Atta flour This is high-extraction (92%) ground

material from wheat, and is used mainly in the Indian

subcontinent for baking chappati. The high degree of

extraction leads to a darker shade and a maximum

ash value of 2.0% (d.b.).

0011 Wholewheat flour Wholewheat flour, also named

‘Graham flour,’ is a product that includes the endo-

sperm, germ, and bran from cleaned kernels. The

FDA indicates that 90% of ground wholewheat

should pass through a 2.38-mm (No. 8 US Standard)

sieve and no less than 50% through an 841-mm (No.

20 US Standard) sieve. Bran particles in the meal

should pass through a 1.191-mm (3/64

) screen.

However, meal granulation is subject to customer

demand and depends on the grinding system. The

product is used in some cases in different ratios with

white flour. The bran particles in the meal interfere

with the gluten structure of the dough causing a re-

duction in baked volume. Usually, to achieve an ac-

ceptable baked product from wholewheat flour, vital

wheat gluten is added to improve the baking perform-

ance. The inclusion of the germ in the product limits

its storability period.

Byproduct Characteristics

0012 Mill products that do not coincide with flour are

considered by the miller as byproducts and are usu-

ally diverted to feed. The total byproducts from the

mill are named ‘Mill Run’ or ‘Wheatfeed.’ Different

combinations of mill byproduct exist in the trade and

vary in name and analysis. Proximate values of feed

products are shown in Table 1.

0013 Middlings These consist of fine particles of wheat

bran, wheat shorts, wheat germ, wheat flour, and

some of the offal from the tail of the mill.

0014 Bran Bran consists of coarse outer layers of the

wheat kernel. Large pieces of bran remain after the

flour has been extracted from the wheat.

0015Shorts Shorts is a mixture of fine bran, endosperm,

and some germ fragments that remain after flour

extraction has been completed and used for animal

feed.

0016Red dog This is a mixture of endosperm and bran

powder, taken from the tail of the mill, that has a high

ash and protein content, poor dress, and is dark in

color. It is a product intermediate between a low-

grade flour and feed.

0017Wheat germ products The germ of the wheat kernel

consists of two parts, the scutellum and the embryo.

The scutellum is softer (with a fat content of about

32%), and the embryo is relatively loosely attached to

the scutellum and covered with the pericarp. In gen-

eral, it is the embryo that is referred to in the trade as

wheat germ. The ‘wheat germ meal’ consists of

embryo together with some bran and shorts, and

contains at least 25% protein. ‘Pure wheat germ’ is

used for human consumption and contains at least

30% protein.

Effect of Wheat as the Raw Material

0018Flour quality depends about 75% on the quality of

the raw wheat and about 25% on mill design proced-

ures and adjustment. The different wheat kinds

vary in the level of protein characteristics, which

are mainly related to the quantity and quality of

insoluble protein that forms the gluten. Wheat

kernels vary in terms of the color of the pericarp.

Accordingly, white wheat has an advantage over

red wheat by allowing a higher flour extraction

with a comparable color. Figure 2 shows the relation

between wheat qualities of flours and their usage for

different end products.

0019The objective of the milling process is to reduce the

wheat kernel and deliver different products of con-

sistent quality to bakers or other end users. To achieve

this objective, the miller mixes wheat and blends final

flours with particular specifications in mind. The

reason is that wheat quality in commerce changes

continuously as a result of variety, soil, and climate.

Even in areas where only the local wheat is used by a

mill, the miller adjusts the quality by mixing.

tbl0001 Table 1 Proximate analysis of mill feed products

Product Minimum protein (%) Minimum fat (%) Maximum fiber (%) Maximum moisture (%) Maximum ash (%) kgm

3

Middlings 13.0 3.0 9.5 14.0 4.2 288–400

Bran 14.0 2.5 12.0 14.0 6.0 256

Shorts 13.0 3.0 7.0 14.0 4.1 160–256

Red dog 15.0 3.0 4.0 14.0 1.64–2.85 352–448

MILLING/Characteristics of Milled Products 3999

Definitions of Wheat Product Variables

0020 The following explanation of milled wheat product

variables that affect characteristics are arranged in

alphabetical order. Different official methods and in-

struments exist to measure flour and other mill prod-

ucts. Flour analysis allows an approximation of the

determined values. Flours with the same analysis have

been found to show different baking characteristics or

end-use performance. The interaction of a few or

more characteristics, rather than one alone, affects

the end-use qualities. (See Wheat: Grain Structure of

Wheat and Wheat-based Products.)

Ash

0021 Ash, or the mineral content of the wheat or its product,

is determined by incinerating the sample and express-

ing the residue as a percentage of the original sample.

The ash content of the wholewheat kernel depends on

the wheat variety as well as the soil, irrigating water,

and growing conditions. However, erroneously, flour

ash is considered as a grading factor by some in the

industry. A better expression of milling performance is

where the whole kernel ash is considered. Accordingly,

ash values of the wheat kernel products should not be

taken as absolute numbers but expressed as a ratio of

flour ash divided by wheat ash (FA/WA).

0022 Where ash is considered as grading factor types

405, 550, 630, 812, and 1050, the ash range for

each grade is quite wide. For example, flour could

be considered as type 550 when the flour’s ash con-

tent is between 0.490 and 0.580% (d.b.). Under regu-

lations in the USA, the ash content added to flours as

iron, or salts of irons, or calcium is excluded in

calculating the ash of the shipped flour.

Baking Quality

0023The baking quality requirement of flour is a function

of specific baking formulas, the baking process, and

the final product requirements. Accordingly, millers

adjust the flour characteristics to accommodate cus-

tomers’ specifications. The miller and the bakers test

the baking qualities of flours in the laboratory. A

scoring method should be developed in the laboratory

to evaluate baked bread objectively.

Color

0024Flour color is one of the main production challenges

to the miller, because it is directly related to the shade

of the final baked product. Flour color evaluation is

performed by the Kent Jones and Martin Color

Grader, the comparison at a certain wavelength of

light or the Tristimulus system and the image analysis

of bran particles. All have their advantages and limi-

tations. However, the miller usually uses the method

that is acceptable to the majority of customers. The

operative miller compares the flours every hour

against an accepted standard. This test is the ‘Pekar

Wholewheat bread

Hearth bread

Pasta

White bread

Noodles

All-purpose flour

Doughnuts

Crackers

Cake

Products

Protein (%)

Soft wheat

Hard winter

Soft

Hardness scale

Hard spring

Durum

Hard

7 8 9 101112131415

fig0002 Figure 2 Schematic diagram of relationship between percentages of wheat protein, wheat type, hardness, and end-product utiliza-

tion. Adapted from Posner ES (2000) Wheat. In: Kulp K and Ponte JG, Jr. (eds) Handbook of Cereal Science and Technology. New York:

Marcel Dekker, with permission.

4000 MILLING/Characteristics of Milled Products

test,’ in which a flattened portion of flour is wet and

dried to exaggerate the color differences.

Falling Number

0025 The falling number value indicates the level of

a-amylase activity in wheat and flours. Sprouting

of wheat in a wet harvest period leads to a high

a-amylase activity and, consequently, a low falling

number. Depending on the amount of wheat germin-

ation, it weakens the gluten, shortens dough fermen-

tation, and increases flour acidity and ash. Upon

germination, the level of a-amylase enzyme activity

increases many-fold. The enzyme breaks down large

starch molecules and thereby reduces the starch

viscosity. Germination increases the diastatic activity

and sugar production during the baking process. Ac-

cordingly, flours that lack gassing power could show

an improvement with some sprouted wheat in the mill

mix. Testing instruments such as the falling number

and amylograph are used to measure the relative

viscosity of the starch in flour and, accordingly, the

level of damage to the wheat.

Fragment Count

0026 The actual number of particles of insect origin, re-

gardless of size, in 50 g of flour or meal is determined

by using a filth test method and by a microscopic

count. FDA regulations for insect and rodent

filth, consider as defective 75 or more insect frag-

ments and one or more rodent hairs per 50 g of

wheat flour.

Lipids

0027 The lipid content of wheat flour depends on the rate

of extraction. The lipid content ranges from about

0.88% in 60% flour extraction to about 2.5% in

whole meal product. Lipids originate during the

milling process mainly from germ particles that end

up in the final flour. The oxidative deterioration of

lipids is the factor limiting the prolonged storage of

flour. (See Fats: Classification.)

Moisture

0028 Moisture in mill products depends on variables such

as the moisture content of the raw wheat, the condi-

tioning process before milling, ambient conditions in

the mill, the design of the milling process, and mill

adjustment.

0029 In general, there is an optimal range of moisture

content in the wheat before milling to achieve optimal

physical attributes such as kernel hardness for

the process, milling performance, and final product

moisture. The miller achieves this by conditioning

of the wheat, which is also the main stage where

physical and chemical changes occur in the raw

material and, subsequently, the characteristics of end

products.

0030All mill product values such as ash and protein

content should be corrected and expressed on a

fixed moisture basis that differs in different parts of

the world. For example, in the USA, flour values

are corrected to a 14% moisture basis, and in others,

to a dry basis. Flour is corrected to that basis because

at an average relative humidity of 70% the amount

of 14% moisture in flour is in equilibrium. In add-

ition, at about 14%, no significant chemical changes

or proliferation of microflora would occur in stored

flour.

Flour Particle-Size Distribution

0031The particle size of flour produced by a commercial

mill ranges between 1 and 200 mm. Flour produced

from soft wheat is finer, whereas that from hard

wheat is coarser. The flour particle-size distribution

is directly related to water absorption, during dough

preparation, and baking performance. The coarsest

fraction in flour is positively related to water absorp-

tion, loaf weight, and crumb quality. Finer fractions

from any given flour absorb water more slowly, pro-

duce a sticky sponge, and show an inferior baking

quality. It is the miller’s objective to optimize the flour

particle size distribution to accommodate end-use

qualities.

0032Whereas the flour particle-size distribution is

within a known range, durum semolina is a wheat

product that is produced within significantly different

particle size ranges. Depending on variables in the

pasta production system, including semolina wetting,

extrusion, and drying, an optimum particle size

distribution is required.

0033The pH levels of flour effect their baking perform-

ance, especially those from soft wheat used for cake

baking. The optimum pH levels of some flours are as

follows: bleached white bread flour, 5.7–5.9; cake

flour, 4.9–5.8; cookie and pie, 4.9–5.8.

Protein

0034Mill product protein content is directly related to that

of the wheat kernel protein. Accordingly, dividing the

straight-grade flour into different products would

result in less protein in the combination of head

streams (patent) and more protein in the combination

of tail-end flour streams (clear). The protein content

of flour is the amount of nitrogen determined, multi-

plied by a factor of 5.7. A factor of 6.25 is used for

wheat bran.

MILLING/Characteristics of Milled Products 4001

0035 The gluten that amounts to about 85% of the total

protein of white flour is made up of two components,

glutenin and gliadin, which are normally present in

roughly equal amounts. Gliadin is a soft, sticky sub-

stance that acts as a bond for the less coherent, but

harder, glutenin. Gluten is insoluble in cold water and

may be recovered from a weighted flour sample. One-

third of the weight of wet gluten approximates the

protein content of flour. Excessive temperatures

during the wheat handling and grinding (above



C) would affect the gluten characteristics and, as

a result, the baking quality of the flour. Accordingly,

millers are sensitive to any rise in grinding roll tem-

perature and, in some designs, can cool them with

internally flowing water.

Rheological Characteristics

0036 Instruments that record rheological properties of

dough made from a flour determine its development

process, rising ability, elasticity, and the quality of the

baked loaf of bread. The instruments generate curves

that allow the quality to be interpreted from the shape

of the curves.

Starch Damage

0037 Mechanically damaged starch granules are suscep-

tible to enzyme action. Starch damage at a desired

level of flour is related to the effective water absorp-

tion and is reflected in an increase in the maltose

value that is responsible for the gassing power during

the baking process. To produce acceptable bread, an

optimum relation should be maintained between the

flour protein content and the level of starch damage.

(See Starch: Functional Properties.)

Water Absorption

0038 The weight of water per weight of flour, expressed as

a percentage, is used to make dough with an optimal

consistency. Various instruments are used to measure

this characteristic of flour. Variation in the flour par-

ticle size distribution, protein, and starch damage

content accounts positively for changes in the water

absorption of flours. Damaged starch absorbs 100%

of its weight in water. Undamaged starch absorbs

only 30% of its weight in water.

Effect of the Mill Design and Adjustment

on Product Characteristics

0039 The design of the mill flow has a decisive effect on

product characteristics. Mill design relates to sieve-

cover aperture, the characteristics of grinding stages,

and the direction of intermediate material flow. Long

or a short mill diagram is characterized by the specific

machine allocations for the milling process. It is based

on a fixed quantity (100 kg of wheat) processed in

24 h. The longer mill flow that contains a larger

number of grinding stages allows a higher degree of

flour extraction and a relatively lower total amount

of cumulative ash content in all flours. This is caused

by a relatively less severe action at each of the grind-

ing stages.

Starch Damage Effect

0040Setting of the grinding rolls, pressure exerted between

them, the rate of feed to the rolls, the surface of

the rolls, and the available sifting area in the mill

affect the level of starch damage in the flour. In

general, the smooth rolls in the mill that reduce

the chunks of endosperm particles to fine flour are

responsible for the main starch damage. Smooth rolls

differ in their surface characteristics; they can be

shiny smooth or matt. Higher starch damage levels

can be achieved with matt surfaces and a greater

differential between the grinding rolls. Rolls are

more effective in generating damage to starch in

flour that impact grinding.

Blending of Flour Stream in the Mill

0041The professional miller is familiar with the quantity

and quality of the individual mill flour streams and

blends them to generate the resulted final flours.

Millers use different methods to achieve desired

final commercial flours.

0042One method is to combine all the flour streams

while grinding a certain wheat mix and subse-

quently make some adjustments in quality before

shipment by blending flours from different wheat

mixes.

0043A second method would be to divide the flour

streams into two or three collecting conveyors.

Those would be the final flours specified as patent,

first clear, etc. Some blending would occur in the

flour handling system before shipment.

0044The third method, named divide or split milling,

would be to group flour streams into ‘groups’ of

certain quality and blend those at the end of the

process at different ratios, to generate required

qualities.

0045A flour stream grouping in a typical spring wheat

mill processing 13.7% protein wheat is shown in

Table 2. The estimated percentage of each of the

individual flour streams and their diversity vary

from one mill to another.

0046The three main groups of flour streams are shown

in Figure 3, which also shows the groups combined to

produce the commercial grades.

4002 MILLING/Characteristics of Milled Products

Product Handling

0047 Depending on the regulations and customer require-

ments, mill products are treated before shipment by

the addition of different additives at specified levels,

moisture adjustment, air classification, or modifica-

tion by heat.

Nutritional Enrichment

0048Since 1940, all white wheat flour or semolina in the

USA has been supplemented with vitamins and essen-

tial minerals that are natural to the wholewheat but

are lost during the milling process (Table 3). Good

manufacturing practices should insure that the

required levels of added vitamins and minerals are

maintained throughout the expected shelf-life of the

food under customary conditions of distribution and

storage. Prepared premixes are available that contain

all the vitamins with added requirements of folic acid,

vitamin A, calcium, and others.

Flour Improvers

0049Flour aging Following the milling process, the flour

goes through an ‘aging’ or maturation process. Aging

enhances the gas-retention properties of the gluten by

oxidizing, among others, the –SH groups. In countries

where additives are not permitted, millers store flours

up to 12 days before shipment to allow the process to

take place. Where permitted, oxidizing agents such as

ascorbic acid (vitamin C), potassium bromate, azo-

dicarbonamide, and chlorine dioxide are used to

improve the bread-making qualities (Table 3).

0050Flour bleaching The yellowish shade of freshly

milled flour is the result of the carotene present.

Bleaching agents are added, where permitted, in the

tbl0002 Table 2 Flour streams grouping in mill grinding 13.7% protein

spring wheat

Stream Flour (%) Ash (%) Protein (%

)

1 M 35.0 0.37 11.9

2 M 12.0 0.39 11.8

3 M 8.0 0.39 11.6

Group I 55.0 0.38 11.8

2 Q 4.0 0.50 12.2

4 M 5.0 0.60 12.5

1 BK 5.0 0.58 14.6

2 BK 7.0 0.54 15.4

3 BK 4.5 0.68 18.0

Break redust 6.5 0.58 14.8

Group II 32.0 0.58 14.7

1 T 1.0 0.75 12.5

5 M 3.0 0.60 12.8

Exhaust 2.5 0.65 14.8

4 BK 2.5 0.95 18.5

Bran duster 1.0 1.20 18.7

Group III 10.0 0.77 15.2

6 M 1.0 1.30 13.2

7 M 0.5 1.50 13.0

Short duster 1.5 1.40 16.5

Low Grade 3.0 1.38 14.8

Adapted from Panter A (1988) Divide milling of Canadian spring wheat

flour. Association of Operative Millers Technical Bulletin December: 5347–

5353, with permission.

Group I

55% flour

0.38% ash

11.8% protein

Group II

32% flour

0.58% ash

14.7% protein

Group III

10% flour

0.77% ash

15.2% protein

Group IV

3% flour

1.38% ash

14.8% protein

10% 10%

45% flour

Top patent

0.38% ash

11.8% protein

52% flour

Fancy clear

0.58% ash

14.2% protein

3% flour

Low grade

1.38% ash

14.8% protein

0% flour

fig0003 Figure 3 Schematic diagram of combining flour groups. Adapted from Panter A (1988) Divide milling of Canadian spring wheat flour.

Association of Operative MillersTechnical Bulletin December: 5347–5353, with permission.

MILLING/Characteristics of Milled Products 4003