Size reduction

Size reduction In Food Processing
Introduction, Principle of size reduction, Forces used in Size Reduction, Energy requirements in Size Reduction, Size reduction equipment,
Kick’s law, Rittinger’s law, Bond’s law

1. Presented By : Maga Ram Patel College of Technology and Engineering, MPUAT , Udaipur Presentation on “ Size reduction In Food Processing ”

2. Point to be covered Introduction, Principle of size reduction Forces used in Size Reduction Energy requirements in Size Reduction Size reduction equipment

3. Introduction Size reduction is a process of reducing large solid unit masses into small unit masses, coarse particles or fine particles. Size reduction process is also termed as Comminution/Diminution/Pulverizations. Solid pieces of food is reduced by the application of grinding, compressing or impact forces. In many food process its frequently necessary to reduce the size of solid materials for different purposes .

4. Benefits in food processing Increase in the surface area to volume ratio of the food. Increases the rate of drying. Increases the rate heating or cooling. Improves the efficiency and rate of extraction. Facilitating mixing and blending. Facilitates heat exchange, chemical and biological reactions.

5. Classification of Size Reduction Chopping, cutting, slicing and dicing: Large to medium Stewing steak Cheese and Sliced fruit for canning Medium to small Sliced green beans and Diced carrot Small to granular Minced or shredded meat Flaked fish or nuts and Shredded vegetables

6. Continued….. Milling to powders or pastes of increasing fineness Grated products Spices Flours Fruit nectars Powdered sugar starches Smooth pastes

7. Continued….. Emulsification and homogenization Milk Essential oils Butter Ice Cream and margarine

8. Forces used in size reduction The type of forces commonly used in food processes are compressive, impact, attrition or shear and cutting In a comminution operation, more than one type of force is usually acting Foe example crushing, grinding and milling take place in powdered sugar, flour, mustard and cocoa production.

9. Principle and mechanisms of comminution of food Compressive force are used for coarse crushing of hard materials Impact forces can be regarded as general purpose forces . Shear or attrition forces are applied in fine pulverization when the size of products can reach the micrometer range . Ultra-fine grinding is associated with processes in which the sub-micron range of particles is attained. Cutting gives a definite particles size and may even produce a definite shape.

10. The Mechanism of Size Reduction In the grinding process, materials are reduced in size by fracturing them. In the process, the materials is stressed by the action of mechanical moving parts in the graining machine and Initially the stress is absorbed internally by the materials as strain energy. Stress Strain Fracture in lines of weakness released heat Some of the energy is taken up in the creation of new surface, but the greater part of it is dissipated as heat.

11. Energy for size reduction Energy required depends upon: The hardness of the materials The tendency of the material to crack (friability). The minimum energy needed to rupture the material. Excess energy is lost as heat. This loss should be kept as low as practicable.

12. Continued….. A three theories depend upon the basic assumption that the energy required to produce a change dX in particles of a typical size dimension X is a simple power function of X: Where, dE is the differential energy required dX is the change in a typical dimension X is the magnitude of a typical length dimension c, n are constants.  

13. Continued….. The energy required to reduce the sixe of solid foods is calculated using one of three equations, as follows: Kick’s law Rittinger’s law Bond’s law

14. Kick’s L aw Kick reckoned that the energy required for a given size reduction was proportional to the size reduction ratio . The value of the power n as 1. In such a way, by integration of Following relation, known as kick’s law is obtained: Where X f / X p is the size reduction ratio. Kick’s law has been found to hold more accurately for coarser crushing.  

15. Rittnger’s Law States that the energy required for size reduction is proportional to the change in surface area of the pieces of food The power n the value of 2 and following relation, known as kick’s law is obtained: E is the energy per unit mass required for the production of a new surface by reduction. c is called Rittinger’s constant and X p and X f = length of product and feed, respectively .  

16. Bond’s Law and Work Index The work necessary for reduction was inversely proportional to the square root of the size produced. In Bond’s considerations n takes the value of 3/2, Where , P = power in, kW f = feed rate, t/ hr D p = 80% of product passes through mesh of dia. D p ( mm) D f = 80% of feed passes through mesh of dia. D f , (mm) w i = work index  

17. Size reduction equipment The principal types of size reduction machines are as follows: Crushers Grinders Hammer mills and impactors Attrition mills Fine grinders Fluid energy mills Agitated mills Ball mills Cutting machines Knife cutters, dicers and slitters

18. Crushers These type of reducing machines squeeze or press the materials until it breaks . Crushers are mostly used to break large pieces of solid materials into small lumps . Crushing rolls Mainly used for extraction of juice from sugarcane Two types Smooth roll crusher Serrated or toothed roll crusher

19. Smooth roll crusher Two heavy smooth faced roll rotating towards each other at same speed on parallel horizontal axes. Size of the material caught by the rolls depends upon the coefficient of friction between the material and the roll surface. Used to make grits or meal from food grains. One of the rolls should be spring loaded to avoid any damage to roll surface.

20. Serrated 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 Size reduction is by compression, impact and shear and not by compression alone, as in the case of smooth roll crushers.

21. Grinders Used to mill the grains into powder Types Attrition mill Hammer mill Impactors Rolling compression mill

22. Attrition 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 rotating disk

23. Continued….. Use the working principle of a shearing and cutting actions . Mills with different patterns of grooves, corrugations on the plates perform a variety of operations Overfeeding: lowers grinders performance and Increases heat generation during milling Disks are 20-137 cm in dia. and operated at 350 to 700 rpm 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

24. Hammer Mills 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 Feed is broken by the fixed or swinging hammers, pinned to a rotor

25. Continued….. The hammers may be replaced by knives or other devices to give the mill the possibility of handling tough, ductile or fibrous materials. The hammer mill is a versatile piece of equipment that gives high reduction ratios Hammers are rotated between 1550 to 4000 rpm, strike and grind the material until it becomes small enough to pass through the bottom screen In the food industry extensive use for grinding spices, sugar agglomerate, dry fruits, dry vegetables

26. Ball 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 the shell.

27. Continued….. C ombination of impact and shearing forces brings about a very effective size reduction. 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 to more distance. Centrifuging : In case of too high speed, balls stick to mill wall and are not released

28. Critical 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 interaction of gravitational and centrifugal forces. Critical speed can be determined by Where , n c = critical speed, revolution/s g = acceleration due to gravity, 9.80 m/s 2 R = radius of the mill, m r = radius of the ball, m  

29. Cutting machine Slicing and flaking equipment Dicing equipment Shredding equipment Pulping equipment

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