4 Size reduction of solids, equipment and methods

166 Size Reduction



●

●

●

●

●



Inertness of the surfaces in contact with the food

Sanitary design, ease of cleaning

Ease of maintenance

Environmental factors (noise, vibration, dust, explosion hazard)

Capital and operating cost (e.g. energy consumption, wear resistance etc.).



Size reduction equipment types may be classified according to the main kind of

action exerted on the processed material, as follows:

●

●

●

●



Main action is impact

Main action is pressure

Main action is attrition

Main action is shearing.



6.4.1 Impact mills

The principal representative of this class is the hammer mill (Figure 6.5).

The crushing elements are hammers fitted on a high-speed rotor inside a cylindrical

chamber. The chamber walls may be smooth or lined with corrugated breaker plates.

The hammers may be fixed or swinging. Swinging hammers are used when it is necessary

to reduce the risk of damage in the case of encounter between the hammer and large,

hard chunks. The principal crushing action takes place as a result of the collision

between the particles and the hammers and chamber walls. The leading face of the

hammers may be blunt or sharp. Knife-like sharp hammers are useful in the case of

fibrous materials where some shearing action is necessary. Normally, the chamber exit

is fitted with interchangeable screens to allow continuous removal of the sufficiently

small particles, while the oversize material is retained for further size reduction.

In the case of heat sensitive materials, the increased residence time of part of the feed

in the mill chamber may be objectionable. In this case, the mill exit is unrestricted.



Figure 6.5 Basic structure of a hammer mill



Size Reduction of Solids, Equipment and Methods 167



An external pneumatic loop is provided for screening the milled material and recycling

the coarse fraction through the mill (Figure 6.6).



6.4.2 Pressure mills

One of the most widespread types of this family is the roller mill (Figure 6.7).

The material is compressed between a pair of counter-rotating heavy rollers with

hardened surfaces to the point of fracture (Gutsche and Fuerstenau, 2004). The rollers

may be smooth or corrugated to reduce slippage. The mill may consist of several rollers with gradually decreasing clearance between the rollers (Figure 6.8). Roller mills

are extensively used in the grain milling industry. Mills equipped with corrugated rollers are used in the initial stages of wheat milling. They are called ‘breaks’ because they

serve to ‘break’ the grain and to separate the feed into the main fractions. Breakers

are also used in the dry milling of corn and for fracturing soybeans prior to flaking for

the production of oil. In wheat flour mills, units with smooth rollers are used as the

further step of size reduction and are called ‘refiners’ as they serve to refine further

the fractions to produce the different flours and other products. Mills consisting of a

series of smooth rollers are standard equipment for refining chocolate mass. Smooth

roller mills are also used as flaking machines, e.g. in the production of corn flakes,

Classifier



Product

Feed



Mill



Figure 6.6 Recycling of coarse material in milling



Figure 6.7 A four-roller mill and a pair of rolls. (Courtesy of Bühler A.G.)



168 Size Reduction



Figure 6.8 Battery of ‘breaks’ in a flour mill. (Courtesy of Bühler A.G.)



Figure 6.9 Attrition mill with flat grinding surface



oatmeal etc. For flaking, the particles are first conditioned (plasticized) by humidification

and heating in order to avoid disintegration.



6.4.3 Attrition mills

This is a very large group of mills. In many of the types belonging to this class, the

material is ground between two corrugated surfaces moving with respect to each other.

The prehistoric stone mill and the familiar household coffee grinder are examples of

this kind. The surfaces may be flat (Figure 6.9) or conical (Figure 6.10), vertical or

horizontal. Usually one of the surfaces rotates and the other is stationary.

Mills with circular grinding surfaces are also known as disc mills (Figure 6.11). The

fineness of grinding is often controllable by adjusting the gap between the surfaces.



Size Reduction of Solids, Equipment and Methods 169



Figure 6.10 Attrition mill with conical grinding surface



Figure 6.11 Disc mill. (Courtesy of Retsch GmbH)



The grinding surfaces may be made of hardened metal or of coarse corundum.

Corundum mills are used for reducing the size of particles in suspension down to colloidal range (micrometer range) and are therefore named colloid mills (Figure 6.12).

They are used in the preparation of fine suspensions such as homogenized strained

infant food, comminuted fruit pastes and mustard.

The main mechanism of action in attrition mills is shear, due to friction between

the particle and the pair of grinding surfaces. Unless the mill is cooled, the temperature rise may be considerable, particularly in the case of dry milling. A different kind

of attrition mill makes use of free ‘grinding media’ instead of fixed grinding surfaces.

One of the representatives of this kind is the ball mill whereby the feed is vigorously

mixed in a vessel containing free spherical bodies (balls) made of materials with different degrees of hardness, from plastic to metal, according to the hardness of the



170 Size Reduction



Toothed mill

Figure 6.12



Corundum stone mill



Colloid mills. (Courtesy of FrymaKoruma GmbH)



Figure 6.13 Dry ball-mill



Bowl



Rotor



Grinding elements



Figure 6.14 Wet ball-mill



particles to be disintegrated. Dry ball-milling (Figure 6.13) is used for fine grinding

of pigments. Wet ball-milling (Woodrow and Quirk, 1982) is one of the methods used

for the disintegration of cells in suspension (Figure 6.14).



6.4.4 Cutters and choppers

Cutting and chopping are size reduction operations based on shearing through the

use of sharp-edged moving elements (knives, blades). The term ‘cutting’ is usually



Size Reduction of Solids, Equipment and Methods 171



Slicing

knife

Circular

knives



Crosscut

knives



Figure 6.15 Cube cutter. (Courtesy of Urshel Laboratories)



reserved for operations resulting in particles with fairly regular geometric forms

(cubes, juliennes, slices), while the term ‘chopping’ is applied mainly to random

cutting. The variety of cutting machines used in the food industry is vast. In the

majority of cases, cutting is done by revolving knives or saws. Following are some

examples of cutting and chopping machines:

●



●



●



Cubing is done by cutting along three mutually perpendicular planes. In the

machine shown in Figure 6.15 the material is first cut into ‘slices’. In the second stage, the slices are cut longitudinally to produce ‘strips’. In the third and

final stage, the strips are shortened to produce ‘cubes’.

An interesting system is used for cutting potatoes in the French-fry industry.

Peeled potatoes are hydraulically conveyed at high velocity through a tube and

thrown against a stationary set of knives in quadratic array. The system has a

number of advantages over other methods of cutting:

1. There are no moving parts

2. Hydraulic conveying orients the potatoes along their long axis. The long

strips thus obtained are preferred by the market

3. Hydraulic conveying provides cutting and washing (removal of released

starch granules) in one step.

The silent cutter (Figure 6.16) is widely used in the meat industry for simultaneous

chopping and mixing. A batch of the material to be processed is placed in a

horizontal revolving dish. The dish circulated the material through a set of

horizontal revolving knives. Another related type of machine is the bowl mixercutter (Figure 6.17), similar in action to the kitchen blender or food processor.



172 Size Reduction



Figure 6.16 Silent cutter. (Courtesy of the Department of Biotech. and Food Engineering, Technion)



Figure 6.17 Bowl mixer-cutter



●



It is extensively used in the meat industry but also as a high-energy blender in

the production of salads and even as a dough kneader.

Meat grinders/choppers: this familiar machine is available in a vast range of

capacities and variations. Basically, a worm (screw) conveyor forces the meat

against one or more revolving knives and perforated plates (Figure 6.18).



The necessity to use knives and blades for cutting presents two problems:

●



The efficiency of the cutting machine strongly depends on the sharpness of the

knives. Although the blades are made of special metals, loss of sharpness is

always a problem, requiring costly maintenance.



Size Reduction of Solids, Equipment and Methods 173



Figure 6.18 Meat grinder. (Courtesy of Hobart)



2



3

1

2



4

5



Figure 6.19 Computer-aided water jet cutting. (Courtesy of Stein.DSI/FMC FoodTech)



●



The knives are often a source of contamination that transfer microorganisms to

the freshly produced new surfaces of the cut food. The steady increase of microbial count in the cut material in the course of a cutting operation is a frequently

observed phenomenon. The problem is particularly serious in the meat and

cold-cuts industry. Compliance with the need for frequent cleaning and sanitation operations is a factor to consider in the selection of cutting equipment.



This last point is elegantly addressed by a ‘knifeless’ cutting method, making use of a

narrow jet of water at very high velocity. In this method, water is pressurized up to many

hundreds of MPa and released as a narrow high velocity jet through nozzles made of

very hard materials, resistant to erosion. The jet is directed to the surface of the material

to be cut. By virtue of its very high kinetic energy, the jet penetrates and cuts through as

a sharp blade. The desired shape of the cut can be obtained by moving either the nozzle

or the material to be cut. The movement is often computer-controlled (Figure 6.19). The

application of water-jet cutting to food processes is still in the introductory stage.



174 Size Reduction



References

Earle, R.L. (1983). Unit Operations in Food Processing, 2nd edn. Pergamon Press,

Oxford, UK.

Gutsche, O. and Fuerstenau, D.W. (2004). Influence of particle size and shape on the

comminution of single particles in a rigidly mounted roll mill. Powder Technol 143,

186–195.

Hassanpour, A., Ghadiri, M., Bentham, A.C. and Papadopoulos, D.G. (2004). Effect

of temperature on the energy utilization in quasi-static crushing of α-lactose

monohydrate. Powder Technol 141(3), 239–243.

Jalabert-Malbos, M.L., Mishellany-Dutour, A., Woda, A. and Peyron, M.A. (2007).

Particle size distribution in the food bolus after mastication of natural foods. Food

Qual Prefer 18(5), 803–812.

Lee, C.C., Chan, L.W. and Heng, P.W. (2003). Use of fluidized bed hammer mill for size

reduction and classification. Pharm Dev Technol 8(4), 431–442.

Loncin, M. and Merson, R.L. (1979). Food Engineering, Principles and Selected

Applications. Academic Press, New York.

Ryan, B. and Tyffany, D.G. (1998). Energy use in Minnesota agriculture. www.ensave.com/

MN_energy_page.pdf

Servais, C., Jones, R. and Roberts, I. (2002). The influence of particle size distribution

on the processing of food. J Food Eng 51(3), 201–208.

Woodrow, J.R. and Quirk, A.V. (1982). Evaluation of the potential of a bead mill for the

release of intracellular bacterial enzymes. Enzyme Microb Technol 4, 385–389.

Yan, H. and Barbosa-Cánovas, G. (1997). Size characterization of selected food powders

by five particle size distribution functions. Food Sci Technol Int 3(5), 361–369.



7



Mixing



7.1 Introduction

The fundamental objective of mixing is to increase the homogeneity of material in

bulk (Uhl and Gray, 1966). In process technology, however, mixing or agitation may

be used to achieve additional effects such as enhancing heat and mass transfer, accelerating reactions, changing the texture etc. Very often, mixing occurs simultaneously

with size reduction, as is the case in foaming, homogenizing and emulsification.

The basic mechanism in mixing consists in moving parts of the material in relation

to each other. It is useful to discuss separately the mixing (blending) of liquids and the

mixing of solids. In liquid–liquid and liquid–gas mixing, the collision between the moving domains results in interchange of momentum or kinetic energy. The scientific discipline most relevant to liquid mixing is therefore fluid mechanics. The energy input per

unit volume is closely related to the quality of mixing. In contrast, the mixing of particulate solids such as powders is governed by the laws of solid physics and statistics.



7.2 Mixing of Fluids (blending)

7.2.1 Types of blenders

The simplest mixer for fluids is the paddle mixer (Figure 7.1), consisting of one and

sometimes two pairs of flat blades mounted on a shaft. For mixing liquids in hemispherical kettles and vessels with a bowl-shaped bottom, anchor mixers (Figure 7.2)

that conform the shape of the vessel walls are used. Paddle and anchor mixers are

usually operated at low speed (tens of revolutions per minute). Anchor mixers are

frequently used in jacketed cooking kettles and are often equipped with wipers that

scrape the product off the heated surface to prevent scorching.

In turbine mixers (Figure 7.3), the impeller consists of a larger number (four or

more) of flat or curved blades, mounted on a (usually vertical) shaft. Similar to the

impeller of a centrifugal pump (see Chapter 2), the turbine may be open, semi-closed

Food Process Engineering and Technology

ISBN: 978-0-12-373660-4



Copyright © 2009, Elsevier Inc.

All rights reserved



176 Mixing



Figure 7.1



Paddle mixer



Figure 7.2



Anchor mixer



B



Open



Semi-closed



Figure 7.3 Turbine mixers



(closed by a plate on one side) or shrouded (closed on both sides with a central opening

or ‘eye’ for circulation). Turbine mixers are usually operated at high speed (hundreds of

revolutions per minute (rpm)). They exert considerable shear on the fluid and are therefore suitable in applications involving mass transfer (e.g. oxygen transfer in fermentors) or phase dispersion (e.g. emulsification and homogenization). The diameter of the

impeller is, typically, one-third to one-half of the diameter of the vessel.

Propeller mixers (Figure 7.4) are primarily used to blend low viscosity liquids.

The shaft is usually coupled directly to the motor. Rotation speed is high, hundreds