1. SIZE REDUCTION OF DRY FOOD

2. SIZE REDUCTION
Size reduction is a unit operation in which raw material often occur in sizes that are too large to be used and, therefore, they must be reduced in size to make various cookable, value added and preservable products.

3. Size reduction also known as ‘Comminution’ or “pulverisation”
This size-reduction operation can be divided into two major categories:
If it is solid, grinding and cutting.
If it is liquid, emulsification or atomization.

4. THEORY OF SIZE REDUCTION
Size reduction is a highly energy inefficient process.
When energy is applied in form of stress on the material, it breaks into two or more pieces resulting in generation of new surface area.
The fracture mechanics of a particle is schematically represented by fig..

6. Relationship between stress and strain force in size reduction
When stress is applied to a food the resulting internal strains are first absorbed, to cause deformation of the tissues.
If the strain does not exceed a certain critical level named the elastic stress limit (E), the tissues return to their original shape when the stress is removed, and the stored energy is released as heat (elastic region(O–E)).

7. If the strain area exceeds the elastic stress limit, the food is permanently deformed. If the stress is continued, the strain reaches a yield point(Y).
Above the yield point the food begins to flow (Y–B) Finally, the breaking stress is exceeded at the breaking point (B) and the food fractures along a line of weakness.
Part of the stored energy is then released as sound and heat.
When no lines of weakness remain, new fissures must be created to reduce the particle size and this requires an additional input of energy.

8. FORCE REQUIRED FOR SIZE REDUCTION
Types of force required for size reduction are:
Compression
Shear
Impact

9. Energy required to reduce the size of solid foods
It is being calculated using one of three equations, as follows:
Kick’s law : This law states that the energy required to reduce the size of particles is proportional to the ratio of the initial size of a dimension to the final size of that dimension i.e,
E= kk ln(d1/d2)
2. Rittinger’s law: This law states that the energy required for size reduction is proportional to the change in surface area of the pieces of food:
E =Kr (1/d2- 1/d1)
3. Bond’s law is used to calculate the energy required for size reduction from:
E/W= (100/d2) – (100/d1)

10. SIZE REDUCTION EQUIPMENTS
A. Crushers (coarse and fine)
Jaw crushers
Gyratory crushers
Crushing rolls
B. Grinders (intermediate and fine)
Hammer mills
Rolling mills
Attrition mills
Ball/Tumbling mills

11. CRUSHERS
Crusher do the heavy work of breaking large pieces of solid material into small lumps.
A primary crusher operates on run-of -mine material accepting anything that comes from mine face and breaking it into 150 to 250 mm lumps.
A secondary crusher reduces these lumps into particles perhaps 6mm in size.

12. Jaw crushers
In a jaw crusher, the material is fed in between two heavy jaws, one fixed and the other reciprocating, so as to work the material down into a narrower and narrower space, crushing it as it goes.

14. ADVANTAGES OF JAW CRUSHER
Firm structure, stable performance, large capacity, even producing granularity
With the increase of production, jaw crusher can greatly reduce the operating costs.
Convenient maintenance and economical running expense.
The jaw has a smaller discharge setting, allowing for a higher degree of flexibility with relative equipment.

15. GYRATORY CRUSHER
The gyrator crusher consists of a truncated conical casing, inside which a crushing head rotates eccentrically. The crushing head is shaped as an inverted cone and the material being crushed is trapped between the outer fixed, and the inner gyrating, cones, and it is again forced into a narrower and narrower space during which time it is crushed.

16. ADVANTAGES OF GYRATORY CRUSHER
Crushing cavity depth, work successive, high productive capacity, the unit consumption is low..
Work smoothly, vibration is lighter, machine’s based weight is smaller. The based weight of gyratory crusher is usually 2-3 times of the machine weight.

18. APPLICATION OF JAW AND GYRATORY CRUSHER
A major application is in the cane sugar industry, where several stages of rolls are used to crush the cane.
Other applications, they are used in metal industry, cement industry etc.

19. CRUSHING ROLLS PRINCIPLE: Compressive forces.
In crushing rolls, two or more counter rotating heavy steel cylinders revolve towards each other, so particles of feed are nipped and pulled through.

20. The roller surface can be smooth or can carry breaker bars, corrugations, teeth, as a device to increase friction and facilitate the trapping of particles between the rolls.

22. ADVANTAGES OF CRUSHING ROLLS
High and constant capacity
Low susceptibility to breakdowns
Easy replacement of wear and spare parts
Wide range of application
High crushing grade with a minimum of fine grains.
DISADVANTAGES OF CRUSHING ROLLS
Cannot handle hard solids.

23. APPLICATION OF CRUSHING ROLLS
Applied in the milling of wheat and in the refining of chocolates.

24. GRINDERS (Intermediate & Fine)
Grinders reduce crushed feed to powder.
The product from an intermediate grinder might pass a 40-mesh screen.
The product from a fine grinder would pass a 200-mesh screen with a 74m m opening.

25. ROLLING –COMPRESSION MILLS
PRINCIPLE: Compression.
Roller mills are similar to roller crushers, but they have smooth or finely fluted rolls, and rotate at differential speeds.

26. Roller Mill

28. ADVANTAGES OF ROLLER MILL
Energy Savings.
More uniform, less fines & oversize particles.
Less Noise, Less Dust.
Works best on easy to grind materials like Corn, cereal grains, extracted meals etc.

29. HAMMER MILLS PRINCIPLE: Impact.
Feed passes into the action zone with the hammers driving the material against the breaker plate and forcing it to pass through a bottom mounted screen by gravity when the particles attain a proper size.

31. HAMMER MILL: USED FOR GRINDING GRAINS

32. ADVANTAGES OF HAMMER MILL
Easy to Use
Versatile as it process a wide range of materials
•Grain, High fiber, High protein
•Can be set up for very fine grinding (90%-60 mesh)
–Must be configured per application

33. DISADVANTAGES OF HAMMER MILL
Heat builtup during milling is more, therefore, product degradation is possible.
Hammer mill cannot apply to sticky, fibrous and hard materials.
The screen may get clogged.
Many hammermills vibrate and are noisy.

34. APPLICATION OF HAMMER MILL
Used in grinding spices, dried milk, sugar agglomerate, cocoa press cake, dry fruit, dry vegetables , extracted bones etc.

35. ATTRITION MILL/DISC MILL
Buhr mill
Single disc mill
Double disc mill

36. The single disc mill has a high speed rotating grooved disc leaving a narrow gap with its fixed casing. Depending on feed size and product requirements, the gap can be adjusted, resulting in an intense shearing action on the feed.
In the double disc mill, the casing contains two counter rotating discs, giving a higher shear intensity compared with the single disc mill.
The Buhr mill, which is the older type of attrition mill consists of two circular stones the upper stone is normally fixed and has a feed entry port, while the lower stone rotates.

38. APPLICATION OF ATTRITION MILL
They have been employed for dry milling of wheat as well as wet milling of corn.
Other applications are:
Breaking of cocoa kernels ,
Preparation of cocoa powder,
Degermination of corn ,
Production of fishmeal,
Grinding of sugar nutmeg, cloves, roasted nuts etc.

39. BALL/TUMBLING MILLS PRINCIPLE: Impact & Attrition.
The reduction mechanism is the following: the grinding medium is lifted up the sides of the cylinder and dropped onto the material being comminuted as the cylinder rotates, filling the void spaces between the medium

40. ADVANTAGES OF BALL MILL
It can produce very fine powder.
Installation , operation and labor costs are low.

41. DISADVANTAGES OF BALL MILL
Ball mill is very noisy machine.
Ball mill is a slow process.
Soft, tacky, fibrous material cannot be milled by ball mill.

42. APPLICATION OF BALL MILL
Breaking of cocoa kernels , preparation of cocoa powder, manufacturing of chocolates , and grinding of sugar nutmeg, cloves, roasted nuts etc.

44. ADVANTGES OF SIZE REDUCTION
The increased surface area will help in enhanced heat and mass transfer, & hence, the rate of processes are enhanced.
The reduced particle size would enable accessibility to the interior of the food materials as in the leaching of spice oleoresins or oil from oil seeds.
Intimate contact with various other ingredient is possible as in case of preparation of soup mixes, etc.

45. DISADVANTAGES OF SIZE REDUCTION
Sometime the generated heat is so much that it is detrimental for product. In such cases preventive measure is “cryogenic grinding.
Disruption of cells allows enzymes and substrates to become more intimately mixed , which result in the development of off-flavors and aromas.

46. Cellular materials provides a suitable substrate for microbial growth.
Increase in surface area of foods during size reduction causes loss of nutritional value due to oxidation of fatty acids and carotenes.

47. Applications Of Size Reduction
Milling of grains , brown rice, pulses , minor millets (like ragi, bajra to make into flour).
Wet grinding of black gram & green gram for making various traditional foods.
Grinding of some non-edible portion of cereals & pulses or minor grains to make cattle feed.
Coarse grinding of spices like chilli , turmeric , pepper & dry ginger to extract oleoresin.
Other applications, they are used in metal industry, cement industry etc.

48. BIBLIOGRAPHY http://www.nzifst.org.nz/unitoperations/sizereduction.htm
LENIGER, H. A. and BEVERLOO, W. A. (1975) Food Process Engineering. D. Reidel, Dordrecht, pp. 169–188.
LEWIS, M. J. (1990) Physical Properties of Foods and Food Processing Systems. Woodhead Publishing,Cambridge, pp. 184–195.
D.G. Rao, Food Engineering, pp