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

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pure endosperm, which flow through the sieves, and

the lighter ones of higher bran content, which tail

over the sieves. The particles of pure endosperm will

go through the sieves at the head-end of the machine,

whereas the lighter ones will float over. Relatively

pure endosperm particles of the head-end are sent to

the reduction rollers for maximum production of

flour. Tailover materials of the sieves are directed to

sizing rollers or tail-end fine breaks.

0018 Basically, the different stages of the process are

grinding, sifting, separation, and regrinding, which

are repeated again and again until the endosperm is

reduced to an acceptable degree of fineness and no

more flour can be separated from the bran (red dog

fraction).

0019 Each flour stream extracted under the sifter is of

different quality, based on its origin within the kernel,

equipment adjustment, and the stage in the milling

process. The collected flours from under the sifter

sections are mixed while transferred in screw convey-

ors (a screw conveyor uses a continuous helical

attached to a drive shaft which runs the length of

the conveyor) and sent to storage bins. Starting

under the sifter, the miller is in position to blend

those flour streams to the desired quality.

0020 The miller also produces granular materials of rela-

tively clean endosperm, which are coarser than flour.

This type of product is called farina, when originated

from hard or soft wheat, or semolina, when origin-

ated from durum wheat.

Experimental Milling

0021Experimental milling is an approach to determine

milling quality of the wheat. The miller uses a small-

scale system with which different settings can be tested

to optimize the processing of the wheat in question.

Results should reveal to the miller the relative

expected performance of the wheat on the commer-

cial mill. With the experimental milling system, the

miller is in a position to learn about the separation of

the bran, the disintegration of the endosperm, and

characteristic qualities and quantities of the inter-

mediate and final products from the wheat sample.

0022A milling system flowsheet, whether experimental

or of a commercial unit, is the ‘road map’ of

the process. The experimental milling flowsheet

(Figure 5) describes the milling by indicating how,

where, and what materials are distributed to the

different machines and stages of the process. In a

flowsheet, grinding rollers are designated by two

adjacent circles. Some pairs are striped and some

are clear, indicating corrugated and smooth rollers,

respectively. A summary of roller characteristics in

an experimental milling system is shown in Table 2.

In the experimental milling process, after each

432 µ

467 µ

478 µ

500 µ

400 µ

432 µ 467 µ 478 µ

375 µ

400 µ

1 Sizf

(b)(a)

1 M

1 Sizc

432 µ 467 µ

3 BKf

1 Sizc

Material entrance:

1st BK 530−300 µ

2nd BK 530−300 µ

Air suction

50 mm WG

50 m

min

−1

fig0004 Figure 4 Schematic view of a purifier: (a) machine cross-section; (b) sieve arrangement. WG, water gauge; BK, break; M, middling;

Sizc, coarse sizing; Sizf, fine sizing.

3984 MILLING/Principles Of Milling

grinding stage, the material is sifted as a batch on a

small-size sifter which includes a stack of sieves of

different apertures.

Flour Handling and Control

0023 Flour quality control is a very important function of

the milling process. Flour color, starch damage,

gluten denaturation, and particle size distribution

can be affected by an uncontrolled milling operation.

The quality of flour, if it is not affected by the milling

process, is largely dependent on the quality of wheat

used for grinding. For example, softer-structured

wheat kernels with a lower protein level will generate

less starch damage in the flour than vitreous kernels

with higher protein content. The mill laboratory rou-

tinely checks different characteristics of raw materials

and finished products to determine whether or not

first, they meet specifications and second, there is any

need to take appropriate measures in wheat blending

or mill adjustments. The final adjustment of flour

quality to customer needs is usually performed by

the miller by blending different flours before ship-

ping. In modern operations, batch-type mixers blend

3–5 t of different flours with optional additives in

a few minutes before they are pneumatically trans-

ported to the customer’s bulk truck. (See Flour: An-

alysis of Wheat Flours.)

0024To measure mill efficiency and control perform-

ance, the miller uses a variable called ash. Ash is

the completely burned remains from a flour

sample, or any other material from the process

burned completely and expressed in percentages of

the original weight. The pericarp and the aleurone

layer, which technically end up in the bran, contain

about six times more minerals than the inner endo-

sperm. The percentage of ash residue from an inter-

mediate material in the mill or the final flour will

indicate the efficiency in separating the bran and

germ from the endosperm. A comparison of flours

cannot be based on ash if the flours are not milled

from the same wheat.

0025In contrast to ash, which is mainly a parameter of

the mill’s technical efficiency, indicating the amount

of bran in flour, color is a quality parameter, which is

significant to the flour end user. Accurate wheat

selection, blending, cleaning, conditioning, and mill

Clean

wheat

SIZ

Germ

20 W

50 GG

10 XX

50 GG

70 GG

50 GG

10 XX

50 GG 10 XX 10 XX

10 XX

70 GG

10 XX

FL FL FL F FL FL FL

1 M 1 T 2 M 3 M

4 M 5 M

1 BK

20 W

50 GG

70 GG

10 XX

FL FL FL FL

Bran

SHORTS

FL FL

20 W

50 GG

70 GG

10 XX

20 W

50 GG

70 GG

10 XX

20 W

50 GG

70 GG

10 XX

70 GG

10 XX 10 XX

Red

dog

2 BK 3 BK 4 BK 2 T LG

fig0005 Figure 5 Experimental milling flowsheet. BK, break; SIZ, sizing; T, tailings; M, middlings or reductions; LG, low grade; FL, flour;

W, metal wire sieving surface; GG, grits gauze sieving cloth; XX, double extra-quality sieving cloth.

tbl0002 Table 2 Experimental milling rolls surface characteristics

Stage Corrugations (cm) Spiral (%) Differential Action

1st Break 4 8 2.5:1 D:D

2nd Break 5 10 2.5:1 D:D

3rd Break 7 11 2.5:1 D:D

4th Break 8 12 2.5:1 D:D

5th Break 11 14 2.5:1 D:D

Reductions Smooth 1.5:1

D, dull edge of a corrugation.

MILLING/Principles Of Milling 3985

adjustment all affect the resulting color and baking

characteristics of the flour.

0026 Flours and other mill byproducts can be mechanic-

ally modified after the milling process to accentuate

certain characteristics or qualities. One of those

systems is air classification of flour. In the case of

flour particle sizes ranging between 1 and 200 mm,

sieving can be accomplished with sieves as fine as

75 mm. Below that size, the efficiency of sieving with

a sieve becomes too low. It is of interest to the miller,

in some cases, to separate endosperm particles below

the range of 75 mm. It was determined that broken-

down endosperm would end up with the fraction in

the range of 1–17 mm, which contains a major part of

the protein matrix in the endosperm. Particles in the

range of 17–37 mm will contain different parts but,

predominantly, whole starch granules. The usage of

air to segregate between particles of endosperm

below 75 mm is used in the flour-milling industry for

two purposes – protein shifting and narrowing the

range of particle size. The principles used in air clas-

sification are based on the differences in size, shape,

and weight of particles, and the characteristic ter-

minal or critical air velocities to segregate between

them. Regrinding of flours on pin mills, before air

classifying, separates more starch granules from the

protein matrix and improves the efficiency of the

system. (See Air Classification: Principles of Oper-

ation; Uses in the Food Industry.)

Important Landmarks in the Industry

0027 The ancient milling process was very simple. Wheat

was crushed between two stones, and then the peri-

carp was sifted out to a limited extent using reels. The

reel is a rotating cylinder sieve, and a counterrotating

internal rotor throws the product against the sieve to

force the fine fraction through, while the coarse

fraction tails over in the end. The same principle is

currently applied in high performance sieves for hard-

to-sieve materials in modern flour-milling systems.

However, stone mills and reels are still assembled

into commercial wheat mills to accommodate specific

quality and taste demands.

0028 Currently, machines are being built at a much

better technical level than during the first half of the

twentieth century. Up to four pairs of rollers are

installed in one machine. High-precision bearings,

better adjustable mechanisms, and higher velocities

currently characterize mill machinery. The result

has been a significant reduction in total roll length

in the mill. Water cooling of the rollers has given the

miller more flexibility in manufacturing desired final

products.

0029In the past, silk screens were used to cover the sieve

frames. Advances in synthetic threads replaced the

silk with synthetic material inert to environmental

conditions and with more stable sieve apertures.

0030In the 1980s, mill automation became the main

concern of the industry, for control of the process as

well as online monitoring of mill products.

0031During the 1990s a new approach to wheat flour

milling technology was established in commercial

mills, abrading and polishing off the bran layers

before the grinding process. This approach causes a

significant reduction in the number of grinding stages

and roller length in the mill. The debranning process

is responsible for an improvement in flour extraction

and quality. The changes in flour quality can be at-

tributed to the heat generated during the debranning

stage of conditioned wheat.

0032One of the important mechanical means used in the

process of flour milling is air as a tool in the process.

In the modern milling industry, almost 10 times more

air than wheat is moved through the operation. Pres-

sure differences created by fans or blowers in pipes

between two points and the movement of an air

quantity at a certain velocity make it possibile to

convey materials by air. The mill, which includes a

large number of machines assembled on several

floors, is an industrial unit in which intermediate

products flow by gravity or are conveyed by air. Air

also has a positive effect on the efficiency of machine

performance and sanitation. Water evaporation off

surfaces of particles improves sieving of the inter-

mediate stocks in the mill.

See also: Air Classification: Principles of Operation;

Uses in the Food Industry; Controlled-atmosphere

Storage: Applications for Bulk Storage of Foodstuffs;

Flour: Roller Milling Operations; Analysis of Wheat

Flours; Insect Pests: Problems Caused by Insects and

Mites; Wheat: The Crop

Further Reading

Kent NL (1966) Technology of Cereals. London: Pergamon

Press.

Posner ES and Deyoe CW (1986) Changes in milling prop-

erties of newly harvested hard wheat during storage.

Cereal Chemistry 63: 451–456.

Posner ES and Hibbs AN (1997) Wheat Flour Milling.

St Paul, MN: American Association of Cereal Chemists.

Wingfield J (1989) Dictionary of Milling Terms and Equip-

ment. Kansas City: Association of Operative Millers.

3986 MILLING/Principles Of Milling

Types of Mill and Their Uses

F Meuser, Technical University of Berlin, Berlin,

Germany

This article is reproduced from Encyclopaedia of Food Science,

Milling and Grinding

0001 The term ‘milling’ covers a wide range of processes

and businesses, including primarily the methods of

processing cereal grains to flours and any other pro-

cessing of metal, cloth, etc., to end products of the

most diverse nature in plants or factories. The plants

are very often described as mills. In the context of this

article, the term ‘milling’ will mainly be limited to

milling operations in the food industry. Emphasis will

be laid on the milling of cereal grains and the grinding

procedures used for this purpose. For this reason, it

is necessary to differentiate between milling and

grinding. Milling includes all operations which lead

to defined end products, and grinding covers those

processes which bring about a particle size reduction.

0002 The grinding of cereal grains can be traced back to

ancient times and is linked with some of the most

important advances made by humankind which

came about through the development of tools for this

process. It all started with the use of stones to crush

cereal grains (Figure 1). This type of crushing led to the

development of the pestle and mortar, and other

stamping tools. The next stage of development was

to grind the grains under pressure between two stone

slabs. This technique was the starting point for all

modern milling processes as it opened up the possibil-

ity of being able to grind the various components

selectively of the cereal grain. It had, of course, always

been possible to use a sieve to separate the ground

material from a pestle and mortar or stamping tool

into a fine fraction containing mainly endosperm

and a coarse fraction consisting principally of husk

and bran. However, it was possible to achieve separ-

ation far more effectively for material obtained from

grinding stones. The main disadvantage of oscillatorily

moved grinding stones was that they could only be

operated manually. The invention of the centrally

holed grinding stone made it possible for the oscilla-

tory grinding motion to be replaced by a circular one.

This development meant that (1) the manual grinding

performance could be increased considerably, and (2)

the required work effort could be obtained at first

from animals and later from wind and water energy.

0003The advantages of this development were that

grinding could take place at a constant speed owing

to the rotary motion of the stones, and that the latter

provided a larger active surface area when compared

with that of grinding stones. From the rotating grind-

ing stone, the grinding cone was evolved, enabling the

size of the grinding surface to be increased in com-

parison with that of the rotating grinding stones.

However, flat milling stones again took the place of

cone mills after the advent of power transmission by

cogwheels, as these enabled wind and water energy to

be used to drive the grinding stones. The resulting

increase in available driving power was used mainly

to boost the speed at which the grinding stones

turned, thus improving the milling performance.

0004Grinding stones driven by wind and water power

remained the most important means of grinding until

the invention of the steam engine enabled the per-

formance to be increased once more. This meant

that grinding rollers (rolls), which had been invented

centuries previously, could now be used for milling

grain. The steam engine enabled two rollers to turn in

opposite directions at speeds which – when the gap

between them was multiplied by the distance covered

by the surface of the roller – resulted in an acting

surface comparable with that of a grinding stone.

Grinding rollers had the advantage that they could

be set according to the fineness of grinding required.

This was a development of considerable importance

when compared with the rather random degree of

fineness obtained by using grinding stones, as the

(a) (b)

fig0001 Figure 1 Historical development of grinding grain. (a) grinding

slabs; (b) grinding stones; (c) grinding cones; (d) grinding rollers.

Reproduced from Milling: Types of Mill and Their Uses, Encyclo-

paedia of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

MILLING/Types of Mill and Their Uses 3987

grinding process could then be subdivided into any

number of stages by altering the size of the gap

between the rollers. Thus it became possible to com-

bine grinding and grading stages in such a way that

the endosperm could be separated almost entirely

from the bran and the germ.

0005 The operations described above, by which flour,

the end product, is obtained and which consists of

grinding cereal grain step by step, with interposed

grading and sifting stages, are referred to as milling

operations. These have assumed various forms as

milling, sieving, and sifting technology has developed

over the past 100 years. As the methods of sieving and

sifting are described elsewhere in the Encyclopedia,

only grinding – the theory behind it and the most

important types of grinders (mills) used in the food

industry – will be dealt with here. (See Flour: Roller

Milling Operations.)

Grinding

Purpose and Aim

0006 The purpose of grinding is to reduce a mass to a

number of parts by the action of mechanical forces

(energy). In the simplest case, the sole aim of the

grinding actions is to achieve a size reduction. This

applies, for example, to the grinding of masses which

are homogeneous with regard to material and struc-

ture. The aim is to reduce the diameter of the mass

concerned and increase its surface area. Size reduc-

tions can be carried out on solids (cereal grain, sugar,

cocoa, spices), liquids (oil droplets in water), and

gases (gas bubbles in liquids).

0007 Grinding solid masses which are inhomogeneous in

material and structure can result in a number of

particles and groups of particles, the composition of

which may differ considerably from the average com-

position of the original mass. One of the most import-

ant aims of grinding is to obtain particles of different

sizes which can then be separated selectively by means

of grading, sedimentation, and sifting. Essentially solid

materials (cereal grain, oil seeds, spices) are first ground

in preparation for these processing stages. It is the

grinding of solid materials which will be dealt with in

this article. (See Ground Nut Oil; Palm Oil; Spices and

Flavoring (Flavouring) Crops: Fruits and Seeds; Leaf

and Floral Structures; Tubers and Roots; Vegetable

Oils: Oil Production and Processing.)

Range of Materials

0008 The range of solid materials which may be ground

covers both hard and soft materials (Table 1). In the

food industry, it is mainly products up to 4 on the

hardness scale which are subjected to grinding.

0009Grinding may also be broken down roughly into

classes according to the particle size of the ground

material (Table 2). Thus it is possible to distinguish

among coarse crushing, fine crushing, coarse grind-

ing, fine grinding, ultrafine grinding, and colloidal

grinding. In relation to the volume per unit weight,

the surface area of the particles increases as their

diameter decreases when ground.

0010Grinding can take place either in a gaseous medium

(air, gas, steam) or a liquid one (water, oil), giving

rise to the terms dry and wet grinding (or milling).

The type of grinding is selected in accordance with

the desired result. Dry grinding is always preferable

where dry end products (flour, cocoa, sugar) are

required and where the products have to remain prac-

tically unaffected by the medium. Where grinding

aims to separate out intermediate products, dry grind-

ing has the disadvantage that it is impossible to set

very high standards for the purity of the individual

particles and groups of particles. In this case, wet

grinding can meet far more stringent requirements

as the medium can be used to refine the particles.

(See Cocoa: Production, Products, and Use; Sugar:

Refining of Sugarbeet and Sugarcane.)

Actions on the Material

0011The grinding process is influenced not only by the

hardness of the material but also by its toughness,

tbl0001Table 1 Grinding classifications according to the hardness of

the material to be ground

Hardness (Mohs scale) Material

6–10 Quartz, cement

3–4 Salts, coal

< 3 Cereal grains, foodstuffs

Gypsum

Data from Zogg M (1987) Einf

hrung in die mechanische Verfahrenstechnik,

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

tbl0002Table 2 Grinding classifications according to the particle size

of the ground material

Ty p e o f gr in d in g Par t i c le s i z e

of product (mm)

Surface area (m

)

of a mass of spherical

particles witha volume of 1m

Crushing

Coarse > 50 < 120

Fine 5–50 120–1200

Grinding

Coarse 0.5–5 1200–1.2 10

Fine 0.05–0.5 1.2 10

–1.2 10

Ultrafine 0.005–0.05 1.2 10

–1.2 10

Colloidal < 0.005 > 1.2 10

Data from Zogg M (1987) Einf

hrung in die mechanische Verfahrenstechnik,

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

3988 MILLING/Types of Mill and Their Uses

inner tension, and surface structure. In addition to

this, the heat capacity and conductivity of the

material, the manner in which they vary according

to temperature, and the type of mechanical action

also affect grinding.

0012 The material may be ground by subjecting it to

various types of action. According to Rumpf, there

are eight types of action which can be divided into

four different groups (Figure 2):

0013 Actions occurring between solid surfaces (com-

pression, friction, shear stress, cutting) which can

be either the surfaces of the grinding tools or those

of an adjacent body. In both cases, the extent of

the actions is limited either by the maximum

amount of force which can be applied, or by the

smallest gap which can be set between the surfaces

of the grinding tools.

0014Actions occurring on contact with a single solid

surface, which could be either that of a tool or of

another particle. In this case, the actions are

limited by the kinetic energy of the movement of

the surfaces in relation to each other.

0015Action of the surrounding medium which results

from the physical state of the medium, or from

changes which occur within it.

0016Application of nonmechanical energy (electrical

discharges). This type of action will not be given

further consideration below.

The various types of action are frequently combined

during grinding operations carried out using grind-

ing machinery. Many grinding machines are even

designed to permit the combined application of sev-

eral types of action. The best example of a machine of

this type is the roller mill, in which compression and

friction, as well as shear and cutting forces, are used

in grinding.

0017The various types of action are combined in order

to obtain the best possible grinding results. The aim is

frequently to obtain a particular grinding result.

Apart from the physical properties of the material

and the type of action to which it is subjected, the

degree of grinding also depends on the intensity of

the action, which is limited by the type of grinding

tool and the capacity of the material to absorb energy.

Grinding Result

0018The grinding result is a collection of particles which

differ in size, shape, and surface structure. It is these

distinctive features which provide the basis upon

which the grinding result is assessed. They are deter-

mined by a variety of methods, of which those used to

measure the particle size are of considerable import-

ance. This may be determined as a function of the

particular size range using various methods of analy-

sis, for which special instruments exist (e.g., oscillat-

ing screen, air jet screen, sedimentation scales, optical

particle spectrometer, laser diffraction spectrometer,

image analyzer).

0019The methods of analysis yield different results for a

single grinding result depending on the measuring

technique used. Consequently, the assessment of the

grinding result is affected by, amongst other things,

the method by which particle size and particle size

distribution are determined. Thus the choice of a

particular method is always a compromise which

must take account of the technical feasibility of the

analyses on the one hand, and the conclusions to be

drawn from the results on the other hand.

0020The means of measuring, e.g., in the case of the

oscillating screen, are frequently smaller versions of

actual sieving machines, so that it is possible to assess

Compression between

surfaces

Impact on a surface

Impact of particles on one

another

Cutting action of surfaces

Shear stress between

surfaces

Action of the

surrounding medium

Action of friction

Action of nonmechanical

energy

P, V

fig0002 Figure 2 Types of grinding actions (according to Rumpf H

(1954) Die Zerkleinerung unter besonderer Beru

cksichtigung

lebensmitteltechnologischer Fragen I: Aufgabenstellungen.

Zerkleinerungsaufgaben in der Lebensmittelindustrie. Fette, Sei-

fen, Anstrichmittel 56: 404–408.

MILLING/Types of Mill and Their Uses 3989

the grinding results solely by analogy. In this case, the

particle sizes are expressed in terms of a single linear

dimension. The specific surface area of the material to

be ground must, however, be determined where infor-

mation on it is required for the purpose of calculating

the energy needed for grinding, or in order to obtain

data on those physical properties of the particles which

are related to size and surface structure. It is usually

obtained after first determining the particle size and

is generally an approximate value. The calculation

includes shape factors determined in separate tests.

Graphical Representation of Particle Size

Distributions

0021 Ground materials, such as flours, are frequently com-

posed of particles of various sizes which cover a range

of values extending over several orders of magnitude.

The frequency with which particles of any one par-

ticular size occur as a result of grinding is random

and follows no universally applicable distribution

law. However, it is possible to describe the particle

size distributions of many ground materials by means

of empirically established function equations which

provide a reasonable match with experimental size

distribution data. A frequently used volume (mass)

distribution function is the Rosin–Rammler–Sperling–

Bennet (RRSB) function which describes skewed

distributions of particle sizes, especially those of

finely ground materials and dusts where significant

quantities of particles exist above and below a

predominant size.

0022 The RRSB function may be expressed as follows:

FdðÞ¼1  RdðÞ¼1  exp  d=d

ðÞ



ð1Þ

In this equation, F(d) is the cumulative weight of

particles passing size d, and R(d) the cumulative

weight of particles coarser than size d. The equation

can be inverted as follows:

log log 1= 1  FdðÞ½

¼ n log d  n log d

þ log log e

¼ n log d þ const: ð2Þ

When this equation is applicable, a straight line, the

RRSB line, results if log log½1=ð1  FÞ is plotted

versus log d. The RRSB line is determined by

the slope n (spread parameter) and the abscissa d

of the point at which d ¼d

. Where d ¼d

FðdÞ¼1  e

1

¼ 0:632 so that d

(position param-

eter) is the diameter of either the throughput of

0.632 ¼63.2% or the residue of 0.368 ¼36.8%.

The diameter is to be found at a point vertically

below that at which the RRSB line intersects the

parallel line at a distance F ¼(0.632) from the ab-

scissa. The slope of the RRSB line can be read from

the specially devised scale at the side of the grid by

parallel displacement of the line through the pole of

the system of coordinates (Figure 3).

Determination of the Specific Surface

Area of Particle Size Distributions

0023The graphical representation of particle size distribu-

tions, such as those according to the RRSB function,

can also be used to obtain an approximate value

of the specific surface area. To illustrate this, an

approximation formula was developed for spherical

particles:

¼ 6:39=r d

ðÞexp 1:795=n



1



ð3Þ

1

) is the surface area as a function of the

mass, and r (kg m

3

) is the density of the mass.

0024As the specific surface area of a mass of spherical

particles is, apart from the density, dependent only

upon the spread parameter (n) which characterizes

the RRSB line and the position parameter (d

), it is

possible to place a scale at the side of the RRSB grid

with which the specific surface area can be calculated

as a function of the volume.

0025In Figure 3, the dimensionless characteristic value

/f is given in the scale at the side of the grid. In

this characteristic value, f represents a form factor

with which the deviation of the particles from the

spherical form is taken into account. Nonspherical

particles have larger surface areas than spherical ones

and the form factors of these are therefore greater

than 1. For example, the form factors for ground

wheat, rye, barley, and oats are 1.2, 1.3, 1.5, and

1.8 respectively. (See Barley; Oats; Rye; Wheat: The

Crop; Grain Structure of Wheat and Wheat-based

Products.)

0026Using the outer scale at the side of the grid, the

surface area is calculated by moving the RRSB line

from the position parameter d

to the pole, keeping it

parallel to its original position. The point at which the

RRSB line intersects the outer scale is the characteris-

tic value of the position and spread parameters. The

specific surface area as a function of the volume S

3

) is determined by dividing the characteristic

value obtained by the position parameter d

and then

multiplying it by the form factor f. The value thus

obtained can be related to the mass by inserting the

density (r):

¼ S

=r m

1



ð4Þ

It should be pointed out that the data for the specific

surface area thus obtained may frequently differ con-

siderably from those determined by other methods

3990 MILLING/Types of Mill and Their Uses

(e.g., adsorption and permeability methods). The

values for specific areas obtained by calculation are

therefore frequently of limited value only.

Energy Required for Grinding

0027 Grinding requires energy which has to be applied

to the material to be ground by means of tools. Part

of the work applied is used to drive the tools and only

the remaining, often much smaller part is used in the

grinding process itself. Several equations have been

developed to estimate the amount of energy required,

based on different concepts of the grinding process.

All formulae are based on model concepts in which it

is assumed that separate particles are to be ground.

They are therefore founded upon very rough simplifi-

cations compared with the actual grinding processes

which take place in grinders. Thus the formulae yield

results which are of value in certain areas of grinding

only. For example, the energy required for grind-

ing according to Rittinger’s law is proportional to

the sum of the newly created surfaces, irrespective of

the type of mechanical action (tension, compression,

shear stress).

0028 Rittinger’s law obeys the following function

equation in which K

is a proportionality factor

(Rittinger’s constant) which depends on the material

and the grinder. Given the same material and grinder,

it is independent of the degree of grinding. D

is the

grain diameter prior to grinding and D

the grain

diameter after grinding. Rittinger’s law is mainly

used to estimate the energy A

required to grind

either fine or brittle materials.

¼ K

1=D

ðÞ1=D

ðÞ ð5Þ

By contrast, Kick’s grinding law states that the energy

required for grinding consists entirely of the elas-

tic and residual deformation of the particles to be

ground, this energy being proportional to their

volume. The function equation is written as follows:

¼ K

log D

ðÞ ð6Þ

is also a proportionality factor (Kick’s constant).

Kick’s law applies above all to coarse materials

with fewer brittle and more elastic characteristics.

In practice, however, it is impossible to obtain an

exact result for the energy required for grinding

by means of these formulae as it is influenced by

too many factors, including the design of the grinder

and its mode of operation. Neither is it possible

to calculate the energy required from the particle

size distributions before and after grinding or from

Equivalent particle diameter (µm)

Mass cumulative distribution

Spread parameter (n)

Dimensionless surface scale (S

d'/φ)

0.001

0.002

0.005

0.01

0.02

0.05

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.95

0.99

0.995

0.999

2.5

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

7.6

100

200

300

500

1000

5000

Pole

Position parameter

fig0003 Figure 3 Graphical representation of particle size distribution, according to the Rosin–Rammler–Sperling–Bennet (RRSB) grid.

Reproduced from Milling: Types of Mill and Their Uses, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

MILLING/Types of Mill and Their Uses 3991

the mechanical strength of the material to be ground,

as a considerable part of the energy applied is used

not for grinding but for moving the material, or is

absorbed by the material which has already been

ground.

Grinders (or Mills) Used in the Food

Industry

0029 In view of the requirements which ground material

has to meet and the large number of materials which

must be ground to produce foodstuffs and similar

goods, the differences in shape, size, composition,

and strength characteristics have led to the develop-

ment of grinders (or mills), the design of which is the

result of the type of grinding operation to be per-

formed. The grinders operate by applying compres-

sive, shear, frictional, and cutting forces or, as is

frequently the case, various combinations thereof

can be applied to the material to be ground either

simultaneously or consecutively, depending on the

design of the grinder.

0030 The application of mechanical energy to the mater-

ial being ground can lead to the latter heating up

considerably. The grinder parts are also heated by

the energy conversion. Where the design and action

of a grinder cannot contribute adequately both to

dissipating the heat produced in the material in

order to preserve its quality and to keeping the oper-

ating temperature of the machine within tolerable

limits, it is necessary to cool the grinding tools, as a

result of which the material is also cooled.

0031 Coolant may therefore be pumped through the

grinding tools and the surrounding sections of

the housing. The rollers of roller mills or the rotor

and stator of stirrer bead mills, for example, may be

cooled in this way. In impact mills, the material being

ground can be cooled by increasing the air flow

through the impellers attached to the shaft of the mill.

0032 Specially constructed impact mills, which are

equipped with heat exchangers, enable the transfer

of evaporable mass from the material being ground

to the airstream. Thus a specific exchange of heat

and mass occurs, during which the material being

ground is dried. The machines in which grinding

and drying take place simultaneously are referred to

as drier-mills.

0033 A few important examples of mills used in the food

industry to process raw materials in order to obtain

specific ground materials are described below. They

have been selected in order to illustrate the most

important principles of grinding and their applica-

tions by means of concrete examples. Hence the list

of mills is not exhaustive, either with regard to the

grinding principles or the design of the mills.

0034The majority of mills work continuously. The ma-

terial to be ground is fed into them by means of

conveyor equipment. The grinding of materials in air

or gas is referred to as dry grinding (or milling) and

that of suspensions as wet grinding (or milling). Some

of the mills described below are designed to be oper-

ated in both dry and wet milling.

Roller Mills

0035Roller mills are used both for solely grinding and for

selectively grinding and milling particles of granular

shape. Their most important application is the select-

ive grinding of cereal grain. In addition to this, roller

mills are also used for fine grinding of cocoa and

spices.

0036The great advantage of milling cereal grains by

means of grinding rollers is that the process can be

subdivided into many stages, the grinding forces

acting on the material only once at each stage. A

further advantage is that the mills can be set

according to the material to be ground and the

required grinding process, because not only can

the distance between the rollers be freely selected,

but also their diameter, surface structure, the speed

at which they turn, and the differential speed.

0037Roller mills consist of a housing in which, as a

general rule, several pairs of rollers in bearing rings

are placed on either side on sliding bearings or on self-

aligning roller bearings. Figure 4 shows a modern

roller mill with four horizontal pairs of rollers which

are used to grind cereal grains. The material to be

ground is fed into the machine by two feed rollers

which are located above the two upper pairs. The

ground product passes, without sifting, directly from

the upper pairs to the two lower pairs of rollers,

where it is subjected to further coarse or fine grinding.

In order to improve the supply of material to the

grinding rollers, the machine is equipped with a fan

to extract the air from the gap between the rollers.

The latter can be set automatically, the automatic

setting device being connected to a computer in

which the optimum roller gap settings for all grinding

phases of a grinding diagram are stored, thus insuring

the maximum flour production for any given cereal or

mixture of cereals.

0038The rollers of roller mills are driven by flat belts or,

in the case of single rollers, by fan belts. The different

roller speeds are produced by a cogwheel or chain

drive. Driving the rollers by chains or stepped gears

enables the distance between the axes of the pairs of

rollers to be adjusted without changing the transmis-

sion wheels.

0039The rollers are made of centrifugally cast, chilled

cast iron which is extremely dense and wear-resistant.

3992 MILLING/Types of Mill and Their Uses

Depending on the hardness of the cast iron, the rollers

are either ground or corrugated by machine to pro-

duce either smooth or corrugated rollers.

0040 The rollers, which are standardized, have a diam-

eter of 220–315 mm, and a length of 315–1500 mm,

and can be machined several times. They may also be

made of porcelain. This type of roller has a working

surface which comprises a porcelain sleeve with a

wall thickness of approximately 50 mm. They are

220–350 mm in diameter and can be up to 1000 mm

in length.

0041 The grinding rollers can heat up considerably

owing to the production of heat caused by friction,

rendering it difficult to set the gap to a fixed distance,

especially when subjected to a high degree of wear. As

this can have a negative effect on the grinding result,

water-cooled grinding rollers have been developed

which are cooled by pumping water over the shaft

through the hollow rollers. Cooling increases the

specific grinding performance and leads to a more

even grinding result.

0042As it is possible to change the working parameters,

the material can be ground between the rollers by

compressive, shear, and cutting forces. The length of

time during which these forces act on the material

depends on the radius of curvature and the absolute

circumferential speed of the rollers as well as on the

ratio between the gap between the rollers and the size

of the material to be ground. In order for the material

to be fed into the gap, the feed angle must be smaller

than the angle of friction. This means that the ratio

between the diameter of the rollers and that of the

material must be large. The larger the diameter of

the rollers, the slower the circumferential speed and

the smaller the gap selected, the greater the forces

acting on the material will be. The forces are limited

by the hardness of the material being ground and its

fracture.

0043Compressive forces only act on the material where

the circumferential speed of the rollers is the same

and the surface of the rollers is smooth. This results in

the product being ground by being deformed to rolled

plate-like particles, especially in the case of soft

products such as cocoa. By comparison, different

roller speeds give rise to shear forces which,

combined with a corrugated surface and the finite

distance between the rollers, results in a slicing

action. This type of grinding occurs above all during

the sharp-to-sharp grinding action of the corruga-

tions, while the dull-to-dull grinding action grinds

the product mainly by compression. Grinding

predominantly follows the product’s own fracture

which can be deduced from its natural structure.

0044This is the case, for example, when grinding many

types of cereal grains: the particle size distributions of

the ground products exhibit characteristic differences

which depend on the hardness of the material. Where

the dull-to-dull grinding action of the corrugations is

applied, it is possible to guide the grinding result

towards the RRSB function. Combined with the

slicing action of the rollers, it is also possible to

impose a characteristic distribution deviating from

the RRSB function on to the particle size of the

ground product. The range of particle sizes and the

shape of each particle are also influenced to a large

extent by the design of the corrugations.

0045The corrugations are shown in section in Figure 5.

The angle of the corrugations is composed of the

cutting angle (a) and the rear angle (b). The slope of

(a)

(b)

fig0004 Figure 4 Roller mills: (a) pair of rollers; (b) eight-roller mill.

Courtesy of Bu

hler AG, Uzwil, Switzerland.

MILLING/Types of Mill and Their Uses 3993