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CHAPTER 20: SIZE REDUCTION – EQUIPMENT FOR SIZE REDUCTION, CARE ANDMAINTENANCEUnit operation in which the average size of solid pieces of food is reduced by theapplication of–grinding–compression–impact forcesRelated terms Homogenization or EmulsificationoReduction in size of globules of immiscible liquids AtomizationoSize reduction of liquids by droplets Extrusion, Agglomeration or FormingoSize enlargementBenefits in food processing Increase in the surface area to volume ratio of the food Increases the rate of drying, heating or cooling improves the efficiency and rate of extraction of liquid components Eg. Fruit juice, cooking oil extraction When combined with screening, a predetermined range of particle sizes isproduced which is important for the correct functional or processing properties ofsome products Eg. Icing sugar, spices, corn starch A similar range of particle sizes allows more complete mixing of ingredients Eg. Dried soup and cake mixesMethods Chopping, cutting, slicing and dicing Large to medium (Cheese and sliced fruit for canning) Medium to small (Diced carrot) Small to granular (minced meat, flaked fish or nuts and shredded vegetables) Milling to powders or pastes Grated products Emulsification and homogenization

TheoryThree types of forces–Compression–Impact–Shearing or Attrition forcesStress is applied to a food; the resulting internal strains are first absorbed to causedeformation of the tissues. Amount of energy needed to fracture a food is determined by itshardness and tendency to crack – in turn depends on the structure of the food. Fewer the linesof weakness in a food, the higher are the energy input needed to cause fracturing. Harder foodsabsorb more energy and require a greater energy input to create fractures. Extent of sizereduction, the energy expanded and the amount of heat generated in the food depend on boththe size of the forces that are applied and the time that food is subjected to the forces.Compression forces are used to fracture friable or crystalline foods. Combined impact andshearing forces are necessary for fibrous foods. Shearing forces are used for fine grinding ofsofter foods.Factors influencing the energy input Moisture contentoWheat is conditioned to optimum moisture contentoMaize is thoroughly soaked and wet milled for complete disintegration Heat sensitivity of the foodoDetermines the permissible temperature rise and the necessity to cool the milloLiquid N 2 or solid CO 2 are mixed with foods before milling to cool the product and toretain volatiles Quantum of the forces that are applied Time that food is subjected to the forcesEquations for energy requirement determinationKicks law–Energy required to reduce the size of particles is directly proportional to the ratioof the initial size of a typical dimension to the final size of that dimensionE K k ln (D f /D p )E – Energy required per mass of feedK k – Kick’s constantD f – Average initial size of feedD p – Average size of product

D f / D p – Size reduction ratio Coarse grinding has RRs below 8:1 Fine grinding can exceed 100:1 Grinding of coarse particles in which the increase in surface area per unit mass isrelatively small, Kick's Law is a reasonable approximation Rittinger’s law–States that the energy required for size reduction is proportional to the change insurface area of the pieces of food E KR11DpDf K R – Rittinger’s constant For the size reduction of fine powders, in which large areas of new surface are beingcreated, Rittinger's Law fits the experimental data better.Bond’s law The work required to form particles of size Dp from very large feed is proportional to thesquare root of the surface to volume ratio of the product P/f 0.3162 wi 1 Dp1 DfP – Power in kWf – feed rate, t/hrDp – 80% of the product passes through mesh of dia Dp, mmDf – 80% of feed passes through mesh of dia, Df, mmWi – Work indexGross energy requirement in kilowatt-hour per tonne of feed needed to reduce a verylarge feed to such a size that 80% of the product passes through a 100 micro meter screen An ideal size reduction equipment should fulfill the following conditions–Large capacity–Should yield a pre desired sized product or range of size–Small power input requirement per unit of product handled–Easy & trouble free operation Amount of power required to create smaller particles Desired uniformity of size

Work required to strain the material is temporarily stored in the form of mechanicalenergy of stress. When external force exceeds the amount of stored mechanical energy, the material isdisturbed beyond its strength and finally broken in to fragments – results in creation ofnew surface Solids have a certain amount of surface energy, thus for creation of new surface, work isrequired and supplied by the release of stress energy when material breaks. Stress energy excess of the new surface energy create is converted into heat energy.Grinding Classified in to two Plain grinding– Milled to a free flowing meal consisting of sufficiently uniform particle sizeSelective grinding–Grinding operation is carried out in various stages depending upon thedifferences in structural and mechanical properties of components of grainDegree of grinding Ratio of the overall surface area of the ground product to the overall surface area of thefeedDg Sp/SfDg – degree of grindingSp – Overall surface area of productSf – Overall surface area of feedMilling efficiency Product of coefficient of hulling (E hulling )and coefficient of wholeness of kernel (E wk) Coefficient of hulling– Percentage of the hulled grains obtained from the total amount of grain inputCoefficient of Wholeness of kernel–Ratio of the amount of kernel, crushed grains and mealy waste obtained by anymilling systemSize reduction machinery Crushers Grinders Fine grinders Cutting machines

Crushers Squeeze or press the material until it breaks Mostly used to break large pieces of solid materials into small lumps Use of crushers in agricultural operations is limitedTypes–Jaw crushers–Gyratory crushersJaw crusher Feed is admitted between two jaws, which are open at the top like V One of the jaws is fixed and vertical, while the other is the swinging jaw This jaw reciprocates in a horizontal plane and makes the angle of 20-30 with the fixedjaw Movable jaw is operated by an eccentric unit so as to impart great compressive force Solids which has to be broken is caught between the two jaws Large lumps of solid materials are caught between the upper parts of the jaws Subsequently broken and dropped into the narrower space below Broken pieces are further reduced next time when jaws come closer. No. of strokes given to the movable jaw ranges between 250 to 400 times per minuteGyratory crusher Jaws between which the solid materials fed are circular Material is being crushed at all times at some point

Solids are caught between V shaped space between the head and casing Material is repeatedly broken in sufficiently small pieces to pass out from the bottom. Speed of crushing ranges between 125 to 425 gyrations per minute Discharge from the gyratory crusher is continuous Less maintenance is required as compared to jaw crusher Power requirement is lowCrushing rolls Mainly used for extraction of juice from sugarcane Two types–Smooth roll crusher–Serrated or toothed roll crusherSmooth roll crusher Two heavy smooth faced roll rotating towards each other at same speed on parallelhorizontal axes Size of the material caught by the rolls depends upon the coefficient of friction betweenthe material and the roll surface Dp 0.04R gDp – maximum size of particleR – roll radiusg – half of the width of gap between the rolls Used to make grits or meal from food grains

One of the rolls should be spring loaded to avoid any damage to roll surface Extensively used for making food grains flakesSerrated or toothed roll crusher Rolls are serrated as per need Much more versatile than smooth roll crusher Best example – Break and reduction rolls of wheat milling Disintegrators are toothed roll crushers in which the corrugated rolls are rotating atdifferent speeds Size reduction is by compression, impact and shear and not by compression alone, as inthe case of smooth roll crushers Can accommodate larger particles than smooth roll crushersCrushing efficiency Ratio of the surface energy created by crushing to the energy absorbed by the solidGrinders Used to mill the grains into powder Types–Attrition mill–Hammer mill–Impactors–Rolling compression millAttrition mill Also known as burr mill Grains are rubbed between the grooved flat faces of rotating circular disks Axis of the roughened disks may be horizontal or vertical One plate is stationary and fixed with the body of the mill while the other one is rotatingdisk Material is fed between the plates and is reduced by crushing and shear Mills with different patterns of grooves, corrugations on the plates perform a variety ofoperations Overfeeding–lowers grinders performance–Increases heat generation during millingDisks are 20-137 cm in dia and operated at 350 to 700 rpm

Used for making whole grain and dehusked grain flour Use in spice grinding is limited Double runner disks type attrition mills are also available Used for grinding of soft materials Both disks are driven at high speed in opposite direction Operated between 1200 to 7000 rpm Capacity is largeSalient features Fineness of grinding is controlled by the type of plates and the gap between them Spacing between the plates is adjustable Arrangement is spring loaded–to avoid damage to plates in case of overloading–to overcome the damage to plates by foreign material coming along with thefeed Lower initial cost Lower power requirementsHammer mill Used for various types of size grinding jobs Size reduction takes place by impact force Consists of high speed rotor rotating inside a cylindrical casing Shaft is usually kept horizontal

Materials are fed into the mill from the top of the casing and is broken by the rotatinghammers and fall out through a screen at the bottom Feed is broken by the fixed or swinging hammers, pinned to a rotor Hammers are rotated between 1550 to 4000 rpm, strike and grind the material until itbecomes small enough to pass through the bottom screen Fineness of grinding is controlled by the screen size There is less chances of damage of hammer in swinging hammer mill Can grind tough fibrous solids, steel chips, food grains, hard rock etc. Assumed to reduce size by impact of hammers

Salient features Simplicity and versatility in design Less chances of damage due to foreign objects High power requirement Capacity and power requirement depend on the nature of feed to be ground Used for poultry feed grinding, spices grinding Suitable for grinding of wet sorghum and millets Also used for potato, tapioca, banana flour makingBall mill Cylindrical or conical shell slowly rotating about a horizontal axis. Half of its volume is filled with solid grinding balls Shell is made of steel lined with high carbon steel plate, porcelain or silica rock. Size reduction is achieved by impact of the balls when they drop from near the top of theshell Energy consumed in lifting the balls is utilized for grinding job When the ball mill is rotated , the balls are carried by the mill wall nearly to the top Balls are released by the gravitational pull and drop to the bottom and picked up again Centrifugal force keeps the ball in contact with the mill wall. Due to centrifugal force, if the speed of rotation of mill is faster, the balls are carried tomore distance. Centrifuging: In case of too high speed, balls stick to mill wall and are not releasedCritical speed: Rotational speed at which centrifuging occurs At this speed, no impact occurs hence little or no grinding results Operating speed must be kept less than the critical speed Speed at which the outermost ball released from the mill wall depends on the interactionof gravitational and centrifugal forces Critical speed can be determined by nc 1/2 g/R-rnc critical speed, revolution/secg acceleration due to gravity, 9.8 m/s2R- radius of the mill, mr radius of the ball, m

Roller mills Roller mills are similar to roller crushers They have smooth or finely fluted rolls, and rotate at differential speeds. They are used very widely to grind flour. Because of their simple geometry, the maximum size of the particle that can passbetween the rolls can be regulated. If the friction coefficient between the rolls and the feed material is known, the largestparticle that will be nipped between the rolls can be calculated, knowing the geometry ofthe particles.