1. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 1 of 91
UNIT OPERATIONS IN FOOD
Notes for Diploma in Food Technology
[Prepared BY:- Mohit Jindal]
Food Technology Department
[Government Polytechnic, Mandi Adampur, HIsar-125052]
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UNIT OPERATIONS IN FOOD
1. Preliminary Unit operation
Cleaning, sorting & Grading - aims, methods and applications
2. Size Reduction and Sieve Analysis
Theory of comminution; Calculation of energy required during size reduction. Crushing efficiency; Size
reduction equipment; Size reduction of fibrous, dry and liquid foods; effects of size reduction on
sensory characteristics and nutritive value of food
Sieving: Separation based on size (mesh size); types of screens; effectiveness of screens
Mixing, Agitating, kneading, blending, homogenization and related equipment
4. Separation Processes
Principles of Filtration, Sedimentation, Crystallization and Distillation and equipment used
LIST OF PRACTICALS
1. Analysis of sampled foods for physical characteristics
2. Determination of critical speed of ball-mill
3. Size reduction and particle size distribution using hammer-mill
4. Steam distillation of herbs
5. Concentration by crystallization
6. Clarification of apple juice using filter press
7. Visit to a public distribution system (PDS) showing storage facilities, warehouse, cold storage,
refrigeration system and slaughter house etc
8. Visit to various food industries for demonstration of various unit operations
1. Handling, Transportation and Storage of Fruits and Vegetables by A Lloyd, Ryall Penizer (AVI
2. Proceedings of Regional Workshop on Warehouse Management of Stored Food Grains by Girish and
Ashok Kumar (UNDP)
3. Modern Potato and Vegetable Storage by Volkind and Roslov (Amerind)
4. Controlled Atmospheric Storage of Fruits by Mettel Skilv
5. Food Grains in Tropical and Sub Tropical Areas by Hall
6. Food Storage Part of a system by Sinha and Muir (AVI)
7. Post Harvest Technology of Fruits and Vegetables – Handling, Processing, Fermentation and Waste
Management by LR Verma and VK Joshi; Indus Publishing com., New Delhi
8. Drying and Storage of Grains and Oilseeds by Brooker & Hall, CBS
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A physical entity, which can be observed and/or measured, is defined qualitatively by a dimension.
For example, time, length, area, volume, mass, force, temperature, and energy are all considered
dimensions like unit of length may be measured as a meter, centimeter, or millimeter.
Primary dimensions, such as length, time, temperature, and mass, express a physical entity.
Secondary dimensions involve a combination of primary dimensions (e.g., volume is length cubed;
velocity is distance divided by time).
Physical quantities are measured by variety of unit systems. The most common systems
include the Imperial (English) system; the centimeter, gram, second (cgs) system; and the meter,
kilogram, second (mks) system. International organizations have attempted to standardize unit
systems, symbols, and their quantities. As a result of international agreements, the Systeme
International d’Unites, or the SI units have emerged. The SI units consist of seven base units, two
supplementary units, and a series of derived units.
The SI system is based on a choice of seven well-defined units, which by convention are regarded as
dimensionally independent. The definitions of these seven base units are as follows:
1. Unit of length (meter): The meter (m) is the length equal to 1,650,763.73 wavelengths in
vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the
2. Unit of mass (kilogram): The kilogram (kg) is equal to the mass of the international prototype
of the kilogram. (The international prototype of the kilogram is a particular cylinder of
platinum-iridium alloy, which is preserved in a vault at Sèvres, France, by the International
Bureau of Weights and Measures.)
3. Unit of time (second): The second (s) is the duration of 9,192,631,770 periods of radiation
corresponding to the transition between the two hyperfine levels of the ground state of the
4. Unit of electric current (ampere): The ampere (A) is the constant current that, if maintained
in two straight parallel conductors of infinite length, of negligible circular cross-section, and
placed 1 m apart in vacuum, would produce between those conductors a force equal to 2*107
newton per meter length.
5. Unit of thermodynamic temperature (Kelvin): The Kelvin (K) is the fraction 1/273.16 of the
thermodynamic temperature of the triple point of water.
6. Unit of amount of substance (mole): The mole (mol) is the amount of substance of a system
that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12.
7. Unit of luminous intensity (candela): The candela (cd) is the luminous intensity, in the
perpendicular direction, of a surface of 1/600,000 m 2 of a blackbody at the temperature of
freezing platinum under a pressure of101, 325 newton/m2
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Derived units are algebraic combinations of base units expressed by means of multiplication and
division. For simplicity, derived units often carry special names and symbols that may be used to
obtain other derived units. Definitions of some commonly used derived units are as follows:
1. Newton (N): The newton is the force that gives to a mass of 1 kg an acceleration of 1 m/s2
2. Joule (J): The joule is the work done when due to force of 1 N the point of application is
displaced by a distance of 1 m in the direction of the force.
3. Watt (W): The watt is the power that gives rise to the production of energy at the rate of 1 J/s.
4. Volt (V): The volt is the difference of electric potential between two points of a conducting
wire carrying a constant current of 1 A, when the power dissipated between these points is
equal to 1 W.
5. Ohm ( Ω): The ohm is the electric resistance between two points of a conductor when a
constant difference of potential of 1 V, applied between these two points, produces in this
conductor a current of 1 A, when this conductor is not being the source of any electromotive
6. Coulomb (C): The coulomb is the quantity of electricity transported in 1 s by a current of 1 A.
7. Farad (F): The farad is the capacitance of a capacitor, between the plates of which there
appears a difference of potential of 1 V when it is charged by a quantity of electricity equal to
8. Henry (H): The henry is the inductance of a closed circuit in which an electromotive force of 1
V is produced when the electric current in the circuit varies uniformly at a rate of 1 A/s.
9. Weber (Wb): The weber is the magnetic flux that, linking a circuit of one turn, produces in it
an electromotive force of 1 V as it is reduced to zero at a uniform rate in 1 s.
10. Lumen (lm): The lumen is the luminous flux emitted in a point solid angle of 1 steradian by a
uniform point source having an intensity of 1 cd.
This class of units contains two purely geometric units, which may be regarded either as base units or
as derived units.
1. Unit of plane angle (radian): The radian (rad) is the plane angle between two radii of a circle
that cut off on the circumference an arc equal in length to the radius.
2. Unit of solid angle (steradian): The steradian (sr) is the solid angle that, having its vertex in
the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with
sides of length equal to the radius of the sphere
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Food engineering is related to the analysis of equipment and systems used to process food on a
commercial production scale. Design of food equipment and processes to insure food quality and
safety we should know the response of the food materials to physical and chemical treatments. Raw
food materials are biological in nature and as such have certain unique characteristics which
distinguish them from other manufactured products. Because food materials are mainly of biological
origin they have
(a) Irregular shapes commonly found in naturally occurring raw materials;
(b) Properties with a non-normal frequency distribution;
(c) Heterogeneous composition;
(d) Composition that varies with variety, growing conditions, maturity and other factors; and they are
(e) Affected by chemical changes, moisture, respiration, and enzymatic activity.
The majority of industrial food processes involve fluid movement. Liquid foods such as milk and
juices have to be pumped through processing equipment or from one container to another. A number
of important unit operations such as filtration, pressing and mixing are, particular applications of fluid
flow. The mechanism and rate of energy and mass transfer are strongly dependent on flow
characteristics. The flow properties and deformation properties of fluids are the science called
‘rheology’ or the relationship between stress and strain is the subject matter of the science known as
Mechanical properties are those properties that determine the behavior of food materials when
subjected to external forces. Mechanical properties are important in processing (conveying, size
reduction) and consumption (texture, mouth feel).
The forces acting on the material are usually expressed as stress, i.e. intensity of the force per unit area
or Pa.). The dimensions and units of stress are like those of pressure.
The response of materials to stress is deformation, expressed as strain. Strain is usually expressed as a
dimensionless ratio, such as the elongation as a percentage of the original length.
We define three ideal types of deformation:
Elastic deformation: deformation appears instantly with the application of stress and
disappears instantly with the removal of stress.
Plastic deformation: deformation does not occur as long as the stress is below a limit value
known as yield stress. Deformation is permanent, i.e. the body does not return to its original
size and shape when the stress is removed.
Viscous deformation: deformation (flow) occurs instantly with the application of stress and it is
permanent. The rate of strain is proportional to the stress
In the food industry every process involves thermal effects such as heating, cooling or phase
transition. The thermal properties of foods are important in food process engineering. The following
properties are of particular importance: thermal conductivity, thermal diffusivity, specific heat, latent
heat of phase transition and emissivity.
The electrical properties of foods are particularly relevant to microwave and ohmic heating of
foods and to the effect of electrostatic forces on the behavior of powders. The most important
properties are electrical conductivity and the dielectric properties. Ohmic heating is a
technique whereby a material is heated by passing an electric current through it.
Size and Shape
The size and shape of a raw food material can vary widely. The variation in shape of a product
may require additional parameters to define its size. The size of spherical particles like peas or
cantaloupes is easily defined by a single characteristic such as its diameter. The size of non-spherical
objects like wheat kernels, bananas, pears, or potatoes may be described by multiple length
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Particle size is used in sieve separation of foreign materials or grading (i.e., grouping into size
categories). Particle size is particularly important in grinding operations to determine the condition of
the final product and determines the required power to reduce the particle’s size.
Various types of cleaning, grading and equipments are designed on the basic physical
properties such as size, shape, specific gravity and colures. The shape of product is the important
parameter which effect covering characteristics of solid materials. The shape is also procedure in
calculation of various cooling and heating of food material.
Size is actually related or correlated to the property weight.
Shape affects the grade given to fresh fruit. To make the highest grade a fruit or vegetable must have
the commonly recognized expected shape of that particular fruit/vegetable.
Roundness, as defined as, “is a measure of the sharpness of the corners of the solid.”
where R in this case is the mean radius of the object and r is the radius of curvature of the sharpest
where: Di = diameter of largest inscribed circle
Dc = diameter of smallest circumscribed circle
Color is an important quality parameter because colour and colour uniformity are vital
components of visual quality of fresh foods and play a major role in consumer choice. Automatic
measurement of color is essential in many process control applications, such as sorting of fruits and
vegetables in packing houses, control of roasting of coffee and nuts, control of frying of potato chips,
oven toasting of breakfast cereals, browning of baked goods etc. However, it may be less important in
raw materials for processing. For low temperature processes such as chilling, freezing or freeze-
drying, the colour changes little during processing, and thus the colour of the raw material is a good
guide to suitability for processing. Any color within the visible range can be represented with the help
of three dimensional coordinates (or three-dimensional color space) L,a,b.
The axis L represents ‘luminosity’ with 0=black and 100=white.
The ‘a ’axis gives the position of the measured color between the two opponent colors red and green,
with red at the positive and green at the negative end.
The ‘b’ axis reflects the position of the color in the yellow (positive) – blue (negative) channel.
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With the help of the L*a*b space system, any color is represented by a simple equation containing the
three parameters. Color measurement instruments (colorimeters) are photoelectric cell-based devices,
capable of reading the L,a,b values and ‘ calculating ’ the color perceived.
Density is defined as objects mass per unit volume. Mass is a property. The symbol most
often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass
divided by volume. It is an indication of how matter is composed in the body material with more
compact density has higher density
The density can be expressed as
ρ = density (kg/m3
m = mass (kg)
V = volume (m3
The SI units for density are kg/m3
. The imperial (U.S.) units are lb/ft3
). While people
often use pounds per cubic foot as a measure of density in the U.S.1 gram/cm3
= 1000 kg/m3
The density of a material is equal to its mass divided by its volume and has SI units of kg m-3
The density of materials is not constant and changes with temperature and pressure. Increasing the
pressure always increases the density of a material. Increasing the temperature generally decreases the
density. Knowledge of the density of foods is important in separation processes and differences in
density can have important effects on the operation of size reduction and mixing equipment. Product
density influences the amount and strength of packaging material. Breakfast cereal boxes contain a
required weight of cereal. More weight of material can be placed into a box if the cereal density is
greater. Also, food density influences its texture or mouth feel. Processing can affect product density
by introducing more air, such as is done in the manufacture of butter or ice cream.
It is the weight of the food material in a unit volume. It is of importance in the packaging,
handling and other operations.
Bulk density is defined as the mass of many particles of the material divided by the
total volume they occupy.
The weight of a material (including solid particles and any contained water) per unit volume including
Bulk density is overall mass of the material divided by the volume occupied by the material
The total volume includes particle volume, inter-particle void volume, and internal pore
Bulk density is not an intrinsic property of a material; it can change depending on how the material is
handled. For example, a powder poured into a cylinder will have a particular bulk density; if the
cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a
higher bulk density. For this reason, the bulk density of powders is usually reported both as "freely
settled" (or "poured" density) and "tapped" density (where the tapped density refers to the bulk density
of the powder after a specified compaction process, usually involving vibration of the container.)
Oil, water and air occupy voids in the soil, called pore spaces.
Bulk density = Oven dry soil weight / volume of soil solids and pores
Particle density is the volumetric mass of the solid soil. It differs from bulk density because the
volume used does not include pore spaces.
Particle density = oven-dry soil weight / volume of soil solids
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The void space can be describing the porosity which is expressed as volume not occupied
as good material. Porosity is the percentage of air between the particles compared to a unit volume of
Porosity is that portion of the material volume occupied by pore spaces. This property does not have
to be measured directly since it can be calculated using values determined for bulk density and particle
density. Finding the ratio of bulk density to particle density and multiplying by 100 calculates the
percent solid space. If subtracting % solid space from 100 gives the % of soil volume that is pore
% solid space = (bulk density / particle density) x 100
% porosity = 100 - (% solid space)
Sample Calculation of Porosity:
A 260 cm3 cylindrical container was used to collect an undisturbed soil sample. The container and soil
weighed 413 g when dried. When empty the container weighed 75 g. What is the bulk density and
porosity of the soil?
To determine bulk density:
Sample Volume = 260 cm3;
Sample Weight = 413 - 75 = 338 g;
Bulk density = 338 g/260 cm3= 1.3 g /cm3
To determine porosity:
Bulk density = 1.3 g /cm3;
Particle density = 2.65 g /cm3;
Porosity = 100 - (1.3/2.65 x 100) = 51%
The Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the
substance to the density of water at a specified temperature. Apparent specific gravity is the ratio of
the weight of a volume of the substance to the weight of an equal volume of the reference substance.
Specific Gravity can be expressed
SG = ρsubstance / ρH2O
SG = Specific Gravity of the substance
ρsubstance = density of the fluid or substance (kg/m3
ρH2O = density of water - normally at temperature 4 o
It is common to use the density of water at 4 o
C because at this point the density of water is at
the highest - 1000 kg/m3
or 62.4 lb/ft3
. Specific gravity can also be calculated from the following
Specific gravity varies with temperature. The reference substance is nearly always water for
liquids or air for gases. Temperature and pressure must be specified for both the sample and the
reference. Pressure is nearly always 1 atm equal to 101.325 kPa. Temperatures for both sample and
reference vary from industry to industry. The density and specific gravity value as a stain and other
communities are used in design of solid storage separation of desired materials cleaning and grading,
texture and softness of food quality, the concentration of solutions of various materials such as brines,
hydrocarbons, sugar solutions (syrups, juices, honeys, brewers wort, must etc.) and acids.
Specific gravity can be measured in a number of ways.
2. Digital density meters
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Thermal conductivity is a measure of the ability of a material to transfer heat. It may be define
as the rate of heat flow through unit thickness of material per unit area normal to direction of heat flow
and per unit time per unit temperature difference is called thermal conductivity.
The thermal conductivity is the heat energy transferred per unit time and per unit surface area,
divided by the temperature difference.
Thermal conductivity, k (also denoted as λ or κ), is the property of a material's ability
to conduct heat. It appears primarily in Fourier's Law for heat conduction. Heat flows at a higher rate
across materials of high thermal conductivity than across materials of low thermal conductivity.
Materials of high thermal conductivity are widely used in heat sink applications and materials of low
thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature
dependent. Thermal energy always moves from that of higher concentration to lower concentration--
that is, from hot to cold.
In the following equation, thermal conductivity is the proportionality factor k. The distance of heat
transfer is defined as ∆x, which is perpendicular to area A. The rate of heat transferred through the
material is Q, from temperature T1 to temperature T2, when T1>T2. SI units for thermal conductivity
watt per meter kelvin W/ (m K), m kg s-3
Viscosity is a resistance of a fluid which is
being deformed by either shear stress or tensile stress. In the other word we can say viscosity is the
property of fluid by virtue of which is opposing its flow.
Viscosity is resistance to flow
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of
Viscosity is an important characteristic of liquid foods in many areas of food processing. For
example the characteristic mouthfeel of food products such as tomato ketchup, cream, syrup and
yoghurt depends on their viscosity (or 'consistency'). The viscosity of many liquids changes during
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heating/cooling or concentration and this has important effects on, for example, the power needed to
pump these products.
A liquid having a series of layers and when it flows over a surface, the uppermost layer flows
fastest and drags the next layer along at a slightly lower velocity, and so on through the layers. The
force that moves the liquid is known as the shearing force or 'shear stress' and the velocity gradient is
known as the 'shear rate'. If shear stress is plotted against shear rate, most simple liquids and gases
show a linear relationship and these are termed 'Newtonian' fluids. Examples include water, most
oils, gases, and simple solutions of sugars and salts. Where the relationship is non-linear the fluids are
termed 'non-Newtonian'. For all liquids, viscosity decreases with an increase in temperature but for
most gases it increases with temperature.(Lewis 1990).
In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction".
Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. All
real fluids have some resistance to stress and therefore are viscous. A fluid which has no resistance to
shear stress is known as an ideal fluid or in viscid fluid. Zero viscosity is observed only at very low
temperatures, in super fluids.
The word "viscosity" is derived from the Latin "viscum", meaning mistletoe and also a
viscous glue (birdlime) made from mistletoe berries. Viscosity represented by the symbol η "eta".
Viscosity is the ratio of the tangential frictional force per unit area. The SI unit of viscosity is
the pascal second [Pa s]. The pascal second is rarely used today the most common unit of viscosity is
the dyne second per square centimeter [dyne s/cm2
], which is given the name poise [P] after the
French physiologist Jean Poiseuille (1799–1869). Ten poise equal one pascal second [Pa s] making
the centipoise [cP] and millipascal second [mPa s] identical.
1 pascal second = 10 poise
1 pascal second = 1,000 millipascal second
1 centipoise = 1 millipascal second
The other quantity called kinematic viscosity (represented by the symbol ν "nu") is the ratio of
the viscosity of a fluid to its density. The SI unit of kinematic viscosity is the square meter per
/s]. A more common unit of kinematic viscosity is the square centimeter per
/s], which is given the name stokes [St] after the Irish mathematician and
physicist George Stokes (1819–1903).
/s = 10,000 cm2
/s = 1,000,000 mm2
/s 1 stokes
/s = 1 centistokes
It is defined as the ratio of thermal conductivity to the ‘volumetric heat capacity’ of the
material. Volumetric heat capacity is obtained by multiplying the mass specific heat c p by the density
It may be calculated by dividing thermal conductivity with the specific heat and density. In heat
transfer analysis, thermal diffusivity usually denoted α but a, κ, k, and D are also used. It has the SI
unit of m²/s. The formula is:
is thermal conductivity (W/(m·K))
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is density (kg/m³)
is specific heat capacity (J/(kg·K))
Thermal conductivity is a property that determines HOW MUCH heat will flow in a material, while
thermal diffusivity determines HOW RAPIDLY heat will flow within it. In a substance with high
thermal diffusivity, heat moves rapidly through because the substance conducts heat quickly relative to
its volumetric heat capacity or 'thermal bulk'. The substance generally does not require much energy
transfer to or from its surroundings to reach thermal equilibriumIt is important to determine heat
transfer rate in solid food material of any shape. It shows capacity of food material to store heat.
In physics, heat is energy in transfer other than as work or by transfer of matter. When there is
a suitable physical pathway, heat flows from a hotter body to a colder one.
A form of energy associated with the motion of atoms or molecules and capable ofbeing trans
mitted through solid and fluid media by conduction, through fluid media byconvection, and through e
mpty space by radiation.
The transfer of energy from one body to another as a result of a difference intemperature or a c
hange in phase.
The specific heat is the amount of heat per unit mass required to raise the temperature by one degree
Celsius without change in surface.
It may be defined as amount of heat that must be added or removed from 1 kg of substance by 1º C
without change in surface. The relationship does not apply if a phase change is encountered, because
the heat added or removed during a phase change does not change the temperature
cp = Q / (mΔT)
cp is the specific heat (kJ/kg o
, kJ/kg o
Q is the heat added(kJ)
m is the mass(kg)
T is the change in temperature (K, o
It is denoted by Cp. SI unit of heat capacity is kJ/(kg K)..Because of the high specific heat of
water relative to other materials, water will change its temperature less when it absorbs or loses a
given amount of heat. The reason you can burn your finger by touching the metal handle of a pot on
the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times
greater than that of iron.
For example, the specific heat of water is around 4180 Joules per kilogram, so it takes 4180J of
energy to raise the temperature of 1kg of water by 1 degree Celsius.
Specific heat of weight agriculture materials is the sum of dry materials and moisture content.
It is an essential part of thermal analysis of food processing or equipment used for heating. Specific
heat can be thought of as a measure of how well a substance resists changing its temperature when it
absorbs or releases heat.
The quantity of heat absorbed or released by a substance undergoing a
change of state, such as
ice changing to water or water to steam, at constant temperature and pressure.
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Latent is the energy released or absorbed by a body or a thermodynamic system during a
Heat absorbed or released as the result of a phase change is called latent heat. There is no temperature
change during a phase change, thus there is no change in the kinetic energy of the particles in the
The term was introduced around 1762 by Scottish chemist Joseph Black. It is derived from the
Latin latere (to lie hidden). The SI unit for specific latent heat is J/kg. Two of the more common forms
of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent
heat of vaporization (boiling). These names describe the direction of energy flow when changing
from one phase to the next: from solid to liquid, and liquid to gas.
A specific latent heat (L) expresses the amount of energy in the form of heat (Q) required to
completely effect a phase change of a unit of mass (m), usually 1kg, of a substance as an intensive
Q is the amount of energy released or absorbed during the change of phase of the
substance (in kJ),
m is the mass of the substance (in kg), and
L is the specific latent heat for a particular substance (kJ-kgm
), either Lf for fusion, or Lv for
The energy released comes from the potential energy stored in the bonds between the particles.
exothermic (warming processes)
endothermic (cooling processes)
Endothermic meaning that the system absorbs energy on going from solid to liquid to gas. The change
is exothermic (the process releases energy) for the opposite direction.
When an object is heated, its temperature rises as heat is added. The increase in heat is called
sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat
removed is also called sensible heat.
Sensible heat is heat exchanged by a body or thermodynamic system that changes the
temperature, and some macroscopic variables of the body, but leaves unchanged certain other
macroscopic variables, such as volume or pressure.
The terms sensible heat and latent heat are not special forms of energy; instead they
characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic
system. A good way to remember the distinction is that a change in sensible heat may be ″sensed″
with a thermometer, and a change in latent heat is invisible to a thermometer – the temperature
reading doesn't change. For example, during a phase change such as the melting of ice, the
temperature of the system containing the ice and the liquid is constant until all ice has melted. The
terms latent and sensible are correlative. Heat that causes a change in temperature in an object is called
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Enthalpythermodynamic function of a system, equivalent to the sum of the internalenergy of t
he system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.
Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is
equal to the internal energy of the system plus the product of pressure and volume
+H indicates that heat is being absorbed in the reaction (it gets cold) and H indicates that heat
is being given off in the reaction (it gets hot). Enthalpy is a defined thermodynamic potential,
designated by the letter "H", that consists of the internal energy of the system (U) plus the product of
pressure (p) and volume (V) of the system. The unit of measurement for enthalpy in the International
System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the
British thermal unit and the calorie. The enthalpy is an extensive property. The enthalpy of a
homogeneous system is defined as:
H is the enthalpy of the system
U is the internal energy of the system
p is the pressure of the system
V is the volume of the system.
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Preliminary Unit operation- Cleaning, sorting & Grading - aims, methods and
PREPARATIVE OPERATIONS IN FOOD INDUSTRY
The preliminary preparative operations in food processing include cleaning, sorting and
grading of food raw material. These may be considered as separation operation. Cleaning involves the
separation of contaminants from the desired raw materials. Sorting involves the separation of
the raw materials into different categories based on their physical characteristics such as size,
shape and colour. Grading involves the separation of the raw materials into categories based on
the differences in their overall quality.
CLEANING OF FOOD RAW MATERIALS
Cleaning is an essential preliminary operation in any food industry. The ultimate quality of the
finished product, storage stability, organoleptic properties, safety from health hazards, and consumer
acceptance depend on cleaning process. The methods adopted depend on the type of raw material, type
and extent of contamination, the degree of cleaning to be achieved and the type of finished product.
Different food raw materials are associated with different types of contaminants. These include
Mineral contaminants- soil, sand, stone metallic particles, grease and oil.
plant part- stalks, pits, husks and rope,
Animal parts and contaminants—excreta, hair, insects eggs and body part
Chemical contamination- sprayed residues of pesticides, insecticides and fertilizers
Microbial contaminants—microorganisms and their metabolites.
The chosen cleaning process must satisfy the following requirements in order to achieve the aforesaid
1. The separation efficiency of the process must be high and consistent and should produce
minimum wastage of good material
2. Damage of cleaned raw material must be avoided.
3. Recontamination of the cleaned food should be avoided by complete removal of the
4. The design of the process equipment should be such that recontamination of the cleaned food
due to flying dust or wash water is prevented.
5. The cleaning process must leave the cleaned surface in acceptable condition,
6. The volume and concentration of liquid effluents must be kept be minimum and the effluents
should be disposed off effectively Complete cleaning of a raw material is not possible and in
practice, a balanced approach, considering the economic aspects of cleaning and the need to
produce good quality food, is usually adopted,
The cleaning methods can be classified into two groups, namely
Dry cleaning methods which include
screening, brushing, aspiration, abrasion and
Wet cleaning methods which include
soaking, spraying, flotation, ultrasonic
cleaning, filtration and settling.
Dry cleaning methods
These methods are relatively cheap and convenient
as the cleaned surface is dry However, a major
drawback is the spread of dust.
15. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 15 of 91
Screening-Screens are primarily size separators or sorting machines but may be used as
cleaning equipment for removing contaminants of different size from that of the raw material. These
machines are useful in cleaning fine materials such as flour and ground spices but must be frequently
cleaned to remove oversized contaminants which may otherwise get pulverized due to abrasion and
spread contamination of the raw material.
Abrasion cleaning- Abrasion between food particles or between the food and moving parts of
cleaning machinery is used to loosen and remove adhering contaminants. Tumblers, vibrators,
abrasive discs and rotating brushes are used for this purpose.
Aspiration cleaning- Aspiration (or
winnowing) is based on the differences in the
aerodynamic properties of materials. The raw
material to be cleaned is fed into a stream of air
flowing at controlled velocity to separate the raw
materials into two or more streams (e.g. light and
heavy streams). The cleaned products are usually
discharged as the middle stream leaving the heavy
debris (stones, pieces of metal or wood) behind
while floating off the light debris such as stalks,
husks and hairs. This method is used in cleaning
cereals, nuts, beans, onions, melon, eggs and other
foods which are not amenable to wetting. The
method cannot be used with oxidation-sensitive
Magnetic cleaning- This type of
cleaning involves where the food
contaminated with high amount of
metallic material. Magnetic separators
used for this type of cleaning include
rotating or stationary magnetic drums,
magnetized belts, magnets located over
belts carrying the food or staggered
magnetized grids through which the food
is passed. A magnetic separator is a piece
of equipment that magnetically attracts
and removes foreign metal pieces from
other materials. The process of magnetic
separation is utilized in many industries,
some of which include:
Food and beverages,
Miscellaneous dry cleaning methods- Such cleaning methods include:
1. Electrostatic cleaning
2. radio isotope separation
16. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 16 of 91
3. X-ray separation.
Electrostatic cleaning- Electrostatic cleaning can be used in a limited number of cases where
the surface charge on raw materials differs from contaminating particles. The principle can be used to
distinguish grains from other seeds of similar geometry but differences in electrostatic charging of
materials under controlled humidity conditions, charged particles being removed by oppositely
charged or earthed rollers, grids, etc. and it has also been described for cleaning tea. The feed is
conveyed on a charged belt and charged particles are attracted to an oppositely charged electrode
according to their surface charge.
Radio isotope separation- Clods of earths and stones may be separated from the potatoes.
X-ray separation- Stones, gloss and metal fragments in foods such as confectionery can be
separated by this method.
Wet cleaning methods-
Wet cleaning has the advantage of removing firmly adherent soils and owing the use of
detergents and sanitizers. However, wet methods have a number of disadvantages such as the use of
large amounts of high quality water and generation of large volume of effluent (about 15,000 liters per
ton of canned food). Wet cleaning methods include soaking, spray washing, flotation washing and
ultrasonic cleaning methods.
Soaking- This is the simplest method and is often used as preliminary stage in the cleaning of
heavily contaminated root vegetables and other foods. Soaking softens adhering soil and also
facilitates the removal of sand, stone, and ether abrasive material. The use of warm water and
detergents increase the efficiency but the use of chemicals may affect the texture of the food, e.g,
sodium hexametaphosphate softens peas while some metal ions toughen peas and peaches destined
for canning, Chlorination is used to decrease bacterial load of water in the soak tank.
Spray washing. This is the most widely used method for wet cleaning of fruits and
vegetables. The surface of the food is subjected to water sprays, The efficiency of spray washing
depends on several parameters such as water pressure, volume of water, temperature, the distance
of the food from jets, the time of spraying and number of spray jets used. A small volume of water at
high pressure is the most effective combination. High pressure sprays may be used to cut out parts
of peaches and tomatoes and to remove adherent soil and black moulds on citrus fruits. It may
damage ripe fruits and vegetables such as straw berries and tomatoes and delicate vegetables such
17. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 17 of 91
The washer is equipped with a central spray rod which is fitted with jets for spraying water.
A rubber disc cleaner requires less amount of water for cleaning. It uses soft rubber discs
spinning axially at about 500 rpm. The soil is collected into the base of the channel. The disc
cleaner uses only about 20 liters of water per ton of fruit while other washers use 1500-5000
Flotation washing- The method depends on the differences in buoyancy of the desired and
undesired parts of the food raw material to be cleaned. For example, bruised or rotten apples
sink in water and can be removed at the base of tank and the good fruit can be collected as
overflow. The flotation washer effectively removes stones, dirt and plant debris from peas,
beans, dried fruits and similar materials. Water requirement is about 4,000-10,000 liters per ton
of raw material to be cleaned.
Froth flotation has been used to separate peas from weed seeds by immersing the peas in
dilute mineral oil-detergent emulsion through which air is blown, the contaminants float on
foam and are removed. The cleaned peas are given a final wash to remove the emulsion.
Dewatering- Wet cleaning results in a cleaned product that may have some excess water
adhering to it. Dewatering may be effected by passing the food over vibratory screens or
specially designed rotary screens. In the case of cleaned peas for freezing, or washed wheat for
milling, centrifuges may be used. Occasionally it may be necessary to resort to drying
procedures, as in the case of cereals or fruits, which arc to be stored or sold as fresh.
The two main objectives of cleaning food raw materials are
1. Removal of contaminants which constitute a health hazard or which are aesthetically
18. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 18 of 91
2. Control of microbiological loads and biochemical reactions which impair subsequent process
effectiveness and product quality.
SORTING OF FOODS
Sorting and grading are terms which are frequently used interchangeably in the food
processing industry, but strictly speaking they are distinct operations.
Sorting is a separation based cm a single measurable property of raw material
units, while grading is the assessment of the overall quality of a food using a number of
attributes". Sorting may be regarded as a separation operation based on the differences in
physical properties of the food raw materials or products such as colour, size, shape or weights
of the food raw material.
Sorting is an important operation in controlling the effectiveness of many processes in
food industry. For example, sorted vegetables and fruits are better suited for mechanized
operations of peeling, pitting and coring or blanching. Similarly, food materials of uniform size
or shape are better suited for efficient heat transfer during sterilization, pasteurization,
dehydration or freezing.
Sorting and grading can both damage the food raw material or product because of
improper handling by human operators (operator damage), dumping (dumping damage) or
dropping of material (drop damage). Such damages can be eliminated or minimized by
choosing effective food process.
Sorting methods include weight sorting, shape sorting, size sorting and photometric or colour
Weight sorting- Weight is usually the most precise method of sorting. The weight of a
food unit is proportional to the cube of its characteristic dimension and hence weight sorting is
more precise compared to dimensional sorting. Meat cuts, fish fillets, fruits such as apples,
pears and citrus fruits,
vegetables such as
potatoes, carrots and
onions and eggs are sorted
by weight using spring-
loaded, strain gauge, or
devices incorporated into
conveying systems. An alternative system is to use the "catapult' principle where units are
thrown into different collecting section, depending on their weight. A disadvantage of weight
sorting is the relatively long time required per unit and other methods are more appropriate
with smaller items such as legumes or cereals, or if faster throughput is required.
Size sorting- Different types of screens are used for size separation of foods,
19. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 19 of 91
The screen designs commonly used in food
industry may be grouped into two types: (i) variable aperture screens using cable, belt, roller or
screw sorters and (ii) fixed aperture screens using stationary, vibratory, rotary, gyratory or
reciprocating screens. Fixed aperture screens of flat-bed type are used in preliminary sorting of
potatoes, carrots and turnips. Multi-deck screens are used in size sorting of cereals, nuts and
also partly processed and finished foods such as flour, sugar, salt, ground spices and herbs.
Drum screens are used for sorting peas, beans and other similar foods capable of withstanding
tumbling action in a rotating drum screen. Variable aperture screens with continuously variable
apertures of roller, belt or screw type find use in size sorting of fruits and vegetables.
Shape sorting- Shape sorting
is adopted when food raw
materials contain undesirable
material even after size or
weight sorting and cleaning.
For example, cleaned and size
or weight sorted wheat may
still contain weed seeds of
similar size and weight
compared to wheat. Shape
sorting on the basis of a
combination of length and diameter is useful under such circumstances. A disc sorter is used
for shape sorting wheat, rice, oats and barley. The principle is that disks or cylinders with
accurately shaped indentations will pick up seeds of the correct shape when rotated through the
stock, while other shapes will remain in the feed.
20. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 20 of 91
Photometric/Color sorting- Photometric sorting uses optical properties of foods to
effect separation of desired material from contaminants. The goal is the separation of items that
are discolored, toxic, not as ripe as required, or still with hull. The color separator separates the
fruits, vegetables or grains due to difference in color or brightness. The color separators are
generally used for larger crop seeds like peas and beans. These seeds differ in color because of
varietal differences and also due to immaturity or disease. Color sorters are also used for color
sorting harvested foodstuffs, such as coffee, nuts, rice, and other cereals such as wheat or rye
Two photocells are fixed at a particular angle, which direct both beams to one point of
the parabolic trajectory of the grains. A needle is placed on the other side, which is connected
to a high voltage source. When a beam falls on a dark object through photoelectric cells,
current is generated on the needle. The needle end receives a charge and imparts it to the dark
seeds. The grains are then passed between two electrodes with a high potential difference
between them. The seed is compared with a selected background or color range, and is
separated into two fractions according to difference in color. Since this machine views each
produce individually, the capacity is low.
Reflectance properties are used to indicate:
1. Raw material maturity (e.g. color of fruit, vegetables and meat indicates ripeness
and freshness characterize ;)
2. the presence of surface defects (e.g. worm holed cereals or nuts and bruised
3. The extent of heat processing (e.g. in manufacture of bread and potato chips or
Other sorting methods- Sorting on the basis of surface roughness or stickiness may be used
for separating seeds. In Surface Texture/Roughness Separator the mixture to be separated is fed over
the centre of an inclined draper belt moving in upward direction. The round and smooth grains roll or
slide down the draper at faster rate than the upward motion of the belt, and these are discharged in a
21. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 21 of 91
hopper. The flat shape or rough surfaced particles are carried to the top of the inclined draper and
dropped off into another hopper.
GRADING OF FOODS
Grading is quality separation on the basis of an overall assessment of those properties, which affect the
acceptance of the food raw material for processing, and finished food product for consumer
acceptance and safety. The grading factors which determine the quality of the food include:
1. Process suitability
2. consumer safety
3. Consumer acceptance.
The grading parameters commonly used in food industry include the following:
size and shape as functional and acceptability factors,
maturity to describe the freshness of eggs, ripeness of fruits and aging of meat,
texture to grade the crumb structure in bread and cakes, crispness in apples and viscosity of
flavour and aroma as indicators of ripeness of fruits as well as effectiveness of processing
colour as indicator for consumer acceptability and effectiveness of process,
Blemishes such as cloudy yolk, blood spot and shell cracks in eggs, bruises in fruits and
insect holes in coffee beans and cereals to indicate their defect and impurity.
Contaminants and undesired parts such as rodent hair and insect parts in flour, soil and spray residues
on fruits and vegetables, microorganisms and their metabolites on meat, toxic metals in shell fish,
hone fragments in meat products, pod residues in peas and beans and stalks and stones in fruits all
these are the adverse qualities of the raw food materials.
Grading methods may be classified into two types:
Quality control procedures in which the quality of the food is determined by laboratory tests on
samples drawn statistically from a batch of food.
Procedures in which the total quantity of food is subjected to physical separation in quality
categories. This grading may be carried out manually or by specialized machines.
For proper grading, the food unit must be presented singly before the human grader or machine for
assessment. These devices may be roller or vibratory tables or rotating wheels equipped peripherally
with pneumatic devices which pick up food pieces, rotate them for viewing and then release them at a
Manual grading is done by trained operators who are able to assess a number of grading
parameters simultaneously. For example, eggs are graded manually by candling.
. Machine grading is only feasible where quality of a food is linked to a single physical property,
and hence a sorting operation leads to different grades of material. But can be carried out by
combining a group of sorting operations so as to separate the food units on quill it basis. Thus wheat of
a particular variety may be graded by a combination of cleaning and sorting operations. Sometimes a
single property may be helpful in grading the food. Thus peas of small size are recognized to be most
tender and of highest quality so that size sorting of cleaned peas results in quality grading. Peas may
also be graded on the basis of their density using flotation in brines of varying densities. Similarly,
potatoes or high density, desirable for manufacturing French fries, potato crisps and dehydrated
mashed potato, may be graded using Rotation in brines. Mechanical grading is cost effective and
22. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 22 of 91
Size Reduction - Theory of Comminution; Calculation of energy required during size
reduction. Crushing efficiency; Size reduction equipment; Size reduction of fibrous, dry and
liquid foods; effects of size reduction on sensory characteristics and nutritive value of food
Size reduction is a process of reducing large solid unit masses-vegetables or chemical substances
into small unit masses, coarse particles or fine particles.
The term size reduction is applied to all the ways in which particles of the food materials is reduced
into smaller size. The size reduction is done for different purpose and by different methods. Crushing,
grinding, hammering and cutt9ng are the main methods of size reduction for food material.
In the case of liquids and semisolids, size reduction operations include mashing, atomizing,
homogenizing etc. The following are some important applications size reduction in the food industry:
Milling of cereal grains to obtain flour
Fine grinding of chocolate mass
Flaking of soybeans prior to solvent extraction
Cutting of vegetables and fruits to desired shapes (Cubes, strips, slices…)
Fine mashing of baby food
Homogenization of milk and cream
Theory of Comminution: It is the process of size reduction. So that the surface area of the produce
increases and solvent can easily interact with the produce. Most of the natural produce is to be dried.
Drying can be done in sun or shade or in the protected area depending upon the type of the
constituents. It is preferred that drying should be slow at low temperature. The dried material is to be
crushed or broken into small parts before extraction/ distillation. During crushing/grinding temperature
of the produce should not be increased. Some of the volatiles get evaporated even at 45o
homogeneity of the ground particle shows the efficacy of the extraction of active ingredient.
Calculation of energy required during size reduction. Crushing efficiency;
Energy and Power for Size reduction:
The cost of power is the major expense in crushing and grinding operation. Thus, accurate
estimation of the energy required is important in the design and selection of size reduction equipment.
During size reduction, the solid particles are first distorted and strained. By applying additional
force, the stressed particles are distorted beyond their ultimate strength and suddenly rupture into
fragments. Thus, new surface is generated. The energy of stress in excess of the new surface energy
created appears as heat.
It is not possible to estimate accurately the power requirement of crushing and grinding
equipment to effect the size reduction of a given material, but a number of empirical laws have been
put forward e.g. Rittinger's law, Kick's law and Bond's law.
Kick's law: Kick’s law states that the work required for crushing a given mass of material is constant
for the same reduction ratio, that is, the ratio of the initial particle size to the final particle size. Kick
assume that the energy required to reduce a material in size was directly proportional to the size
reduction ratio dL/L.
E = KKfc loge (L1/L2)
Kk is called Kick’s constant
fc is called the crushing strength of the material
23. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 23 of 91
Equation of Kick's Law implies that the specific energy required to crush a material, for
example from 10 cm down to 5 cm, is the same as the energy required to crush the same material from
5 mm to 2.5 mm.
Rittinger's theory: Von Rittinger proposed a theory stating that the energy consumed in comminution
is proportional to new surface produced. Rittinger on the other hand, assumed that the energy
required for size reduction is directly proportional, not to the change in length dimensions, but to the
change in surface area.
E = KRfc(1/L2– 1/L1)
KR is called Rittinger's constant
fc is called the crushing strength of the material
Equation of Rittinger's Law means that energy required to reduce L for a mass of particles from 10 cm
to 5 cm would be the same as that required to reduce, for example, the same mass of 5 mm particles
down to 4.7 mm. This is a very much smaller reduction, in terms of energy per unit mass for the
smaller particles, than that predicted by Kick's Law.
Bond's theory: Bond's so called third theory of comminution states that the energy required is
proportional to the length of crack initiating breakage
E = Ei (100/L2)1/2
[1 - (1/q1/2)
Ei is the amount of energy to reduce unit mass of the material from an infinitely large particle size
down to a particle size of 100 mm.
It appears that Kick's results apply better to coarser particles, Rittinger's to fine ones with Bond's being
It is defined as the ratio of the surface energy created by crushing to the energy absorbed by the solid
Ƞc = es (Ab-Aa)
Ƞc = crushing efficiency
Wn = energy absorbed by material, J/kg
es = surface energy per unit area, J/m2
Ab = area of product, m2
Aa = area of feed, m2
The energy created by fracture is very small as compared to the energy stored in the material at
the time of rupture, and most of the mechanical energy stored in the material is converted into heat.
Crushing efficiencies are thus low.
24. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 24 of 91
The energy absorbed by the solid (Wn) is less than the energy supplied to the machine (W).Part
of the total energy input to the machine is utilized to overcome the friction in the bearings and other
moving parts and the remaining part is available for crushing. The mechanical efficiency is the ratio of
the energy absorbed to the energy input.
The minimum energy required for crushing is the energy required for creating fresh surface. In
addition, energy is absorbed by the particulate material due to deformation, friction, etc., which results
in an increase of the material temperature.
Advantages of size reduction
1. Size reduction increases the digestion of the food.
2. smaller particles are easy to pack
3. Facilitating separation of different parts of a material (milling wheat to obtain flour and bran
4. Accelerating heat and mass transfer (atomization of milk as a fine spray into hot air in spray
5. Size reduction also increase the reactivity of solids
6. Size reduction make the food eatable
7. Facilitating mixing and dispersion
8. Size reduction also reduces the bulk of fibre material.
9. Obtaining pieces and particles of defined shapes.
10. Easy to handle and pack
11. To improve blending efficiency of formulations, composites e.g insecticides, dyes, paints
Disadvantages of size reduction
The destruction of cells resulting increased in surface are and promotes oxidation deterioration.
Due to this high microbiological deterioration and increased the enzyme activity which effect the
quality aroma and texture.
Criteria for size reduction
An ideal crusher would (1) have a large capacity; (2) require a small power input per unit of product;
and (3) yield a product of the single size distribution desired.
25. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 25 of 91
Principles of size reduction
Most size reduction machines are based on the following principles:
3. Attrition or rubbing
1. Compression/Crushing: - When an external force is applied on a material in excess of its
strength, the material fails because of its rupture in many directions. The particles produced
after crushing are irregular in shape and size. The type of material and method of force
application affects the characteristics of new surfaces and particles. Food grain flour, grits and
meal, ground feed for livestock are made by crushing process. Crushing is also used to extract
oil from oilseeds and juice from sugarcane.
2. Impact: - When a material is subjected to sudden blow of force in excess of its strength, it
fails, like cracking of nut with the help of a hammer. Operation of hammer mill is an example
of dynamic force application by impact method.
3. Attrition/ Shearing: - It is a process of size reduction which combines cutting and crushing.
4. Cutting: In this method, size reduction is accomplished by forcing a sharp and thin knife
through the material. In the process minimum deformation and rupture of the material results
and the new surface created is more or less undamaged. An ideal cutting device is a knife of
excellent sharpness and it should be as thin as practicable. The size of vegetables and fruits are
reduced by cutting.
SIZE REDUCTION EQUIPMENT
Size-reduction equipment is divided into crushers, grinders, ultrafine grinders, and cutting
Crushers do the heavy work of breaking large pieces of solid material into small lumps. A primary
crusher breaking it into 150 to 250 mm lumps. A secondary crusher reduces these lumps to particles
perhaps 6 mm in size. Example: -
1. Jaw crushers
2. Gyratory crushers
3. Crushing rolls
4. Cone Crushers
Intermediate Crusher: - Feed size is about 50mm to 5 mm and final product size may be 5 to 0.1mm
1. Hammer mills; impactors
2. Rolling-compression mills
Fine Crusher/ Grinders reduce crushed feed to powder. The product from an intermediate grinder
might pass a 4O mesh screen; most of the product from a fine grinder would pass a 200-mesh screen
and the feed size may be in the range of 5-2mm.
1. Attrition mills
4. Tumbling mills
a) Rod mills
b) Ball mills; pebble mills
c) Tube mills; compartment mills
Ultrafine grinder accepts feed particles no larger than 6 mm; the product size is typically 1 to 50 μm.
1. Hammer mills (Fine Impact Mill)
2. Fluid-energy mills
26. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 26 of 91
Cutters give particles of definite size and shape, 2 to 10 mm in length.
1. Knife cutters; dicers; slitters
These machines do their work in distinctly different ways. Compression is the characteristic action of
crushers. Grinders employ impact and attrition, sometimes combined with compression; ultrafine
grinders operate principally by attrition. A cutting action is of course characteristic of cutters, dicers,
Crushers are slow-speed machines for coarse reduction of large quantities of solids. The main
types are jaw crushers, gyratory crushers, smooth-roll crushers, and toothed-roll crushers.
The first three operate by compression and can break large lumps of very hard materials, as in
the primary and secondary reduction.
Toothed-roll crushers tear the feed apart as
well as crushing it; they handle softer feeds
like coal, bone, and soft shale.
Jaw crushers: In a jaw crusher feed is
admitted between two jaws, set to form a V
open at the top. A jaw crusher consists of a vertical
fixed jaw and another swinging jaw moving in the
horizontal plane. The two jaws make 20-30o
between them. The jaw faces may be flat or slightly bulged. Feed is admitted between the jaws. Large
lumps caught between the upper parts of the jaws are broken; drop into the narrower space below. It is
crushed several times between the jaws before it is discharged at the bottom opening. After sufficient
reduction they drop out the bottom of the machine. The jaws open and close 250 to 400 times per
minute. A jaw crusher is a primary crusher which produces a course product
27. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 27 of 91
Gyratory crushers: A gyratory crusher is
similar in basic concept to a jaw crusher.
This type of crushers consist a concave
surface and a conical head and both surfaces
are rough and typically lined. The inner cone
has a slight circular movement, but does not
rotate. An eccentric drives the bottom end of
the shaft. A gyratory crusher is one of the
main types of primary crushers in a mine or
ore processing plant.
The crushing action is caused by the
closing of the gap between the movable
center surface and main frame of the
crusher. The gap is opened and closed by an
eccentric on the bottom of the spindle that
causes the central vertical spindle to gyrate.
The speed of the crushing head is typically 125 to 425 gyrations per minute. Less maintenance is
required than with a jaw crusher; and the power requirement per ton of material crushed is smaller.
a. Smooth-roll crushers
b. Toothed-roll crushers
Smooth-roll crushers: In this type of crushers two heavy smooth-faced metal rolls are present, which
are mounted horizontally. The size of the rollers may be from a few centimeters to meters to
diameters. Generally, one of the rollers is driven directly, while the second one runs freely. The
material to be crushed is feed form the hopper into the gap between the two rollers. Due to rotation of
these rollers the material is crushed. Typical rolls are 600 mm to 2000 mm in diameter. Rollers speed
range from 50 to 300 r/min. Smooth-roll crushers are secondary crushers, with feeds 12 to 75 mm in
size and products 12 mm to about 1 mm. The limiting size Dp.max. of particles that can be nipped by
the rolls depends on the coefficient of friction between the particle and the roll surface, but in most
cases, it can be estimated from the simple relation.
28. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 28 of 91
Dp.max. = 0.04R + d5.1 (5.1)
Where R = roll radius
d = half the width of the gap between the
The maximum size of the
product is approximately equal to 2d. The particle
size of the product depends on the spacing between
the rolls, as does the capacity of a given machine.
Smooth-roll crushers give few fines and virtually no
oversize. To avoid damaging machine, at least one
roll must be spring mounted.
Toothed Roll Crusher: -Toothed roll
crusher is widely used in coal, metallurgy, mining,
chemical industry, building materials and other
industries, and it is more suitable to crush coal in
large coal mines or coal preparation plant so that it also can be called coal crusher. Toothed roll
crusher has high crushing capacity. The distance between the rollers can be adjusted by hydraulic
pressure. Such crushers may contain two rolls, as in smooth-roll crushers. A single-roll toothed
crusher is also used. Some crushing rolls for coarse feeds
carry heavy pyramidal teeth.
Toothed-roll crushers are much more versatile than
smooth-roll crushers, within the limitation that they cannot
handle very hard solids. They operate by compression,
impact, and shear, not by compression alone, as do
smooth-roll machines. Some heavy-duty toothed double-
roll crushers are used for the primary reduction of coal and
similar materials. The particle size of the feed to these
machines may be as great as 500 mm; their capacity
ranges up to 500 tons/h.
The term grinder describes a variety of size-reduction machines for intermediate duty. The product
from a crusher is often fed to a grinder, in which it is reduced to powder. The chief types of
commercial grinders described in this section are hammer mills and impactors, rolling-compression
machines, attrition mills, and tumbling mills.
1. Hammer mills:
Principle: - It operates on the principle of impact between rapidly moving hammers mounted on rotor
and the stationary powder material.
These mills all contain a high-speed rotor turning inside a cylindrical casing. The shaft is
usually horizontal Feed dropped into the top of the casing is broken and falls out through a bottom
opening. In a hammer mill the particles are broken by sets of swing hammers attached to a rotor disk.
A particle of feed entering the grinding and shatters into pieces by hammers, then the material pushed
through a screen that covers the discharge opening.
Several rotor disks of 150 to 450 mm diameter carrying four to eight swing hammers are often
mounted on the same shaft.. Intermediate hammer mills yield a product 25 mm to 20-mesh in particle
size. Hammer mills grind almost anything tough fibrous solids like bark or leather, steel turnings, soft
wet pastes, sticky clay, hard rock. For fine reduction they are limited to the softer materials.
29. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 29 of 91
Particles are broken by impact without rubbing action in a hammer mill.
2. Rolling-compression machines:
Material is compressed by application of stress and attrition. Stress is applied by rotating heavy
wheels, Muller or Rollers.
Roller mills are similar to roller crushers, but 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 pass between the rolls can be
regulated. If the friction coefficient between the rolls and the feed material is known, the
largest particle that will be nipped between the rolls can be calculated, knowing the geometry
of the particles. In this kind of mill the solid particles are caught and crushed between a rolling
member and the face of a ring or casing. The most common types are rolling-ring pulverizers,
bowl mills, and roller mills. They pulverize up to 50 ton/h. When classification is used, the
product may be as fine as 99 percent through a 200-mesh screen.
3. Attrition mills: In attrition mill particles of soft solids are rubbed between the grooved flat
faces of rotating circular disks. The axis of the disks is usually horizontal, sometimes vertical.
In a single-runner mill one disk is stationary and one rotates; in a double-runner machine both
disks are driven at high speed in opposite directions. Feed enters through an opening in the hub
of one of the disks; it passes outward through the narrow gap between, the disks and discharges
from the periphery into a stationary casing. The width of the gap, within limits, is adjustable.
Mills with different patterns of grooves, corrugations, or teeth on the disks perform a variety of
operations, including grinding, cracking, granulating, and shredding, and even some operations
not related to size reduction at all, such as blending. There are two type of mills Single-runner
mills and double run mill. The disks of a single-runner mill are 250 to 1400 mm in diameter;
turning at 350 to 700 r/min. Disks in double-runner mills turn faster, at 1200 to 7000 r/min.
The disks may be cooled with water or refrigerated brine. Cooling is essential with heat-
30. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 30 of 91
sensitive solids like spices, rubber which would otherwise be destroyed. Attrition mills grind
from 1 to 8 ton/h to products that will pass a 200-mesh screen.
Attrition mills are used for fine grinding operations in the production of spices (pepper, cinnamon, and
paprika), food (peanuts, grain, cereal), fibers (chips, cork, cellulose) and blending (face powders,
insecticides). The pictures below show pepper and cinnamon, finished products from attrition milling.
Finely ground products.
Large range of sizes available.
Needs specific input size.
4. Tumbling/Ball mills: tumbling mills is basically of three types
a) Rod mills
b) Ball mills; pebble mills
c) Tube mills; compartment mills
It operates on the principle of impact and attrition.
A cylindrical shell slowly turning about a horizontal axis and filled to about half its volume with a
solid grinding medium forms a tumbling mill. The shell is usually steel, lined with high-carbon steel
plate, porcelain, silica rock, or rubber. The grinding medium is metal rods in a rod mill, lengths of
chain or balls of metal, rubber, or wood in a ball mill, flint pebbles or porcelain or zircon spheres in a
pebble mill. Tumbling mills may be continuous or batch. In a batch machine solid to be ground is
loaded into the mill through an opening in the shell. The opening is then closed and the mill turned on
for several hours; it is then stopped and the product is discharged. In a continuous mill the solid flows
steadily through the revolving shell, entering at one end through a hollow turn-on and leaving at the
other end through the turn-on or through peripheral openings in the shell. In all tumbling mills the
grinding rods are usually steel, 25 to 125 mm in diameter.
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In a ball mill or pebble mill most of the reduction is done by impact as the balls or pebbles drop
from near the top of the shell. In a large ball mill the shell might be 3 m in diameter and 4.25 m long.
The balls are 25 to 125 mm in diameter; the pebbles in a pebble mill are 50 to 175 mm in size.
A tube mill is a continuous mill with a long cylindrical shell, in which material is ground for 2 to 5
times as long as in the shorter ball mill. Tube mills are excellent for grinding to very fine powders in a
single pass where the amount of energy consumed is not of primary importance. Putting slotted
transverse partitions in a tube mill converts it into a compartment mill.
One compartment may contain large balls, other small balls, and a third pebbles. The amount of
energy expended is suited to the difficulty of the breaking operation, increasing the efficiency of the
Vertical spindle mills are used in the mineral industry to grind materials such as phosphate, limestone,
magnesite, and bauxite.
Rings and rollers wear easily.
Many commercial powders must contain particles averaging 1 to 20 μm in size, with substantially all
particles passing a standard 325-mesh screen that has openings 44 μm wide. Mills that reduce solids to
such fine particles are called ultra fine grinders. Ultrafine grinding of dry powder is done by grinders,
such as high-speed hammer mills, provided with internal or external classification, and by fluid-energy
or jet mills. Ultrafine wet grinding is done in agitated mills.
1. Hammer mills: As like given above
2. Fluid energy mills:
Principle: It operates on the principle of impact and attrition.
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In these mills the particles are suspended in a high - velocity gas stream. The reduction occurs
when the particles strike or rub against the walls of the confining chamber, but most of the reduction is
believed to be caused by interparticle attrition. Internal classification keeps the larger particles in the
mill until they are reduced to the desired size. Gas is usually compressed air or superheated steam,
admitted at a pressure of 7 atm through energizing nozzles.
The grinding chamber is an oval loop of pipe 25 to 200 mm in diameter and 1.2 to 2.4 m high.
Feed enters near the bottom of the loop through a venturi injector. Classification of the ground
particles takes place at the upper bend of the loop. As the gas stream flows around this bend at high
speed, the coarser particles are thrown outward against the outer wall while the fines congregate at the
inner wall. A discharge opening in the inner wall at this point leads to a cyclone separator and a bag
collector for the product. They reduce up to 1 ton/h of non sticky solid to particles averaging! to 10
11m in diameter, using 1 to 4 kg of steam or 6 to 9 kg of air per kilogram of product. Loop mills can
process up to 6000 kg/h.
Pulverizers are commonly used for chemicals, pigments and food processing. The
microscale air impact pulverizer is used in laboratories, where small samples are needed.
Air needed is free.
Large range of sizes available.
3. Agitated mills: For some ultrafine grinding operations, small batch non rotary mills containing
a solid grinding medium are available. The medium consists of hard solid elements such as
balls, pellets, or sand grains. These mills are vertical vessels 4 to 1200 L in capacity, filled with
liquid in which the grinding medium is suspended. In some designs the charge is agitated with
a multiarmed impeller; in others, used especially for grinding hard materials (such as silica or
titanium dioxide), a reciprocating central column “vibrates” the vessel contents at about 20 Hz.
Concentrated feed slurry is admitted at the top, and product (with some liquid) is withdrawn
through a screen at the bottom. Agitated mills are especially useful in producing particles 1 /lm
in size or finer.
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4. Colloid mills: In a colloid mill,
intense fluid shear in a high-
velocity stream is used to disperse
particles or liquid droplets to form
a stable suspension or emulsion.
The final size of the particles or
droplets is usually less than 5 /lm..
Syrups, milk, purees, ointments,
paints, and greases are typical
products processed in this way. In
most colloid mills the feed liquid is
pumped between closely spaced
surfaces one of which is moving
relative to the other at speeds of 50
m/s or more. In the mill the liquid
passes through the narrow spaces
between the disk-shaped rotor and
the casing. The clearances are adjustable down to 25 /lm. Often cooling is required to remove
the heat generated. The capacities of colloid mills are relatively low, ranging from 2 or 3 L/min
for small mills up to 440 L/min for the largest units.
Colloid mills are used largely in asphalt production and grease manufacturing. They are also used in a
wide variety of industries, such as paints, pigments, food and cosmetics, such as in the production of
the lipstick. In the food processing industry, colloid mills are used in the production of mayonnaise,
peanut butter, salad dressings, buttered syrups, and chocolate toppings.Pin mills are commonly used to
produce talc, clays, resins, flour and starch.
Rugged and durable.
Wide variety of uses.
In colloid mills, the feed must be in a
Pins in pin mills wear easily.
Unlike most types of mills, cone mills can be used for hard to grind products while using less energy
than other types of mills. Cone mills are preferred in some industries because they produce less noise,
dust, and heat than traditional milling equipment.
Material is fed into the conical chamber by gravity or conveying it. Inside the chamber is a rotor that
spins at a low velocity and forces the material against the wall. The rotor has two paddles that pass
over the material on the wall, inducing a shear force on it. This shear force breaks apart the material
and when the particles are small enough they pass through the holes in the wall and fall into a
collection container. Since the rotor is spinning at a low velocity the particles that pass through the
34. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 34 of 91
wall tend to have a uniform size and the rotor generates little heat. This system is completely enclosed
so that little noise and dust is generated.
Cone milling is used in the pharmaceutical, food, and chemical industries. It is widely used in
pharmaceuticals for wet and dry granulation. In the food industry it is many used for grinding of foods
such as sugar, candy, and chocolate.
Low heat generation
Low noise and dust emissions
Can mill sticky materials
Easy to clean
In some size-reduction problems the feed
stocks are too tenacious or too resilient to be
broken by compression, impact, or attrition. In
other problems the feed must be reduced to
particles of fixed dimensions. These requirements
are met by devices that cut, chop, or tear the feed
into a product with the desired characteristics.
The saw-toothed crushers mentioned above do
much of their work in this way. True cutting
machines include rotary knife cutters and
granulators. These devices find application in a
variety of processes but are especially well
adapted to size reduction problems in the
manufacture of rubber and plastics.
Principle of rotary cutters
Size Reduction involves successive cutting / Shearing the feed material with help of sharp knife
Knife cutters: A rotary knife cutter contains a horizontal rotor turning at 200 to 900 r/min in a
cylindrical chamber. On the rotor are 2 to 12 flying knives with edges of tempered steel or satellite
passing with close clearance over 1 to 7 stationary bed knives. Feed particles entering the chamber
from above are cut several hundred times per minute and emerge at the bottom through a screen with 5
to 8 mm openings. Sometimes the flying knives are parallel with the bed knives; sometimes,
depending on the properties of the feed, they cut at an angle. Rotary cutters and granulators are similar
in design. A granulator yields more or less irregular pieces; a cutter may yield cubes, thin squares, or
Size reduction of fibrous foods
Most fruits and vegetables fall into the general category of ‘fibrous’ foods. Fruits and vegetables have
an inherently firmer texture and are cut at ambient or chill temperatures. There are five main types of
size reduction equipment, classified in order of decreasing particle size, as follows.
1. Slicing equipment consists of rotating or reciprocating blades which cut the food as it passes
beneath. In some designs food (Figure 6.1) is held against the blades by centrifugal force. In other (for
slicing meats) the food is held on a carriage as it travels across the blade. Harder fruits such as apples
35. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 35 of 91
are simultaneously sliced and de-cored as they are forced over stationary knives fitted inside a tube. In
a similar design (the hydro cutter) foods are conveyed by water at high speed over fixed blades.
2. Dicing equipment is for vegetables, fruits and meats. The food is first sliced and then cut into strips
by rotating blades. The strips are fed to a second set a rotating knives which operate at right angles to
the first set and cut the strips into cubes (Figure 6.2).
3. Flaking equipment for flaked nuts, fish or meat is similar to slicing equipment. Adjustment of the
blade type and spacing is used to produce the flakes.
4. Shredding equipment. Typical equipment is a modified hammer mill in which knives are used
instead of hammers to produce a flailing or cutting action. A second type of shredder is known as the
squirrel cage disintegrator. Here two concentric cylindrical cages inside a casing are fitted with knife
blades along their length. The two cages rotate in opposite directions and food is subjected to powerful
shearing and cutting forces as it passes between them.
5. Pulping equipment is used for juice extraction from fruits or vegetables and for pureed and pulped
meats. A combination of compression and shearing forces is used in each type of equipment. A rotary
grape crusher consists of a cylindrical metal screen fitted internally with high-speed rotating brushes
or paddles. Grapes are heated if necessary to soften the tissues, and pulp is forced through the
perforations of the screen by the brushes. The size of the perforations determines the fineness of the
pulp. Skins, stalks and seeds discarded from the end of the screen. Other types of pulper, including
roller presses and screw presses are used for juice expression.
A bowl chopper is used to chop meat and harder fruits and vegetables into a coarse pulp (for
example for sausage meat or mincemeat preserve). A horizontal, slowly rotating bowl moves the
ingredients beneath a set of high-speed rotating blades. Food may be passed several times beneath the
knives until required degree of size reduction and mixing has been achieved.
Size reduction of Liquid Foods (Emulsification and Homogenization)
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The terms emulsifiers and homogenizers are often used interchangeably for equipment used to
produce emulsions. Emulsification is the formation of a stable emulsion by the intimate mixing of two
or more immiscible liquids, so that one (the dispersed phase) is dispersed in the form of very small
droplets within the second (the continuous phase). Homogenization is the reduction in size (to 0.5-
3μm) and increase in number of solid or liquid particles of the dispersed phase, by the application of
intense shearing forces, to increase the intimacy and stability of the two substances. Homogenization
is therefore a more severe operation than emulsification. Both operations are used to change the
functional properties or eating quality of foods. They have little or no effect on nutritional value or
The four main types of homogenizer are as follows:
1. High-speed mixers;
2. Pressure homogenizers;
3. Colloid mills;
4. Ultrasonic homogenizers.
1. High-speed mixers
Turbine or propeller-type high-speed mixers are used to pre-mix emulsions of low-viscosity
liquids. They operate by shearing action on the food at the edges and tips of the blades.
2. Pressure homogenizers
These consist of a high-pressure pump, operating at 10,000-70,000kPa, which is fitted with a
homogenizing valve on the discharge side. When liquid is pumped through the small adjustable gap
(300μm) between the valve and the valve seat, the high-pressure results in a high liquid velocity (8400
ms−1). There is then an almost instantaneous drop in velocity as the liquid emerges from the valve.
These extreme conditions of turbulence produce powerful shearing force. In some foods (for example
milk products) there may be inadequate distribution of the emulsifying agent over the newly formed
surfaces, which causes fat globules to clump together. Pressure homogenizers are widely used before
pasteurization and ultrahigh temperature sterilization of milk, and in the production of salad creams,
ice cream and some sauces.
3. Colloidal mills
These homogenizers are essentially disc mills. The small (0.05-1.3 mm) gap between a vertical
disc which rotates at 3000-15000 rev min−1 and a similar sized stationary disc creates high shearing
forces. Size reduction takes place between a stationary part (stator) and a rotating cone (rotor). The
premix is feed into the area between the rotor and stator by centrifugal force. With the high peripheral
speed, the rotor generates high shear fields within the fluid in the working area. They are more
37. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 37 of 91
effective than pressure homogenizers for high-viscosity liquids, but with intermediate viscosity liquids
they tend to produce larger droplet sizes than pressure homogenizers do. Numerous designs of disc,
including flat, corrugated and conical shapes, are available for different applications.
For highly viscous foods (for example peanut butter, meat or fish pastes) the discs may be
mounted horizontally (the paste mill). The greater friction created in viscous foods may require these
mills to be cooled by recirculating water.
4. Ultrasonic homogenizers
High-frequency sound waves (18-30 kHz) cause alternate cycles of compression and tension in
low-viscosity liquids and capitation of air bubbles, to form an emulsion with droplet sizes of 1-2μm.
In. operation, the dispersed phase of an emulsion is added to the continuous phase and both are
pumped through the homogenizers at pressures of 340-1400kPa. The ultrasonic energy is produced by
a metal blade, which vibrates at its resonant frequency. Vibration is produced either electrically or by
the liquid movement (Figure 6.8). The frequency is controlled by adjusting the clamping position of
the blade. This type of homogenizer is used for the production of salad creams, ice cream, synthetic
creams and essential oil emulsions. It is also used for dispersing powders in liquids
Size reduction of dry foods
There are a large number of mills available for application to specific types of food.
1. Ball mills
This type of mill consists of a slowly rotating, horizontal steel cylinder which is half filled with
steel balls 2.5-15cm in diameter. At low speeds or when small balls are used, shearing forces
predominate. With larger balls or at higher speeds, impact forces become more important. A
modification of the ball mill named the rod mill has rod instead of balls to overcome problems
associated with the balls sticking in adhesive foods.
2. Disc mills
A disc mill is a type of
crusher can be used to grind,
cut, shear, crack, rub, curl,
twist, hull, blend or refine. It
works in a similar manner to
the ancient Burhstone mill in
that the feedstock is fed
between opposing discs or
plates. The disc may be
grooved, or spiked. There are
a large number of designs of
disc mill. Each type employs
shearing forces for fine
grinding or shearing and impact forces for coarser grinding. For example,
1. single-disc mills in which food passes through an adjustable gap between a stationary
casing and a grooved disc which rotates at high speed,
2. double-disc mills in which two discs rotate in opposite directions to produce greater
38. Unit operations in food processing by Mohit Jindal Vol. 1.2 Page 38 of 91
3. Hammer mills
A horizontal cylindrical chamber is lined with a toughened steel breaker plate. A high-speed
rotor inside the chamber is fitted with hammers along its length. In operation, food is disintegrated
mainly by impact as the hammers drive it against the breaker plate. In some designs the exit from the
mill is restricted by a screen and food remains in the mill until the particles are sufficiently small do
pass through the screen apertures. Under these ‘choke’ conditions; shearing forces play a larger part in
the size reduction. Two or more steel rollers revolve towards each other and pull particles of food
through the ‘nip’ (the space between the rollers) (Figure 6.6). The main force is compression but, if the
rollers are rotated at different speeds, or if the rollers are fluted (shallow ridges along the length of the
roller), there is an additional shearing force exerted on the food. The size of the nip is adjustable for
different foods and overload springs protect against accidental damage from metal or stones.
4. Roller mills
Two or more steel rollers revolve towards each other and pull particles of food through the ‘nip’ (the
space between the rollers). The main force is compression but, if the rollers are rotated at different
speeds, or if the rollers are fluted (shallow ridges along the length of the roller), there is an additional
shearing force exerted on the food. The size of the nip is adjustable for different foods and overload
springs protect against accidental damage from metal or stones.
Effect on the sensory characteristics
Size reduction is used in processing to control the textural or theological
properties of foods and to improve the efficiency of mixing and heat transfer,
the texture of many foods (for example bread, hamburgers and juices) is
controlled by the condition used during size reduction of the ingredients. There
is also art indirect effect on the aroma and flavour of some foods. The disruption of
cells and resulting increase in surface area promotes oxidative deterioration and
higher rates of microbiological and enzymic activity. Oxidation of carotenes
bleaches colours and flavour and reduce the nutritive value. There is a less of
volatile compounds form spices and some nuts. That may be due to the
expose of new surface or due to rise in temperature during milling.
Size reduction therefore has little or no preservative effect there may be
small change in sensory characteristics during size reduction.
In most of food the destruction of cell allows enzymes and
substrate to be come more thoroughly mixed which cause increase in
deterioration aroma and flavour. Additionally, the release of cellular material
provides a substance for the microbial growth and this can also result in the
development of off flavours.
The texture of food is greatly changed by sized reduction both by
physical reduction in the size of tissues and also by the release by the
The speed and duration of size reduction and gap between
completion of size reduction and after their processing are closely controlled
to achieved the desire texture
The increase in surface area of foods during size reduction causes loss of nutritional value due
to oxidation of fatty acids and carotenes. Losses of vitamin C and thiamin in chopped or sliced fruits
and vegetables are substantial. Losses during storage depend on the temperature and moisture content
of the food and on the concentration of oxygen.
Factors related to nature of raw materials affecting size reduction
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Hardness- It is easier to break soft material than hard materials. Ex: For iodine hammer mill is used.
Fibrous- These are tough in nature. A soft, tough material has more difficulty than a hard, brittle
substance. Ex: Ginger. Here cutters can be used.
Friable-These tend to fracture along well-defined planes. Brittle substances can be easily converted into
fine particles. Ex: Sucrose. Mechanism used is attrition, impact and pressure.
Elastic/ Sticky-Become soft during milling. Ex: synthetic gums, waxes, resins. Low melting substances
should be chilled before milling. These are milled using hammer, colloid or fluid energy mill.
Melting point- Waxy substances, fats and oils are softened during size reduction due to heat generated.
This is avoided by cooling the mill and the substance.
Hygroscopic- Certain substances absorb moisture content rapidly. This wet mass hampers the milling
process. Ex: Potassium carbonate. Closed system such as porcelain ball mill is used
Thermoability- Certain Substances are degraded by hydrolysis and oxidation, due to moisture and
atmospheric oxygen. Heat produced on milling enhances these reactions. Closed system is used here
with an inert atmosphere of CO2 and N. Vitamins and antibiotics are milled using fluid energy and ball
Other Factors affecting size reduction
Purity required- The size reduction of such hard substances leads to the abrasive wear of milling parts,
causing contamination. Such mills are to be avoided. The mills should be thoroughly cleansed between
Flammability- Under certain conditions fine dust such as dextrin, starch, sulphur are potential explosive
mixtures. All electrical switches should be explosive proof and mill should be well grounded
Particle size- The feed should be of proper size and enter the equipment at a uniform rate to get a fine
powder. Several stages are carried out in size reduction process. Pretreatment of fibrous materials with
pressure rollers and cutters facilitates further Comminution.
Moisture content- Presence of more than 5% moisture influences hardness, toughness, stickiness of
substance. In general, materials with moisture content below 5% are suitable for dry grinding and above
50 % for wet grinding.
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Sieving: Separation based on size (mesh size); types of screens;
effectiveness of screens
The basic purpose of any screen is to separate a mixture of particles / items of different sizes
into two distinct fractions. These fractions are,
(1) the underflow, the particles that pass through the screen,
(2) the overflow or oversize, the materials that are retained over the screen.
A screen can be termed as ideal screen that separates the mixture in such a way that the largest
particle of underflow is just smaller than screen opening, while the smallest particle of overflow is just
larger than the screen opening. But in practice a given screen does not gives perfect separation as
stated above, and is called actual screen. The underflow may contain material coarser than screen size,
whereas the overflow may contain particles smaller than screen size. In most screens the grain/ seed
drops through the screen opening by gravity. Coarse grains drop quickly and easily through large
opening in a stationary surface. With finer particles, the screening surface must be agitated in some
The common ways are
(1) revolving a cylindrical screen about a horizontal axis
(2) shaking, gyrating or vibrating the flat screens.
Screen showing how a feed is separated into two products, the oversize (overflow) and the undersize
(underflow or fines).
To get the maximum, minimum and other particle sizes, you would need to pass the material through a
series of screens. The amount retained in each screen, that is size fraction, is weighed and its
percentage calculated from the total mass of sample. This operation is called screen analysis.
Industrial screening can be done using structures made up of any of the following
spaced metal bars
perforated or slotted plates
woven wire or fabric screens
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When metal screening material is to be used, selection must be based on how compatible that metal is
with the material being screened and also on the strength of screen required For example, if you are
screening very heavy feed, you will need a strong screen. Commonly used metals include steel,
stainless steel, bronze, copper and nickel.
Screening can be done wet or dry. Screen structures include: metal bars, perforated or slotted
plates, woven wire or fabric. Metals used include steel, stainless steel, bronze, copper, nickel.
Selection of material is based on compatibility with materials being screen and strength required.
1. Woven screen sizes
Woven screens are commonly used in industry. To
describe the size of woven screen material, two
Aperture: This is the minimum clear space in mm or
mm between the edges of the opening.
Mesh: This is the number of apertures per linear inch,
i.e. number of apertures in 25.4mm along the wire. If
you count the number of openings from one wire along the inch perpendicular to the wire, the number
you get is the mesh of that screen. Screen analysis data is given in either mesh or aperture sizes.
2. Perforated Metal Screens
Round openings: The round openings in a perforated sheet metal screen are
measured by the diameter (mm or in.) of the openings. For example, 1/18
screen has round perforation of 1/18 in. in diameter or 2 mm.
Oblong openings: The oblong or slotted openings in a perforated sheet metal
screen are designated by two dimensions; the width and length of the opening.
While mentioning oblong openings the dimension of width is listed first then
the length as 1.8 x 20 mm. Generally, the direction of the oblong opening is
kept in the direction of the grain flow over the screen.
Triangular openings: There are two different systems used to measure
triangular perforations. The most commonly used system is to mention
the length of each side of the triangle in mm, it means, 9 mm triangle
has 3 equal sides each 9 mm long. The second system is to mention
openings according to the diameter in mm that can be inscribed inside
the triangle. This system is identified by the letter Vas 9V, l0V etc.
3. Wire mesh Screens
Square mesh: The square openings in wire mesh are measured by
the number of openings per inch in each direction. A 9×9 screen has 9
openings per inch.
Rectangular mesh: the rectangular openings in wire mesh screens
are measured in the same way as square wire mesh screen. A 3×6
rectangular wire mesh screen will have 3 openings per inch in one
direction and 6 openings per inch in the other direction. The rectangles
formed by the wire mesh are parallel to the direction of grain flow.