Size reduction of foods through various mills hammer mill, burr mill, ball mill, energy consumption in size reduction

1. SIZE REDUCTION FOOD PRODUCTS
DEVINDER DHINGRA

2. Size reduction of solids – converting large particles into smaller/fine
particles of the same material.
Size reduction is brought by application of force on larger particles.
Types of forces applied:
(i) Compression (ii) Impact, and (iii) Shear
Size reduction process is also known as comminution or grinding.
Reduction in particle size by mechanical means is known as milling.
SIZE REDUCTION

3. Classification of Size Reduction
• Chopping, Cutting, Slicing and Dicing
• Slicing / Cutting of fruits / vegetables (canning, minimal
processing, value addition-chips)
• Mincing of meat
• Dicing of carrots
• Shredding of vegetables
 Milling into powders /pastes
• Spices
• Flour
• Smooth pastes (peanut butter)
 Homogenisation
 Milk (reduction in particle size of fat globules)

4. Compressive forces are generally used for the coarse crushing of hard
materials. Careful application of compressive forces enables control to be
exercised over the breakdown of the material, e.g. to crack open grains of
wheat to facilitate separation of the endosperm from the Bran.
Impact forces are used to mill a wide variety of materials, including
fibrous foods.
Shear forces are best applied to relatively soft materials, again including
fibrous foods.
All three types of force are generated in most types of mills, but generally
one predominates. For example, in most roller mills compression is the
dominant force, impact forces feature strongly in hammer mills and shear
forces are dominant in disc attrition

5. Roller Mill
Roller surface could be plain or serrated depending on the requirement
Wheat Milling in a roller flour mill is done by such machines and wheat
grains are converted into flour through number of stages of roller mills

6. Hammer Mill
Predominant Force: Impact
Commonly used for spices: turmeric, pepper, chillies etc.

7. Disc Mill
Interacting surfaces of discs are rough/corrugated
The ground particles leave the edge of the discs
Marge particles are fed at the centre
The discs could be vertical or horizontal
Example: Atta Chakkis
Predominant force: Shear (& compression)

8. Reduction Ratio
The extent of the reduction in size of the feed particles/raw material can be
expressed by the reduction ratio
𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 =
Average size of the feed particles(larger size)
Average size of the product particles (smaller size)
The term average size depends on the method of measurement. In the
food industry, screening or sieving is widely used to determine particle size
distribution in granular materials and powders. In this case, the average
diameter of the particles is related to the aperture sizes of the screens
used.
Size reduction ratios vary from below 8:1 in coarse crushing to more than
100:1 in fine grinding.

9. Why Reduction in Size
• Size reduction may aid the extraction of a desired constituent from a
composite structure, e.g. flour from wheat grains or juice from sugar cane.
• Reduction to a definite size range may be a specific product requirement; e.g.
as in the manufacture of icing sugar, in the preparation of spices and in
chocolate refining.
• A decrease in particle size of a given mass of material leads to an increase in
surface of the solid. This increase in surface is of assistance in many rate
process, e.g.
– The drying time for moist solids is much reduced by increasing the surface
area of the solid.
– The rate of extraction of a desired solute is increased by increasing the
contact area between solid and solvent.
– Process time required for certain operation such as cooking, blanching,
etc.-can be reduced by cutting, shredding or dicing the process material.
– Thorough mixing or blending is usually easier with small size ranges of
particles, an important consideration in the production of formulated
packaged soups, cake mixes, etc.

10. Size reduction
• It is desirable to produce particles in the
specified size range (to meet the
standards/requirements of further processing
operations etc.)
• Screens are used to obtain particles in the
specified size range.
• Multiple stages / machines and screens may
be used in size reduction, depending on the
raw material and finished product.

11. Application of Force causes strain

12. fracture occurs along these fissures. Larger particles will contain more fissures than
smaller ones and hence will fracture more easily. In the case of small particles, new
crack tips may need to be created during the milling operation. Thus, the breaking
strength of smaller particles is higher than the larger ones. The energy required for
particle breakdown increases with decrease in the size of the particles.
In the limit of very fine particles, only intermolecular forces must be overcome and
further size reduction is very difficult to achieve. However, such very fine grinding is
seldom required in food applications. Only a very small proportion of the energy
supplied to a size reduction plant is used in creating new surfaces. Literature values
range from 2.0% down to less than 0.1%. Most of the energy is used up by elastic and
inelastic deformation of the particles, elastic distortion of the equipment, friction
between particles and between particles and the equipment, friction losses in the
equipment and the heat, noise and vibration generated by the equipment.

13. Mathematical Equations for Energy Analysis in size reduction
• The relationship between the comminution energy and the product
size obtained for a given feed size has been researched extensively
over the last century. Theoretical and empirical energy-size
reduction equations were proposed by Rittinger (1867), Kick (1885)
and Bond (1952), known as the three theories of comminution; and
their general formulation by Walker et al. (1937)
𝑑𝐸 = −𝐶
𝑑𝑥
𝑥 𝑛 …………………….(1)
Where E is the net specific energy; x is the characteristic dimension of
the product; n is the exponent; and C is a constant related to the
material. Equation [1] states that the required energy for a differential
decrease in size is proportional to the size change (dx) and inversely
proportional to the size to some power n. If the exponent n in
Equation [1] is replaced by the values of 2, 1 and 1.5 and then
integrated, the well-known equations of Rittinger, Kick and Bond, are
obtained respectively

14. • Rittinger (1867) stated that the energy required
for size reduction is proportional to the new
surface area generated. Since the specific surface
area is inversely
proportional to the particle size, Rittinger’s
hypothesis can be written in the following form:
𝐸 = 𝐾1
1
𝑥 𝑝
−
1
𝑥 𝑓
where E is energy per unit mass required, K1 is
constant and xp and xf are avg particle size of product
and feed. Observed to be good for fine grinding

15. • Kick (1885) proposed the theory that the equivalent relative reductions in
sizes require equal energy. Kick’s equation is as follows:
𝐸 = 𝐾2 ln
𝑥 𝑓
𝑥 𝑝
where E is energy per unit mass required, K2 is constant and xp and xf are avg
particle size of product and feed
Observed to be good for coarse grinding

16. • Bond (1952) proposed the ‘Third Law’ of grinding. The Third Law states
that the net energy required in comminution is proportional to the total
length of the new cracks formed. The resulting equation is:
𝐸 = 2𝐾3
1
𝑥 𝑝
−
1
𝑥 𝑓
where E is energy per unit mass required, K3 is constant and xp and xf are avg
particle size of product and feed
Only a very small proportion of the energy supplied to a size reduction plant
is used in creating new surfaces. Literature values range from 2.0% down to
less than 0.1%. Most of the energy is used up by elastic and inelastic
deformation of the particles, elastic distortion of the equipment, friction
between particles and between particles and the equipment, friction losses in
the equipment and the heat, noise and vibration generated by the
equipment.

17. SIZE REDUCTION EQUIPMENT

18. SIZE REDUCTION EQUIPMENT
a) Roller mill
A common type of roller mill consists of two cylindrical steel rolls,
mounted on horizontal axes and rotating towards each other. The
particles of feed are directed between the rollers from above. They are
nipped and pulled through the rolls where they are subjected to
compressive forces, which bring about their breakdown. If the rolls
turn at different speeds shear forces may be generated which will also
contribute to the breakdown of the feed particles .

19. Flour Roller Mill
Roller surface
SIZE REDUCTION EQUIPMENT

20. Hammer mill
SIZE REDUCTION EQUIPMENT

21. Ball Mill
• In the ball mill both shearing and impact forces are utilized
in the size reduction. The unit consists of a horizontal, slow
speed-rotating cylinder containing a charge of steel balls or
flint stones. As the cylinder rotates balls are lifted up the
sides of the cylinder and drop on to the material being
comminuted, which fills the void spaces between the balls.
The balls also tumble over each other, exerting a shearing
action on the feed material. This combination of impact
and shearing forces brings about a very effective size
reduction. Ball sizes are usually in the range 1 – 6 inches.
Small balls give more point contacts but larger balls give
greater impact. As with all grinding mills, working surfaces
gradually wear, so product contamination must be guarded
against.
SIZE REDUCTION EQUIPMENT

22. • At low speeds of rotation the balls are not lifted very far up the walls of
the cylinder. The balls tumble over each other and shear forces
predominate. At faster speeds the balls are lifted further and the impact
forces increase. Attrition and impact forces play a part in reduction. At
high speeds the balls can be carried round at the wall of the mill under the
influence of centrifugal force. Under these conditions grinding ceases. For
efficient milling the critical speed should not be exceeded. This is defined
as the speed at which a small sphere inside the mill just begins to
centrifuge.
• The critical speed is the speed at which centrifugal forces equal
gravitatinal forces at the inside surface of the shell of ball mill and no balls
will fall from its position to the shell. Effective grinding will not happen.
• It can be shown that the critical speed Nc in r.p.m., is given by:
• 𝑁𝑐 =
1
2𝜋
𝑔
𝑅−𝑟
(derivation on next slide)
• Where, R is the radius of the mill, in m
In practice, the optimum operating speed is about 75% of the critical
speed and should be determined under the plant operating conditions.

23. Critical speed
ϴ
R
mg
mV2/(R-r)
R = radius of ball mill shell
R=radius of ball
N= rotational speed of ball mill shell (revolutions per second)
m= mass of ball
R-r = effective radius for balancing forces
V=linear peripheral velocity=2π (R-r)N
g= Acceleration due to gravity m/min
ϴ
Critical Speed
Centrifugal force = gravitational force
mV2/(R-r) cos ϴ = mg
At ϴ=0, the ball will be at the top and will not fall down at critical speed (N=Nc)
Solving above equation yields 𝑁𝑐 =
1
2𝜋
𝑔
𝑅−𝑟
Critical speed (rpm) = Nc*60

24. • Ball Mill

25. Pin Mill
In one type of pin mill a stationary disc and rotating disc are located facing each other,
separated by a small clearance. Both discs have concentric rows of pins, pegs or teeth.
The rows of one disc fit alternately into the rows of the other disc. The pins may be of
different shapes; round, square or in the form of blades. The feed in introduced
through the centre of the stationary disc and passes radially outwards through the mill
where it is subjected
to impact and shear forces between the stationary and rotating pins. The mill may be
operated in a choke feed mode by having a screen fitted over the whole or part of the
periphery.
SIZE REDUCTION EQUIPMENT

26. Attrition mills
SIZE REDUCTION EQUIPMENT

27. • Mechanical Properties of the Feed
• Moisture Content of the Feed
• Temperature Sensitivity of the Feed
Selection Criteria of Size Reduction Equipment

29. Size reduction of fat globules
• Homogenizers use a high pressure
pump, operating at 100–700 bar,
Which is fitted with a homogenizing
valve(s) (two-stage homogenization)
on the discharge side.
• When liquid is pumped through the
small adjustable gap (< 300 μm)
between the valve and the valve seat.
• The high pressure produces a high
liquid velocity (80–150 m/s).

30. • Thank you