SIZE REDUCTION

Prepared by: Azwin bt Ahmad

INTRODUCTIONThe size reduction or ‘comminution’ is a unit operation in which the average size of solid pieces of food is reduced by application of grading, compression or impact forces.

However, screening is the unit operation in which a mixture of various sizes of solid particles is separated into two or more fractions by passing over a screen.

The breaking of large pieces of coal, or stone by a primary breaker, or of small pieces by grinding equipment.

DISISTEGRATION OF FIBRIOUS MATERIALSDISISTEGRATION OF FIBRIOUS MATERIALS

SLICING

Sliced fruits are much in demand as a desert dish. They present an attractive appearance, the portions produced being of convenient size for eating. Rotary cutting knives are usually employed, the knives being set to cut the material being presented to them often on a vibrating belt into parallel slices or of the desired thickness. In other fruit slicing operations the fruit is forced through a tube containing stationary knife edges arranged radially along the length of the tube. This type of assembly can produce decored, wedge shaped sections from firmer fruits such as pineapple.

Slicing equipment

DICING

Dicing is the cutting of materials into cubes that usually follows a preliminary slicing operation which produces slices of the desired thickness. These slices are then fed onto a conveyer belt containing a series of studs which hold the slices in position as the belt carries them against the rotary knife assembly which cuts them into strips. The strips then pass through a further cutting zone at right to the stripping section. This produces the required cube. Dicing Is a method of food preparation in which the food item is cut into small blocks or dice.

Dicing equipment

SHREDDING

Shredding is torn into small fragments, the average size of the pieces depending on the type of machine and on residence time in the action zone.

Shredding often precedes dehydration, the increase in surface aiding the rate process. Hammer mills are commonly used as shredders.

PULPING

Pulping is widely used in the processing of lower grade fruits separated during grading.

The fruit, which is edible but unacceptable for whole fruit processing, may be used in jam manufacture.

A combination of pulping and sieving is used with many fruits and vegetables.

A common equipment used for pulping consists of a cylindrical perforated screen containing high speed rotating brushes.

Material is fed to the inside of the cylinder, the brushes forcing pulped material through the perforations.

PRINCIPLES OF SIZE REDUCTION

The breakdown of solid materials through the application of mechanical forces is a requirement in many food processing operations. The reason for size reduction are varied.

A) Size reduction may aid the extraction of a desired constituent from a composition structure, for example flour from wheat grains or juice from sugar cane.

B) Reduction to a definite size range may be a specific product requirement. For example as in the manufacture of icing sugar, in the preparation of spices and in chocolate refining.

C) A decrease in particle size of a material leads to an increase in surface of solid. This increase in surface is of assistance in many rate processes.

D) Intimate mixing or blending is usually easier with smaller size range of particles, an important consideration in the production of formulated, packaged soups, mixes, etc.

For example:A) The drying time for moist solids is much reduced by

increasing the surface area of the solid.

B) The rate of extraction of a desired solute is increased by increasing the contact area between solid and solvent.

C) Process time require for certain operations such as cooking, blanching, etc than can be reduced by cutting, shredding, or dicing the process.

D) Intimate mixing or blending is usually easier with smaller size ranges of particles, an important consideration in the production of formulated, package soups, cake mixes, etc.

Force that involve in size reduction

Force Principle Machine

Compressive Nutcracker Crushing rolls

Impact Hammer Hammer mill

Shear Grindstone Disc attrition mill

• Compressive – used for the coarse crushing of hard materials.

• Impact - can be regarded as general purpose forces and are used for coarse, medium and fine grinding of a variety of food materials.

• Shear force – for fine grinding

Reduction ratioThe ratio: average size of feed average size of productKnown as the reduction ratio and can be use in

predicting the likely performance of a particular machine.

Coarse crushers have size reduction ratio below 8:1 but for fine grinding, ratio as high as 100:1 can be realised.

The ratio depends on machine, feed materials and method of measurement.

Consideration governing equipmentselection

• The aim of an economic size reduction process is to achieve the desired reduction at minimum cost.

• Characteristics of the feed materials, possible machines and of the product material will be necessary.

Hardness and abrasiveness of feed

• Hard materials are more difficult to comminute.• This may necessitate either:a. A slower throughput for a given millb. A larger capacity mill to satisfy a particular duty c. These surfaces should be fabricated in hard

wearing materials of construction, such as manganese steel, and should be easily replaceable

Mechanical structure of feedMechanical structure of feed

• A knowledge of the mechanical structure of the feed material can indicate the type of force most likely to effect disintegration.

• If the material is friable, or has a crystalline structure, fracture may occur easily along cleavage planes, larger particles fracturing more readily than smaller ones. In these cases, crushing using compressive force is employed

• If few cleavage plans are present and new crack tips have to be formed, impact and shear forces may be better.

MoistureMoisture

The presence of water can both aid and The presence of water can both aid and hinder a comminution process.hinder a comminution process.Moisture content in excess of 2%-3% can Moisture content in excess of 2%-3% can be lead to clogging of the mill.be lead to clogging of the mill.Agglomeration or balling can also occur in Agglomeration or balling can also occur in the presence of moisture.the presence of moisture.This is undesirable if a free flowing, finely This is undesirable if a free flowing, finely powdered food material is required.powdered food material is required.

Temperature sensitivity of Temperature sensitivity of feedfeed

►Heat arising can lead to a considerable Heat arising can lead to a considerable rise in temperature of the material rise in temperature of the material being processed and degradation being processed and degradation could occur.could occur.

►Heat generated to development of a Heat generated to development of a sticky charge, clogging of the mill and sticky charge, clogging of the mill and a reduction in efficiency of the process a reduction in efficiency of the process may well result.may well result.

Size reduction equipmentSize reduction equipment► Crushing rollsCrushing rolls► Two or more heavy steel Two or more heavy steel

cylinders revolve cylinders revolve towards each other.towards each other.

► Particles of feed are Particles of feed are nipped and pulled nipped and pulled through the rolls, through the rolls, experiencing a experiencing a compressive force with compressive force with crushes them.crushes them.

► In some machines a In some machines a differential speed is differential speed is maintained between rolls maintained between rolls and shearing force also and shearing force also arise.arise.

Crushing rollsCrushing rolls

► Hammer millHammer mill► A high speed rotor carries a A high speed rotor carries a

collar bearing a number of collar bearing a number of hammers around its hammers around its periphery.periphery.

► Feed passed into the action Feed passed into the action zone where the hammers zone where the hammers drive the materials against drive the materials against the beaker plate.the beaker plate.

► Reduction is mainly due to Reduction is mainly due to impact force.impact force.

► The hammers are often The hammers are often replaced by cutter or by replaced by cutter or by bars as in the beater bar bars as in the beater bar mill.mill.

Hammer millHammer mill

Calculation for size Calculation for size reduction of solids using reduction of solids using

Rittinger’s LawRittinger’s Law

12

11xx

KE

Where,X1 = The average initial feed size

X2 = The average final product size E = Energy per unit mass required for the

production of this new surfaceK = Rittinger’s constant (particular machine and

material)

Example:Example:Calculation for size reduction of solids Calculation for size reduction of solids

using Rittinger’s Law :using Rittinger’s Law :E=K [1/xE=K [1/x11 – 1/x – 1/x22]]XX1 1 == The average initial feed sizeThe average initial feed sizeXX2 2 == The average final productThe average final productE = Energy per unit mass required for E = Energy per unit mass required for

the production of this new surfacethe production of this new surfaceK = Rittinger’s constantK = Rittinger’s constant

Suatu bahan makanan dikisar daripada 6 x 10-3 m kepada 0.0012 x 10-3m menggunakan motor yang berkuasa 10hp. Adakah motor ini mampu untuk

mengecilkan saiz partikel kepada 0.0008 x 10-3m

1hp = 745.7w N.P = [ 6 x 10-3 / 0.0012x 10-3] = 5000

Penyelesaian :E=K [ 1/x2 – 1/ x1] L1 7457 = K [ 1/ 0.0012x10-3 - 1/6x10-3]L2 K = 7457 / 833166.67 = 8.95 x 10-3 L3 E = 8.95 x 10-3 [ 1/0.0008 x 10-3 -1/6 x 10-3] E = 11186.0w/ 11.12 kw /15hpMotor tersebut tidak sesuai digunakan.

Bahan serbuk dengan hablur bersaiz 800µm dikisar kepada saiz 125µm dengan menggunakan pengisar berkuasa 8.5 hp. Jika bahan hendak dikisar kepada 160 µm , apakah pengisar mampu untuk

melakukan pengisaran tersebut.NP = x1/x2 = 800/125 = 6.4 (Hukum Kick)

E = K ln(x1/x2)

K = 6.34 x 103

ln(800/125) = 3.415 x 10-3

Jika bahan dikisar kepada 160 µm

E=K ln(x1/x2)

E=3.415 x 103 ln(800/160) = 5.496 x 103 w = 7.37 hp

SIZE REDUCTION EQUIPMENT