Food Factors Affecting the Selection of the Size Reduction Process:

Criteria for the Selection of Equipment

Food processors have to design a size reduction process and decide on the appropri-ate equipment. When selecting a size reduction machine, the following factors of the food material feed should be considered: (a) mechanical and structural properties, (b) moisture content, (c) temperature sensitivity, and (d) the size distribution of the feed as well as of the product (Barbosa-Canovas et al., 2005b; Brennan, 2006).

On the basis of these criteria, the most appropriate equipment can be selected in each case. Additionally, the size and other features, such as the capacity and rotational velocity of the machine, should be evaluated and its operating variables should be normally adjusted. Finally, good practices regarding loading, operation, or stopping, which do not exceed the critical speed, cleaning, maintenance, and so on, should be followed to exploit the complete grinding capacity of the mill.

2.4.5.1.1 Mechanical and Structural Properties of the Feed

The mechanical and structural properties of the feed material (i.e., hardness and fri-ability) are of major importance in selecting the suitable equipment for size reduction.

The knowledge of these parameters is useful to determine the type of force needed in performing the size reduction. Moreover, the design of a size reduction operation for a certain food material may require experimental determination of the process-ing parameters. Accordprocess-ingly, requirements for the wearprocess-ing material, the workprocess-ing surfaces, the easy replacement of moving parts, and the entire construction of mills should be met.

Friable and crystalline materials may fracture easily, with larger particles break-ing down more easily than smaller particles, startbreak-ing from the current cleavage planes and then creating new crack tips. So, crushing using compressive forces is initially recommended followed by impact and shear forces. Roller mills are usually employed for such materials.

Hard food materials may be either brittle, fracturing rapidly above the elastic limit, or ductile, deforming extensively before breakdown. As already mentioned, the harder the material, the more difficult it is to break down and more energy is required. This should be taken into account in the case of very hard materials, where larger mills or extended milling may be required. Since many dry food materials are brittle and fragile, compressive forces are recommended. Finally, for most solid foods, ball mills, hammer mills, roller mills, and attrition mills are commonly used in the food industry.

Foods with fibrous structure exhibit increased toughness and resistance to break-down, requiring cutting than crushing (i.e., cutting of roasted whole beans to ground coffee with particles of suitable size and shape). Disk mills, pin-disk mills, or cutting devices are usually employed to break down such materials.

2.4.5.1.2 Moisture Content of the Feed

The moisture content and distribution significantly affect both the degree of size reduction and the mechanism of breakdown in some foods. Each food material must have the optimum moisture content for milling to achieve the expedient performance;

Size Reduction 41

if it is above or below that limit, problems may arise in the product’s quality or the mill’s safety. Therefore, before milling, the food materials are subjected to condi-tioning or soaking to obtain the optimum moisture content; such examples are the dry milling of wheat after conditioning and the wet milling of maize after soak-ing to attain complete disintegration of the starchy materials. When exceedsoak-ing the optimum moisture content upward or downward, agglomeration of the product’s particles, which generates difficulty in the product’s free flowing and blocking of the mill, or problems due to dust formation associated to human health or inflam-mation and explosion may arise, respectively. Anyway, in both cases, these have an adverse impact on the efficiency of the process. Thus, in the case of wheat, excessive moisture content may deform rather than break, without allowing the release of the endosperm, while, in the case of dry milling, fine particles of bran are created that cannot be separated by the screens, thereby contaminating the white flour.

2.4.5.1.3 Temperature Sensitivity

Considerable amounts of heat, generated during the mill’s operation, lead to a con-siderable temperature increase that can adversely affect the quality of food mate-rials subjected to grinding. To protect the heat-sensitive and volatile components of foods, milling should be performed under reduced temperatures. For this rea-son, mills equipped with cooling systems or cryogenic milling (by mixing the feed with solid carbon dioxide or liquid nitrogen before milling) are recommended. Such examples are the milling of fibrous materials such as meat or the fine grinding of roasted coffee that is facilitated under cryogenic conditions and the cool milling of spices to prevent the undesirable heating effects on sensitive volatile components.

Also, the increase of surface temperature during grinding of sugar may lead to over-stepping, locally, the glass transition temperature. This results in the generation of an amorphous surface layer that is hygroscopic and may induce recrystallization and consequently agglomeration of the sugar particles.

2.4.5.1.4 Size Distribution of Feed and Product

Each type of crusher or grinder is intended for a certain particle size of the feed and product. Recommendations for selecting the appropriate equipment in relation to the food material and the reduction range, as shown in Table 2.1, should be taken into account. Size control and/or screening of the feed material allow the proper opera-tion of the equipment, thereby protecting from jamming, and so on, while monitor-ing of the product’s size and distribution is necessary, in particular for the quality control of the final product.

2.4.5.2 Mills

2.4.5.2.1 Classification

There are various types and sizes of equipment available for use in size reduction or comminution operations in the food- processing industry. They are commonly clas-sified according to their basic function into one of three classes:

(a) Crushers that use compressive forces and are mainly used for coarse reduction;

42 Food Engineering Handbook

TABLE 2.1 Size Reduction Equipment: Types, Characteristics, and Applications in Food Processing Generic Name of EquipmentType of EquipmentOperating ForceFineness RangeTypical Products CompressionImpactShearingAttritionCuttingCoarse lumps1Coarse grits2Interme -diate3Fine–Ultra fine4 CrushersCrushing rolls**Cane sugar, wheat Grinders— mills(i) Roller mills (crushing rolls) Roller mills*****Wheat, roasted coffee, and chocolate refining (ii) Impact (percussion) mills Hammer mill***Spices, sugar, dried milk, cocoa press cake, dry fruits, and vegetables Beater bar mill****Dried vegetables, spices Comminuting mill**Fruits, vegetables Pin (pin-disk) mill****Sugar, starch, roasted nuts, cocoa powder, and spices

Size Reduction 43

(iii) Attrition mills Single-disk attrition mill***Wheat, corn, cocoa kernels, sugar, spices, and nuts Double-disk attrition mill*** Buhrstone mill*** Colloid mill*Pastes and purees (iv) Tumbling mills Ball mills***Fine grinding, cocoa mass Rod mills***Sticky products Cutters(i) Slicing and flaking equipment**Meat, fruits and vegetables, and dry particulate foods (ii) Dicing equipment** (iii) Shredding equipment*** (iv) Pulping equipment Bowl chopper*** 1:1–0.1 m, 2:10–1 cm, 3:10–1 mm, and 4:100–10 µm.

44 Food Engineering Handbook (b) Grinders or mills that combine shear and impact with compressive forces.

They can be additionally divided into force grinders, employed in interme-diate and fine reduction, or ultrafine grinders; and

(c) Cutting machines used for exact reduction.

Another classification of the size reduction equipment can be made according to the feed material into: (a) an equipment for size reduction of dry foods (including cat-egories A and B of the previous classification) and (b) equipment for size reduction of fibrous foods (including category C of the previous classification).

There are several types of machines included in each category of the above clas-sification (Barbosa-Canovas et  al., 2005b; Brennan et  al., 1990; Brennan, 2006;

Fellows, 2000; Leniger and Beverloo, 1975; Meuser, 2003). The appropriate equip-ment is selected according to its operation principles and individual characteristics as well as the desired reduction range for its applications in specific food materials as shown in Table 2.1. Nowadays, to address claims, such as reduced noise, operat-ing temperatures, and dust generation, or to satisfy the requirements of modern food industries (i.e., controlled comminution, high speed of production, micro-fine pow-ders specification), new, improved types of size reduction equipment (i.e., ultrasonic cutters) are commercially available.

2.4.5.2.1.1 Crushers

Jaw and gyratory crushers: The jaw crusher consists of two heavy jaws, one fixed and the other reciprocating (placed sideways). The feed material is introduced between the jaws and as it proceeds downward, into the nar-rowest space of the machine, it is forced to crush. The gyratory crusher consists of a truncated conical fixed casing and inside this, a crushing head, shaped as an inverted cone, which is rotated eccentrically. The material is fed between the outer and the inner gyrating cones and crushed as it is forced into a narrowest space of the machine. These crushers are no longer widely used in the food industry (Earle, 1983).

Crushing rolls: This type of crusher consists of two large horizontal, smooth-surfaced cylinders, placed close together in parallel, which are driven in opposite directions at the same or different speeds. The material is fed into the space between the rotating rolls, and as it passes through it is nipped and crushed. Such crushers usually achieve an RR of 4 or lower. An example of their use is with the cane sugar industry.

2.4.5.2.1.2 Grinders or Mills

Roller mills (crushing rolls): A roller mill contains two (or more) smooth or finely fluted, corrugated, and grooved steel horizontal rolls, placed close together in parallel and rotating at different speeds. The food particles are fed from above in the space between the rollers (nip), then nipped and pulled through (Figure 2.2). The main force contributing to the breakdown is compression; if the rollers are rotated at different speeds, or if the rollers are fluted, then there is the addition of shear force. The nip is adjustable,

Size Reduction 45

depending on the particle size of the feed, and their overloading must be avoided, protecting the rollers against damage. If necessary, the surface of the rolls may be cooled or heated (Barbosa-Canovas et al., 2005b; Brennan, 2006; Fellows, 2000; Meuser, 2003). There are alternative roller mills that carry breaker bars or teeth on the roller surfaces, increasing the friction and facilitating the trapping of particles, or, as in the case of edge mills, they consist of one roll operating against a flat or curved stationary plate (Brennan, 2006).

The surface of the rolls are selected and their operating speeds are adjusted according to the feed and product material (i.e., fluted, corrugated, or grooved surfaces for high RR whereas smooth surfaces for fine milling). For a certain roller mill system, the maximum permissible particle size of the feed and the theoretical mass flow rate of the product can be estimated. In practice, the available roller mills have diameters >500 mm and rotating speeds in the range of 50–300 rpm; smaller rolls may operate at higher speeds. Roller mills are widely applied in the milling of wheat, in roasted coffee grinding, and in the refining of chocolate. Usually, high RRs are achieved using two or more pairs of rolls in sequence with increasing smoothness and decreasing nip size from one pair to the next, with each milling step to be accompanied by sieving. Wheat grains are milled by this process.

Impact (percussion) mills: A hammer mill contains a high-speed rotor mounted on a horizontal shaft inside a cylinder casing; a toughened plate (breaker plate) is also fitted inside the casing. The rotor carries swinging hammerheads

Feed

Roll

Relief spring

Product

FIGURE 2.2 A roller mill (crushing rolls).

46 Food Engineering Handbook around its periphery that pass within a small clearance of the casing (Figure 2.3). As the hammers drive, the fed material is forced against the breaker plate. The ground particles may pass through a screen mounted in the bot-tom of the casing; so, a product of a certain diameter is obtained (choke feeding). Fracture of particles is mainly by impact forces, although shear forces or even attrition forces under choke feeding can also evolve. Hammer mills are usually heavy devices of large capacity, with rotors rotating at 1.000–1.200 rpm. The operation of mills with fibrous or sticky materials may be associated with screen blockage; therefore, mills must be equipped with removable screens and/or with cooling systems. Also, a high propor-tion of fine particles in the product may result in the grinding of friable materials (Barbosa-Canovas et al., 2005b; Brennan, 2006; Fellows, 2000;

Meuser, 2003).

A wide variety of materials can be handled in hammer mills, from hard and friable to fibrous and sticky; for fibrous materials, knives instead of ham-mers may be used to give a cutting action. They are widely used in the food industry for various materials including spices, sugar, dried milk, cocoa press cake, dry fruits, and vegetables.

The beater bar mill carries bars instead of hammers, the tips of which pass within a small clearance of the casing.

Feed

Hammer

Screen

Product

FIGURE 2.3 A hammer mill.

Size Reduction 47

The comminuting mill carries knives instead of bars or hammers that can be either hinged to the shaft and swing out as it rotates or be rigidly fixed to it. They are used in relatively soft materials, such as fruits and vegetables.

Moreover, the knives may be sharp on one edge and blunt on the other;

when they rotate in the first direction, they perform a cutting action while on the opposite direction, they act as beater bars (Earle, 1983).

A pin (pin-disk) mill contains two disks, a stationary and a rotating, located with a small clearance between them. Both disks have pins, pegs, or teeth attached on the surface so that the rows of one disk fit alternately into the rows of the other disk. The material is subjected to impact and shear forces between the stationary and rotating pins (Figure 2.4). Disk speeds may be up to 10.000 rpm. In other types of mills, it is possible to operate in a choke-feed mode (with the product’s screen attached), to rotate the two disks in the same direction at different speeds, or in opposite directions, to adjust the clearance between the disks, to feed the material by air stream, or to control the temperature. Improved effectiveness is attained in pin-and-disk mills with intermeshing pins fixed either to the single disk and casing or to double disks. Such mills are used for fine grinding of friable materials and for breaking down fibrous substances (i.e., sugar, starch, roasted nuts,

FIGURE 2.4 A pin mill.

48 Food Engineering Handbook cocoa powder, and spices) (Barbosa-Canovas et al., 2005b; Brennan, 2006;

Fellows, 2000; Meuser, 2003).

In fluid energy (jet) mill, the feed material is suspended in a high-velocity gas stream into a chamber where the breakdown occurs through the impact between individual particles and the chamber wall, while the product is recovered by an air–solids separation cyclone system.

Attrition mills: There are several designs of attrition mills (disk or plate mills).

They use shear forces for fine grinding or shearing and impact forces for coarser grinding. The attrition mills are employed in the dry milling of wheat and wet milling of corn. Other applications include grinding of cocoa kernels, sugar, spices, and nuts (Barbosa-Canovas et al., 2005b; Brennan, 2006; Fellows, 2000; Meuser, 2003).

A single-disk attrition mill has a high-speed rotating grooved disk close to a stationary disk with matching grooves. The material is fed at the center of the stationary disk, passing through the disks and breaks due to intense shearing between the disks and pressure subjected (Figure 2.5a). The gap between the two disks is adjustable, and a product screen may be attached (Brennan, 2006; Fellows, 2000).

A double-disk attrition mill contains two counter-rotating disks with match-ing grooves, located close to each other. The breakdown of the material is caused by shear forces and the product is discharged, passing through a screen; however, a greater degree of shear is achieved compared to the single-disk mill (Figure 2.5b). Both types of disk attrition mills are used for corn and rice milling (Brennan, 2006; Fellows, 2000).

The Buhrstone mill consists of two circular stones mounted on a vertical axis of which the upper stone is fixed, while the bottom stone rotates. The feed material moving between the grooved stones is gradually ground by a scis-sor action and is discharged over the edge of the lower stone (Figure 2.5c).

In some Buhrstone mills, both stones rotate, in opposite directions. It is the oldest attrition mill, used for wheat milling for centuries, and is still used today for wet milling of corn and whole-grain flour production.

The Foos mill is a modified pin mill with studs instead of grooves, with similar applications as the other disk mills.

The colloid mill is also an attrition mill used for emulsification as well as in pastes and purees.

Tumbling mills: Tumbling mills consist of a horizontal slow-speed rotating cylin-der, sometimes with conical ends, half-filled with a solid-grinding medium, most commonly balls or rods. As the cylinder shell rotates, the grinding medium units are lifted up the sides of the shell and then dropped onto the material being milled, which fills the void spaces between the grinding medium units. In this method, effective size reduction is achieved by exert-ing impact and shearexert-ing action on the feed material (Figure 2.6). A com-mon problem in tumbling milling is the product’s contamination because of

Size Reduction 49

the wear of the grinding medium. A usual application of tumbling mills is in fine grinding (Barbosa-Canovas et al., 2005b; Brennan, 2006; Fellows, 2000; Meuser, 2003).

In ball mills, the cylinder shell is commonly made of steel lined with carbon-steel plate, porcelain, silica rock, or rubber while the grinding medium are steel (or porcelain or zircon) balls (25–150 mm in diameter), occupying 50% of the shell volume. Pebble mills use flint pebbles as grinding materi-als. At low rotation speeds or if the balls used are small, shearing forces predominate. At higher rotation speeds or with the use of larger balls, impact forces are generated, contributing to the size reduction of the feed material (Brennan, 2006; Fellows, 2000). Ball mills operate under batch or continuous processes. They are used for fine grinding, both under dry or wet conditions (i.e., fluid cocoa mass).

Feed (c)

Product Product

Feed Rotating disk

Rotating disk

Rotating disk Fixed

disk

Product

(a) (b)

Product Feed

FIGURE 2.5 Disk attrition mills: (a) single-disk mill; (b) double-disk mill; and (c) Buhr mill.

50 Food Engineering Handbook Another type is the trunnion overflow mill in which the feed’s material passes

through a hollow trunnion at one end, and the ground product overflows at the opposite end (Figure 2.6a). In compartment mills, grinding can be performed over sequential compartments of the mill using different grind-ing medium (large balls, small balls, and pebbles), thus increasgrind-ing the effi-ciency of the milling operation (Figure 2.6b). Finally, the conical ball mills (Figure 2.6c) consist of a single cone-shaped section that contains balls of different sizes. The material is fed through the widest part of the mill while the product leaves through the narrowest part of the cone. As the shell rotates, the large balls move toward the feed point while the small balls migrate toward the discharge outlet. The initial breaking of the feed mate-rial is thereby performed by the largest balls dropping the greatest distance, whereas the final reduction of small particles is done by small balls drop-ping a smaller distance (Barbosa-Canovas et al., 2005b; Brennan, 2006).

Feed

Feed

Feedinlet

Rotating cylinder

Rotating cylinder

Large balls

Large balls

Medium balls Small balls

Small balls

Product

Product

Product Conical grate Drive gear

(c)

Balls (a)

(b)

FIGURE 2.6 Tumbling mills: (a) trunnion overflow mill; (b) compartment mill; and (c) conical mill.

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In vibration-type ball mills, impact forces predominate and very fine grind-ing is attainable. In the attritor mill, grindgrind-ing is achieved by slow stirrgrind-ing of the balls together with the feed particles, which generates shear and probably impact forces. It is commonly used in wet milling (i.e., chocolate manufacture).

In rod mills, instead of balls, rods (of circular, square, or hexagonal cross

In rod mills, instead of balls, rods (of circular, square, or hexagonal cross

In document Food Engineering Handbook- Food Process Engineering (2014) (2014) (Page 64-76)