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

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objectives of agitation are to keep solids in suspension

and to disperse them uniformly so that any draw-off

would have an identical solid concentration. Impel-

ler-agitated devices, described earlier, are generally

useful for these purposes; propellers, in particular,

are known to be efficient for suspending solids in

low-viscosity liquids. For a given impeller and solid–

liquid system, there is a minimum rotational speed

below which the impeller does not produce sufficient

‘lift force’ to suspend the solids. The threshold rota-

tional speed, in general, depends on the size, shape

and density of the solid particles, the solid concen-

tration, the density and viscosity of the liquid, and the

geometry of the mixing vessel. In food systems, the

density difference between the particles to be sus-

pended and the liquid phase is generally not high. It

follows that the minimum rotational speed for solid

suspension is not high. Nevertheless, it is quite prob-

lematic to ensure homogeneity after the particles have

been suspended, especially when the particles are sen-

sitive to collision and high shear. While information

on threshold suspension speeds is readily available,

the number of studies on the homogeneity of such

dispersions in mixing vessels is very limited.

Air–Liquid Agitation

0033 During the agitation of food products, it is generally

desirable to exclude air, since entrained air can cause

spoilage during storage. However, there are certain

situations where aeration is desirable: aerobic fer-

mentations generally demand a continuous supply

of air; processes such as cream whipping and ice-

cream preparation involve air inclusion. Both

aeration and deaeration are therefore important to

food processing.

0034 Aeration, whether desirable or not, can occur from

the surface of an impeller-agitated vessel. The action

of the impeller induces circulation and turbulence in

the liquid. When strong eddies are generated at the

surface, air is entrained from the head-space to form

bubbles, which are then dragged into the bulk by

circulation currents. These bubbles are stabilized

by surface-active agents that are invariably present in

food systems, and the two phases coexist. It has been

noted that surface aeration occurs above a minimum

impeller speed, and it is possible to estimate its value

for a variety of systems. If aeration is detrimental to

product storage, then either the impeller speed should

be less than the minimum speed for surface aeration,

or agitation should be followed by de-aeration.

0035 Aerobic fermenters invariably need a continuous

supply of air to sustain microbial growth. The main

effect of sparging air into an impeller-agitated tank is

to lower its power consumption. The reduction in

power is a consequence of the formation of stable

air cavities behind the impeller blades; the extent of

the reduction depends on the size and shape of the

cavities. At a given impeller speed, an increase in air

flow rate results in an increase in cavity size. How-

ever, the cavity size cannot increase indefinitely: once

the cavities have grown to their maximum size at a

given flow rate, further increases in air flow rate cause

some of the excess air to bypass the cavities and hence

the impeller blades. The cavities, in certain cases, may

also coalesce. As a consequence, the impeller virtually

stops pumping, and the phenomenon is described as

‘impeller flooding.’ An impeller-agitated fermenter

operates well away from the flooding point. In prac-

tise, the disc turbine of Figure 2a is the preferred

impeller used in aerobic fermentation, mainly owing

to dispersion ability. This impeller, however, suffers

from certain drawbacks: it is a high-shear impeller,

and therefore causes shear damage to certain sub-

stances, its power consumption is very high, and,

more importantly, a large fraction of the supplied

power is dissipated in a relatively low volume fraction

of the vessel. These disadvantages have been found to

be critical to several processes, and there is an attempt

to use alternative devices, such as the hydrofoil

impeller of Figure 2d.

Agitation of Particulate Material

0036Particle mixing is an extensive food processing oper-

ation used for mixing materials that include flour,

sugar, dried milk, salt, flavoring materials, cereal

flakes, etc. Wide differences among properties such

as particle size, shape, density, and surface character-

istics (e.g., frictional and electrostatic), make particle

mixing a difficult and a complex operation. The pro-

cess can be further complicated in food systems by a

high moisture content, friability, complex flow prop-

erties, and agglomeration or segregation. The desired

end point of solid-phase mixing is the attainment of

a truly random distribution.

0037The following differences between mixing in

particulate systems and fluid systems have been

recognized:

0038There is no particulate motion equivalent to

molecular diffusive transport in liquids and

gases. Thus, when two miscible liquids or gases

are in contact with each other, complete mixing

eventually occurs. However, blending of particu-

late materials cannot occur without some input

from external energy.

0039Unlike fluids, mixing of particles is reversible, i.e.,

mixed solids, on storage, tend to segregate, pri-

marily because of size differences; even marginal

differences of 15–20% can cause ‘unmixing.’ Dif-

ferences in other properties, such as density and

94 AGITATION

shape, can only accelerate the process of unmix-

ing. It has been recognized that, in general, heav-

ier, smaller, or smoother and rounder particles

tend to percolate and sink through lighter, larger

or jagged particles.

0040 Mixing in liquids and gases is far more intimate

than in solids. The ultimate mixing elements of a

particulate mixture have a coarser texture, and are

of a poorer quality.

Particulate materials may be broadly classified into

two groups: (1) free-flowing or cohesionless powders

and (2) cohesive powders. Cohesionless powders are

mixed by convective transport, surface mixing and/or

interparticle percolation; mixing and segregation

occur simultaneously. Particles in cohesive powders

move in clusters.

0041 Mixers used for solids are normally batch type.

Solid mixers can be broadly classified as: (1) tumbler

mixers, (2) convective mixers, and (3) hopper mixers.

A V-shaped tumbler mixer is shown in Figure 6. The

vessel is typically filled to about half its volumetric

capacity and is rotated on an axis between bearings,

thus causing the particles to tumble and roll over each

other. In the case of convective mixers, the mixing

vessels are stationary, and the solids are agitated by

impeller blades, usually of a ribbon or screw type.

Cohesionless powders can also be mixed by discharg-

ing a mixture of the components through hoppers.

Radial and axial mixing is achieved by returning the

discharged material to the top of the hopper and

repeating the process until the solids are completely

homogeneous.

Mixing of Doughs and Batters

0042 Dough and batter, used in the making of bakery

products, are formed by a series of agitation-induced

interactions of such diverse components as water,

flour, lipids, enzymes, salt, sweeteners, yeast, air,

and oxidizing and reducing agents. When classified

on the basis of the desired agitating action, most

bakery products fall into two broad categories, exten-

sible doughs, such as those used to make bread or puff

pastry, and flowable or friable mixtures, used in the

preparation of cake batters, icings, etc. (See Bread:

Dough Mixing and Testing Operations.)

0043 Mixing of doughs is more complex and energy-

intensive. Although continuous processes are now

being developed, mixing of doughs is invariably

carried out batchwise in one or many stages. Single-

stage mixing is mainly adopted by smaller bakeries,

and it aims to complete dough development within its

duration. However, in a two-stage process, the initial

stage has a shorter duration, and it accomplishes

blending and flour hydration (or hydration of

the gluten proteins). Dough development and

strengthening of the gluten network occurs more

gradually during the second stage, promoted by

repeated folding and stretching action caused by the

mixer. An undesirable effect of the intense shearing

action, especially in high-speed mixers, is the rapid

increase in dough temperature. Provision for simul-

taneous cooling therefore exists in most mixers. Care-

ful control of the process is necessary to obtain a

dough structure optimum for baking purposes: at

the right end point, the dough has maximum consist-

ency or minimum mobility; it appears smooth with a

dry surface, and its elastic character is optimized. The

end point is reached when the dough starts to pull

away from the mixer walls. Overmixed dough, how-

ever, is sticky and difficult to handle, and its surface

possesses a characteristic sheen.

0044The agitator in a dough mixer is mounted either on

a horizontal shaft or on a vertical shaft. Vertical shafts

generally rotate about a fixed axis; in some cases,

planetary motion is also superimposed. In either case,

there is a close clearance between the agitating blades

and the mixing vessel, in order to eliminate stagnant

regions and build-up of sticky material on the wall.

See also: Aerated Foods; Bread: Dough Mixing and

Testing Operations; Emulsifiers: Organic Emulsifiers;

Plant Design: Designing for Hygienic Operation

Further Reading

Harnby N, Edwards MF and Nienow AW (eds) (1992)

Mixing in the Process Industries, 2nd edn. Oxford:

Butterworth-Heinemann.

Jones RL (1985) Mixing equipment for powders and pastes.

The Chemical Engineer 419: 41, 43.

Lindlay JA (1991a) Mixing for agricultural and food

materials: 1. Fundamentals of mixing. Journal of

Agricultural and Engineering Research 48: 153–170.

Lindlay JA (1991b) Mixing for agricultural and food ma-

terials: 2. Highly viscous liquids and cohesive materials.

Journal of Agricultural and Engineering Research 48:

229–247.

Lindlay JA (1991c) Mixing for agricultural and food mater-

ials: 3. Powders and particulates. Journal of Agricultural

and Engineering Research 49: 1–19.

Niranjan K, Smith DLO, Rielly CD, Lindlay JA and Phillips

VR (1994) Mixing for agricultural and food materials:

5. Review of mixer types. Journal of Agricultural and

Engineering Research 59: 145–161.

Niranjan K (1995) An apprisal of the characteristics of food

mixing. In: Singh RK (ed.) Food Process Design and

Evaluation, pp. 47–67. Lancaster, PA: Technomic.

Rielly CD, Smith DLO, Lindlay JA, Niranjan K and Phillips

VR (1994) Mixing for agricultural and food materials:

4. Assessment and monitoring of mixing systems. Jour-

nal of Agricultural and Engineering Research 59: 1–18.

AGITATION 95

AIDS See HIV Disease and Nutrition

AIR CLASSIFICATION

Contents

Principles of Operation

Uses in the Food Industry

Principles of Operation

P Fedec, POS Pilot Plant Corporation, Saskatoon,

Saskatchewan, Canada

Background

0001 In simplistic terms, air classification is a means of

using air to effect a dry separation of objects having

certain characteristics. These characteristics include

physical properties such as size, shape, density, and

physicochemical nature. Air classification is an oper-

ation that applies the technology to fractionation of

nonhomogenous particles, suspended in an air stream,

into classes of fairly uniform size based on a common

criterion of density or mass. Whereas some segrega-

tion of light and heavy particles can be achieved by

aspiration where airflow patterns are generated by

only a vacuum, true air classification makes use of

airflow induced through centrifugal motion with

the assistance of a vacuum. Generally, air classifiers

will segregate a heterogeneous particulate into two

subclasses, one primarily below a targeted particle

size (fine fraction) and the remainder above this

size (coarse fraction). Air classification becomes a

possible alternative when ordinary sieving fails to

effect separation below 40 mm. Applications of air

classification in the food industry separate par-

ticulates in the range of 2–60 mm, seldom exceeding

100 mm.

Factors Affecting Classification

0002 In the ideal case, classification can be best achieved if

the particles are uniform and homogeneous, spherical

and smooth, and dry and easily dispersible. Practic-

ally, very few finely granulated mixtures exhibit these

ideal properties. Other features such as particle geom-

etry can affect the tragectory of particles in an air

stream. Classification is also affected if some of the

particles are porous, allowing fine particles to hide

within the pores of the coarser particles.

0003Dispersibility is a key issue during classification.

Materials containing moisture or oil, or exhibiting

hygroscopic tendencies, are likely to agglomerate

and resist classification. To minimize these effects,

pre-classification techniques need to be considered,

which may include additional grinding, vacuum

drying, extraction by solvent, blending, dispersion,

or deagglomeration.

0004The cell structure of food materials is one of the

most important factors for the mechanical separation

or dry processing. Application has primarily been in

protein and starch separations of legume and cereal

flours. Within the cell, the protein forms a matrix in

which the starch granules are embedded. During

comminution, the breakdown of this matrix into

wedge protein or granular protein (and its size or

homogeneity) is important and will affect the effi-

ciency of classification (Figure 1). The cell matrix

must shatter easily, and in addition, the protein

matrix must disintegrate readily. If compound starch

granules are present, they must remain intact. A sig-

nificant number of particles (e.g., starch granules)

must be above the selected cut point. Typically,

impact milling is the comminution of choice.

0005Three particle-related factors of primary import-

ance in achieving separation include: particle density,

shape, and size. The difference between density of

starch (1500 kg m

3

) and protein (1300 kg m

3

)is

too small for density to have a significant role.

Shape plays some role; in particular, starch granules

flattened by impact milling exhibit increased air drag

and unfortunately may concentrate in the fine frac-

tion. Experience has shown that particle size is

the predominating influence on the quality of air

classification.

0006The particle size of the starting material is an

important factor in that, generally, the particle must

96 AIR CLASSIFICATION/Principles of Operation

be less than 100 mm and preferably lower, with a

significant number being below the selected cut size.

For example, if the granular and wedge protein can be

pulverized into aggregates of 2–10 mm in size, while

the starch particles remain intact and are in the range

of 15–45 mm, separation is possible (Figure 1). Some

wedge protein of larger size will spill over into the

coarse particle cut, and some small granules will spill

into the protein or fine fraction. Products that do

not have a significant difference in the sizes of

components are not suitable candidates for air classi-

fication.

0007 The moisture content of grains and legumes affects

the ability to disintegrate the cell contents and dis-

lodge the protein from the starch. Practice suggests

that the moisture content for satisfactory comminu-

tion preceding air classification should be around

8%. Moisture contents above this value do not lead

to an ideal size reduction, but a moisture below 6%

can lead to starch granule fragmentation and subse-

quent spill-over into the fine fraction.

0008 No well-documented research has been carried out

on the influence of fat content of the starting material

on air classification, but in general, fat contents have

to be kept low in order to achieve dispersion in the

centrifugal zone and minimize bridging in and around

rotors, vanes, and passage-ways. Generally, a 1–2%

fat content is the limit, although some results have

indicated that flours having up to 7% fat are separ-

able by some classifier models, but prolonged operat-

ing times with sustained quality of separation have

not been established.

0009Sharpness of classification is affected by the feed

rate, and generally, as rates are elevated above opti-

mal, the sharpness decreases.

Air-classifier Operations

0010Air-classifier operation is influenced by air-flow dy-

namics on suspended/dispersed particles of typically

less than 100 mm. The term ‘cut size’ is used in con-

junction with discussing the segregation of the two

fractions.

0011If a predetermined particle size were selected (e.g.,

typically 15 mm in flour applications), a theoretical

ideal classification would permit a subdivision of a

mixed particle feed stock into two fractions: a fine

fraction below the predetermined size and a coarse

fraction above it. Figure 2a depicts such a segregation

of fine (shaded) and coarse (clear) fractions.

0012Under practical conditions, such a sharp separation

cannot be achieved, and the separation is more realis-

tically observed as shown by the curves in Figure 2b.

A single ‘cut size’ is not achieved; rather, there is a

range of particle sizes present in both the fines and

coarse fractions, and these overlap in the region of a

nominal cut size. The less the overlap, the better is the

separation efficiency.

0013The two major forces affecting air classification are

the drag or frictional force exerted on the particle by

the air flow and an inertial force exerted by the accel-

erated motion of the particles. The latter is generally

achieved by a centrifugal field or rotational air flow

through use of a rotor. A typical example of forces

Impact

milling

Endosperm cell

Free protein

(incl. wedge protein)

= 0.5 − 5 mm

Wedge protein, clusters

and detached small

granules

= 2 −15 mm

Detached large

starch granules

= 15 − 35 mm

Starch and protein

clusters

>35mm

fig0001 Figure 1 Conceptualized presentation of endosperm cells subjected to milling and subsequent fragmentation.

AIR CLASSIFICATION/Principles of Operation 97

acting within a classifier is shown in Figure 3.A

particle (‘G’) of a certain diameter and density is

exposed to two opposing forces, the centrifugal force

(F) produced by a rotating assembly and frictional

force (R) produced by an air flow. Additional and

more complex mathematical models can be found in

articles by Vose and de Silva. As the air flows inwards

in a spiral path, particles entrained in the flow are

subjected to the opposing forces F and R. Larger

particles are dominated by the mass-dependent cen-

trifugal force, whereas small particles are influenced

by the frictional force. When the forces are equal, a

cut point or particle of a definite size can be estab-

lished. The cut size can be adjusted by manipulating

these forces within the classifier.

Design Styles of Air Classifiers

0014Many air-classifier variations are on the market. They

can be subdivided into types based on their principles

of operation. Some of the common types are detailed

below.

x min

q(x)

x max

Fines

Coarse

x min

q(x)

x max

Fines Coarse

(a) (b)

fig0002 Figure 2 Classification phenomena: (a) an ideal classification; (b) a real classification. Adapted from de Silva SR (1983) Develop-

ments in air classifier theory and practice. In: Institution of Chemical Engineers (Gr. Br.) Symposium Series, vol. 69, pp. 387–410, with

permission.

SS SSS

SSSSSS

= Centrifugal forceF

= Frictional forceR

= Cut size particleG

= Circular path

= Spiral flow line

= Air velocity

= Peripheral component

= Radial componentK

fig0003 Figure 3 Forces interacting in the spiral flow of an air classifier. From Hosokawa Micron International Inc. Catalogue 31/6e. Summit,

NJ with permission.

98 AIR CLASSIFICATION/Principles of Operation

Gravity Principle

0015 This is the simplest design, has no moving parts, and

consists of a zigzag-shaped classifying channel with

internal baffling. It is generally used for gross separ-

ations, often for dissimilar materials that may be

difficult to separate by simple separation techniques

such as screening.

Free Vortex Principle

0016 Air classification is achieved by adjusting a set of

louvers located at the periphery of the classifying

zone, which allows the superimposed radial air flow

to enter the zone at an angle. Typical of this group are

designs such as Pallmann Galaxy and Alpine Mikro-

plex. The latter (Figure 4) has been used in a large

number of research studies, and significant data have

been published. This design incorporates a rotor on a

horizontal axis, resulting in a vertical plane of centri-

fugal force, whereas most others use a horizontal

plane that favors greater throughput but with a re-

duced precision in cut size. The Mikroplex MPS

model has been discontinued, but many are still in

industrial operation. Feed rates vary from 50 to

5000 kg h

1

, depending on the model size. Classifiers

manufactured by Larox operate on the same

principle, but the peripheral vanes are not adjustable.

The cut size is adjusted by manipulating the air flow

produced by a blower.

Forced Vortex Principle

0017The force is provided by means of a turbine or rotor

that disperses the particles into an air stream applied

by a suction fan. Variations in equipment design then

use air streams to select fine or coarse particles. The

Bauer Centri-Sonic was typical of this design and was

used commercially for cake flour separation as well as

much research, but its precision honeycomb rotor

was costly to manufacture, and the equipment has

been discontinued. A new breed of classifier, often

referred to as the turbine classifier, is now being util-

ized for fine powder separation (3–150 mm). These

may incorporate single or multiple turbine classifier

wheels in either vertical or horizontal configuration.

The MikroPul Micron Separator illustrates this con-

cept (Figure 5). Other manufacturers using this

principle include Hosokawa Micron, ABB Raymond,

and Alpine ATP series classifiers, as well as those

manufactured by Nisshin, Sturtevant, and Matter

and Partner.

0018Additional but less popular classifier designs used

mixed vortex, countercurrent, cocurrent, or cross-

flow principles. Some are experimental, and others

have not been applied to the food industry.

Combined Mill/classifier Units

0019In the food applications area, no appreciable techno-

logical changes to classifier design have been made in

Spiral air flow

Air guide vane

Coarse fraction

discharge worm

Coarse fraction

knife edge

Fine fraction

outlet

Coarse particles

Fine particles

Inlet for material

to be classified

Classifying

chamber

fig0004 Figure 4 Frontal cross-section view of a Mikroplex MP spiral classifier.

, coarse particles;

, fine particles. From Hosokawa Micron

International Inc. Catalogue 31/6e. Summit, NJ with permission.

AIR CLASSIFICATION/Principles of Operation 99

recent years, primarily because the profit margins

are low in this industry, thus providing little incentive

for spending resources on further development and

refinement.

0020 To reduce operating costs, interest has focused on

equipment that combines the required fine grinding

with classification. One such unit is the Mikropul

ACM Air Classifier Mill. The basic operation in-

cludes a high-speed impact mill through which an

air stream is drawn containing the raw material.

The product/air mixture enters the milling chamber,

after which the product is then accelerated by the

rotor towards peripheral guide vanes into the classifi-

cation zone. Here, the particles are acted upon by

centrifugal force in a centripetal airstream, and

lower-mass fine particles are separated and conveyed

out of the mill to a collector. The coarse particles are

directed back to the grinding zone for further size

reduction. The rotor speed and air flow are adjusted

to maximize mill performance in a given application.

Plant System Designs

0021The typical installation consists of a centrifugal fan

generating a vacuum draw to a baghouse collector.

An additional scrubber for fine particulates may be

included after the baghouse filter, and some designs

use an optional cyclone ahead of the baghouse col-

lector. The fines separated by the classifier are drawn

into the collector, whereas the coarse particles are

usually rejected by one of several ways, either in an

auger system, in an air stream to a cyclone, or within

a cyclone built into the classifier. The classifier may

be fed from a silo of premilled material or may be

directly coupled to a fine grinding mill. This direct

coupling method has received criticism in that

changes in material composition or particle size,

owing to stratification, affect the classifier cut fraction

and thereby the final product composition, especially

if the latter is being packaged directly from the pro-

cess line. Ancillary equipment includes rotary air

Fine product

discharge

Secondary

air inlet

Feed and

primary air

Coarse product

discharge

fig0005 Figure 5 Typical illustration of a turbine-type fine powder classifier (MikroPul Micron Separator MS). From Hosokawa Micron

International Inc. (1988) MikroPul Micron Separators Cat. No. HMS-1 5M-4/88. Summit, NJ with permission.

100 AIR CLASSIFICATION/Principles of Operation

locks for the collectors, filter assemblies, discharge

devices, blow-out relief doors, monitoring and con-

trol devices, blowers, silencing devices, and pulse jet

filter cleaning apparatus.

0022 Some operating facilities, in particular for legumes,

have used a double milling, double classification

design sequence, where the initial mill is followed by

a classifier, the coarse fraction remilled and reclassi-

fied, thereby yielding a final coarse fraction and two

fine fractions that are generally pooled. This plan

strives to increase the quantity recovered as fines,

although their quality may be compromised, and to

improve the purity of the coarse fraction.

0023 Safety factors that should be considered in an in-

stallation include dust inhalation, noise hazard,

vibration, and dangers of explosion from fine dusts,

which are ignitable under certain conditions.

Operating Conditions

0024 In addition to the features of the feed material as given

by the composition, nature of particles, product uni-

formity, and premilling influences, the variation of cut

point, and, subsequently, the quality/content of the

separated fractions can be manipulated by classifier

variables such as the rotational speed of the rotor or

disintegrator, the amount of air flow through the clas-

sifying chamber and the velocity of the air flow (both

can be altered by either adjusting air intake or the

external booster fan), degree or angle of vane setting,

which deflects flow, and finally the rate of feed de-

livered to the classifier. These options vary in both

availability and degree of adjustment, depending on

the manufacturer of the equipment. Some classifiers

allow for easy adjustment of fineness from an external

position while the unit is operating, whereas others

can be adjusted only after partial disassembly (i.e.,

such as alteration or adjustment of numbers or sizes

of selection plates). However, with proper manipula-

tion of these parameters, the classifier can produce a

quality separation of high efficiency and yield.

0025 Supplemental options are available for special ap-

plications whereby water cooling may be applied to

the outside cone of certain separators to satisfy the

needs of the cooling product (prior to packaging) that

has developed a temperature rise during the milling

and classification sequence.

0026 When selecting systems for an application, there is

no substitute for actually trying out a system on the

materials intended for classification. Most manufac-

turers maintain laboratory and pilot plant test facil-

ities, where various types and sizes of classifiers may

be tested for separation efficiencies. Some independ-

ent contract research centers are also able to conduct

exploration and demonstration testing.

See also: Air Classification: Uses in the Food Industry;

Plant Design: Basic Principles; Separation and

Clarification

Further Reading

Alpine Aktiengesellshaft (1978) Alpine Mikroplex Spiral

Air Classifiers MP, MPS. Alpine Bulletin 31/6e. Augs-

berg, Germany.

de Silva SR (1983) Developments in air classifier theory and

practice. In: Institution of Chemical Engineers (Gr. Br.)

Symposium Series, vol. 69, pp. 387–410.

Galk J (1998) Production of fine powder with the air classi-

fier mill ACM or the newly developed multistage impact

mill MSM. Powder Handling Process 10(1): 11–13.

Hixon L (1992) Sizing up air classifiers. Chemical Engin-

eering Progress 88(7): 59–62.

Hosokawa Micron International Inc. (1988) MikroPul

Micron Separators Cat. No. HMS-1 5M-4/88. Summit,

NJ.

Jones CR (1960) New developments in milling processes.

Protein displacement. Milling 135: 494–498.

Klumpar IV, Currier FN and Ring TA (1986) Air classifiers.

Chemical Engineering 93: 77–92.

Lauer O and Prem H (1978) Protein enrichment in vege-

table products by air classification. Zeitschrift fu

Lebensmittel - Technologie und - Verfahrenstechnik 29:

212–215.

Schubert H and Heideker H-Th (1986) Protein separation

from vegetable sources by selective commination and air

classification. In: Le Maguer M and Jelen P (eds) Food

Engineering and Process Applications II, Unit Oper-

ations, pp. 293–303. London: Elsevier.

Thomas ND (1999) The MikroPul air classifier mill. Inter-

national Sugar Journal 101: 480–481.

Vose JR (1978) Separating grain components by air classifi-

cation. Separation and Purification Methods 7: 1–29.

Uses in the Food Industry

P Fedec, POS Pilot Plant Corporation, Saskatoon,

Saskatchewan, Canada

Background

0001Air classification is a technology that is based on

physical principles, and therefore, the use of

chemicals is precluded, and protein denaturation is

avoided, as is the formation of artifacts. This dry

treatment also avoids the production of polluting

effluents and minimizes or obviates drying costs.

Two of its negative aspects are that there may be a

retention of antinutritional or undesirable compon-

ents in one or both fractions, and neither of the

AIR CLASSIFICATION/Uses in the Food Industry 101

fractions produced is ‘pure’ when compared to

those obtained through conventional wet processing;

hence, applications for products produced often need

to be developed.

Application to Grain Legumes

0002 Considerable research work on the milling and clas-

sification of nonoleaginous, starchy legume seeds has

been carried out in Canada at the National Research

council Prairie Regional laboratory and the Univer-

sity of Saskatchewan, both in Saskatoon. Although a

wide number of species were studied for their poten-

tial to commercialization, the focus on field peas led

to the adoption of this dry separation process by

industry. Similar procedures were adopted in France

for the production of fababean protein concentrate.

Other centers that have also explored fine grinding

and air classification in legumes include the Agricul-

tural and Food Research Council Food Research

Center and the University of Reading in the UK, the

Technical University of Denmark, the Michigan State

University in the USA, and the Korean Food Research

Institute. Most researchers have employed pin mills

for fine grinding of the seed, and a majority have

focused on using the Alpine Mikroplex MPS series

classifiers, although Alpine Zig Zag and Bauer Cen-

tri-Sonic series were also used. Recently, studies have

shown that the use of jet air mills has given flour

grinds having a mean particle size of < 40 mm, which

has improved separation of flour into protein and

starch fractions.

0003 A flow diagram for the process developed at the

Canadian Prairie Regional Laboratory and applied to

legumes is shown in Figure 1. Researchers have found

that the milling of the seed is of overriding import-

ance to the air-classification process. Efficient size

reduction in the initial milled flour should lead to

improved separation of the protein and starch com-

ponents. Examining Figure 1, whole or dehulled seeds

are finely ground in a pin mill and the resulting flour

classified in a spiral air stream classifier, set to a

predetermined cut point, resulting in the production

of a fine (protein) fraction (F) and a coarse (starch)

fraction (C). After the first pass, some protein bodies

still remain attached to the starch granule surface,

and some agglomerates remain, having starch gran-

ules embedded in a protein matrix. A second milling

of the starch fraction disrupts these complexes fur-

ther, and a second classification generates additional

protein (G), albeit at a reduced yield and protein cont

ent, and a final starch fraction (E) with low levels of

protein.

0004 In a typical example, the two-run process on field

peas generated a combined protein fraction of 34.1%

yield and a protein content of 56.6% (Figure 1). For

fababeans, a protein fraction of 37.2% yield and a

protein content of 68.1% was produced. Starch frac-

tions contained 6–7% residual protein. Repeated pin

milling and air classification studies show that the

starch fraction can be further purified, but even

after four runs, the residual protein is still around

3% for field peas. It follows that little is to be gained

from a process based on more than two runs, and,

depending on economics, it may be sufficient to use

only a single-run procedure.

0005A comprehensive review of the published data

would be beyond this article. However, to give the

reader an overview of what may be achieved/expected

from various legume flours, a summary table (Table 1)

has been prepared. By way of caution, it should

be noted that the data are drawn from a range of

classifier types and operating parameters on starting

flours with varying initial protein levels prepared by

employing a wide assortment of grinding mills from

seed with varying moisture contents. The data are

presented as min–max ranges recorded in the litera-

ture. All data shown are based on a single impact

milling followed by a single air classification, gener-

ally using dehulled seeds.

0006Low fine fraction yields have a correspondingly

higher level of protein content as well as a higher

protein level in the coarse (starch) fraction. As the

yield of fines increases, the protein content of the

fine fraction decreases, and the quantity of the coarse

fraction will show a decrease. The protein content in

the coarse fraction will decrease, and correspond-

ingly, the starch proportion will increase. In all cases,

the recovery of protein into the fine fraction is not as

efficient as the recovery of the starch into the coarse

fraction.

0007The initial protein content of the legume flour has

an influence on the classification result. At the same

yield or split ratio, higher protein levels in the start-

ing flour will result in a higher protein content in the

fines fraction. Alternatively, if the protein content in

the fines fraction is kept constant, a higher yield of

that fraction can be expected. Finely milled, defatted

soybean meal has been added to Table 1 for compara-

tive purposes and serves to indicate that, unless there

is a high proportion of relatively large and uniform

starch granules, a shift of protein cannot be accom-

plished, despite achieving a separation into two

fractions.

0008Comparing the legumes, good separation may be

expected with faba, lima, mung, navy, Great North-

ern beans, lentils, peas, and cowpeas. Especially high

protein concentrates have been achieved from finely

ground fababean flour. Chickpeas are a poor candi-

date for air classification, presumably because of the

102 AIR CLASSIFICATION/Uses in the Food Industry

higher oil content in the flour and a high proportion

of small starch granules.

0009 The separation efficiency (i.e., the percentage of

total protein recovered in the fine fraction or the

percentage of total starch recovered in the coarse

fraction) is dependent upon moisture at the time the

legumes are milled. In peas, an increase in moisture to

14% from 10% reduces the separation efficiency by

20%. As the seed moisture decreases, there is a de-

crease in the starch yield, in the protein content of the

starch fraction and in the protein content of the pro-

tein fraction. However, there is an increase in the

yield of the protein fraction as well as the protein

separation efficiency. It has been suggested that a

moisture range of 8–10% is ideal for an impact-

milling, air-classification process operation in

legumes.

0010The effect of seed maturity on air classification

suggests that the presence of immature seeds, even

at high levels, has little effect on subsequent milling

and classification. There is concern that green and

shrunken seeds may make dehulling difficult and

translate to high fiber levels in the starch fraction.

0011The protein content of the starting flour, and ini-

tially of the seed supply prior to milling, has a signifi-

cant effect on fractionation. In a study of widely

varying protein content pea flours, it was noted that

the percentage protein in all air-classified fractions

Field peas

Dehulled seed

65.9 kg Pea starch (6.2% protein)

Air

classifier

Air

classifier

22.3 kg Protein I (59.2% protein)

11.8 kg Protein II (51.8% protein)

34.1 kg Pea protein (56.6% protein)

B 100 kg Pea flour (23% protein)

77.7 kg Starch I (12.4% protein)

Remilled starch I

Fababeans

Dehulled seed

62.8 kg Bean starch (7.2% protein)

21.1 kg Protein I (71.3% protein)F

16.1 kg Protein II (63.8% protein)

37.2 kg Bean protein (68.1% protein)

B 100 kg Bean flour (30.4% protein)

78.9 kg Starch (19.6% protein)

Remilled starch I

mill

fig0001 Figure 1 Double-pass milling and air classification sequence for legumes. Adapted from Tyler RT, Youngs CG and Sosulski FW

(1981) Air classification of legumes. I. Separation efficiency, yield and composition of the starch and protein fractions. Cereal

Chemistry 58: 144–148, with permission.

AIR CLASSIFICATION/Uses in the Food Industry 103