Introduction to Unit Operations in Food Industry

1. Introduction to Unit Operations:Introduction to Unit Operations:
Fundamental ConceptsFundamental Concepts

2.  Unit operations In transforming matter from inexpensive
raw materials to highly desired products, Food engineers
became very familiar with the physical and chemical
operations.
 Examples of this include: filtration drying distillation
crystallization grinding sedimentation combustion catalysis
heat exchange coating, and so on. Physical Chemical
operations
Unit operation in Food engineering

3. Thermal properties such as specific heat, conductivity,
diffusivity, and boiling point rise, freezing point depression.
Optical properties, primarily color, but also gloss and
translucency. Electrical properties, primarily conductivity
and permittivity
 Structural and geometrical properties such as density,
particle size, shape, porosity, surface roughness, and
cellularity
Mechanical properties such as textural (including strength,
compressibility, and deformability) and rheological
properties (such as viscosity)
Others, including mass transfer related properties
(diffusivity, permeability), surface tension, cloud stability,
gelling ability, and radiation absorbance.
Properties of Food Materials

4. The main objective of Unit operation in Food engineering:
i.to study the principles and laws governing the physical,
chemical, or biochemical stages of different processes, and
the apparatus or equipment by which such stages are
industrially carried out.
i.The studies should be focused on the transformation
processes of agricultural raw materials into final products, or
on conservation of materials and products

5. Production of Fruit Juice Concentrate

6. Unit Operations: Classifications
 Physical stages: grinding, sieving, mixture, fluidization,
sedimentation, flotation, filtration, absorption, extraction,
adsorption, heat exchange, evaporation, drying, etc.
 Chemical stages: refining, chemical peeling.
 Biochemical stages: fermentation, sterilization,
pasteurization, enzymatic peeling

7. Other Unit Operations:
Momentum Transfer Unit Operations:
These operations are controlled by the diffusion of a
component within a mixture.
Internal circulation of fluids: study of the movement of
fluids through the interior of the tubing.
External circulation of fluids: the fluid circulates through
the external part of a solid. This circulation includes the flow
of fluids through porous fixed beds, fluidized beds
(fluidization), and pneumatic transport.
Solids movement within fluids: the base for separation of
solids within a fluid. This type of separation includes:
sedimentation, filtration, and ultrafiltration, among others.

8. Mass Transfer Unit Operations
These operations are controlled by the diffusion of a
component within a mixture.
 Distillation: separation of one or more components by
taking advantage of vapor pressure differences.
Absorption: a component of a gas mixture is absorbed by a
liquid according to the solubility of the gas in the liquid.
Absorption may occur with or without chemical reaction.
The opposite process is called desorption.

9.  Extraction: based on the dissolution of a mixture (liquid or
solid) in a selective solvent, which can be liquid–liquid or
solid–liquid. The latter is also called washing etc.
 Adsorption: also called sorption, adsorption involves the
elimination of one or more components of a fluid (liquid or
gas) by retention on the surface of a solid.
 Ionic exchange: substitution of one or more ions of a
solution with another exchange agent.

10. Heat Transfer Unit Operations
These operations are controlled by temperature gradients.
They depend on the mechanism by which heat is transferred:
 Conduction: in continuous material media, heat flows in the
direction of temperature decrease and there is no macroscopic
movement of mass.
 Convection: the enthalpy flow associated with a moving
fluid is called convective flow of heat. Convection can be
natural or forced.

11.  Radiation: energy transmission by electromagnetic
waves. No material media are needed for its transmission.
 Thermal treatments (sterilization and pasteurization),
evaporation, heat exchangers, ovens, solar plates, etc. are
studied based on these heat transfer mechanisms.

12. Simultaneous Mass–Heat Transfer Unit Operations
In these operations a concentration and a temperature
gradient exist at them same time:
Humidification and dehumidification: include the
objectives of humidification and dehumidification of a gas
and cooling of a liquid.
Crystallization: formation of solid glassy particles within a
homogeneous liquid phase.

13. Dehydration:
elimination of a liquid contained within a solid. The
application of heat changes the liquid, contained in a solid,
into a vapor phase. In freeze-drying, the liquid in solid phase
is removed by sublimation, i.e., by changing it into a vapor
phase.
Complementary Unit Operations
One series of operations is not included in this classification
because these are not based on any of the transport
phenomena cited previously. These operations include
grinding, milling, sieving, mixing of solids and pastes, etc.

14. Thermal Properties of FoodThermal Properties of Food
materialsmaterials

15. Thermal Properties
Processing and Storage of Ag Products
– Heating
– Cooling
– Combination of heating and cooling
Grain dried for storage
Noodles dried
Fruits/Vegetables rapidly cooled
Vegetables are blanched, maybe cooked and canned
Powders such as spices and milk: dehydrated
Cooking, cooling, baking, pasteurization, freezing, dehydration: all involve heat
transfer
Design of such processes require knowledge of thermal properties of material

16. Continue….
Heat is transferred by
Conduction: Temperature gradient exists within a body…heat transfer within
the body
Convection: Heat transfer from one body to another by virtue that one body is
moving relative to the other
Radiation: Transfer of heat from one body to another that are
separated in space in a vacuum. (blackbody heat transfer)
We’ll consider
 Conduction w/in the product
 Convection: transfer by forced convection from product to moving fluid
Moisture movement through agricultural product is similar to movement of heat
by conduction
 Moisture diffusivity
 Volume change due to moisture content change

17. Continue….
Terms used to define thermal properties
 Specific heat
 Thermal conductivity
 Thermal diffusivity
 Thermal expansion coefficient
 Surface heat transfer coefficient
 Sensible and Latent heat
 Enthalpy

18. Specific Heat

19. Specific Heat

20. Specific heat
Eq. for calculating Cp based on moisture Content
For liquid H2O
• Cp = 0.837 + 3.348 M above freezing
For solid H2O
• Cp = 0.837 + 1.256 M below freezing
Eq. based on composition
Cp=4.18Xw+1.711Xp+1.928Xf+1.547 Xc+0.908Xa
X is the mass or weight fraction of each component
The subscript denote following components: w=water, p= protein, f=fat, c=
carbohydrate, a=ash

21. Thermal Conductivity (k)

22. Thermal Conductivity (k)
k water =0.566 at 0°C
= 0.602 at 20°C
= 0.654 at 60°C
At room temp. value of k for endosperm of cereal grains,
flesh of fruits and veg., dairy pdts, fats and oil and sugar are
less than that of water.
Higher the moisture content higher will be thermal
conductivity of food product
Another factor is porosity e.g. freeze dried products and
porous fruits like apple have low thermal conductivity.

23. Thermal Conductivity (k)

24. Thermal Diffusivity (α)

25. Surface heat transfer coefficient (h)

26. Sensible and Latent heat
Sensible heat: Temperature that can be
sensed by touch or measured with a
thermometer. Temperature change due to heat
transfer into or out of product
Latent heat: Transfer of heat energy with no
accompanying change in temperature.
Happens during a phase change...solid to
liquid...liquid to gas...solid to gas

27. Latent heat (L)
Latent Heat, L, (kJ/kg or BTU/lb)
Heat that is exchanged during a change in phase
Dominated by the moisture content of foods
Requires more energy to freeze foods than to cool foods (90kJ removed
to lower 1 kg of water from room T to 0 °C and 4x that amount to freeze
food)
420 kJ to raise T of water from 0 ° C to 100 ° C, 5x that to evaporate 1
kg of water
Heat of vaporization is about 7x greater than heat of
fusion (freezing)
Therefore, evaporation of water is energy intensive (concentrating
juices, dehydrating foods…)

28. Enthalpy (h)
Units: (kJ/kg or BTU/lb)
Heat content of a material.
Used frequently to evaluate changes in heat content of steam or moist
air
Combines latent heat and sensible heat changes
ΔQ = M(h2-h1)
Where,
ΔQ = amount of heat needed to raise temperature from T1 to T2
M = mass of product
h2= enthalpy at temp T1
h1 = enthalpy at temp T2

29. Enthalpy (h)
Approach useful when one of the temperature is below freezing
Measurements based on zero values of enthalpy at a specified temperature e.g. at -40°C,
-18°C or 0°C.
Enthalpy changes rapidly near the freezing point
Change in enthalpy of a frozen food can be calculated from eq. below:
Δh = M cp(T2 – T1) + MXw L
Xwis the mass fraction of water that undergoes phase change(frozen fraction)
L is the latent heat of fusion of water
M is the mass of product
Δh = Change in enthalpy of frozen food

30. 1. specific heat
is the amount of heat required to increase the temperature of a unit
mass by 1o
C or 1o
F. This is denoted by Cp
Q = MCp∆T
Factors that influence the specific Heat
 Moisture content
 Temperature
 Pressure
2. Thermal conductivity
thermal conductivity of a product gives in quantitatives
terms the rate of heat that will be conducted through a unit thickness
of the materials if a unit temperature gradient exists across that
thickness.
Q = KA dT/dx
Where
K= thermal conductivity
A = area through which heat flows
X = length
T = Temperature

31. Applications of thermal conductivity
•Prediction of processing time
• Heat calculation
• thermophysical properties –predictions
3. Thermal diffusivity
Applications
• estimating the processing time i.e canning, cooking,
freezing etc
• the higher value of TD indicates high heat energy will
passes through the process.

33. • Heat exchanger, (pasteurization, blanching,
sterilization.) designing and problems solutions.
• Evaporation systems designing and problems
solutions.
• Filtration, Centrifugations, Sedimentations,
Distillation, Extractions, Ultra-filtrations systems
operations, designing and problems solutions
• Material handling, Conveyor, Mixing operations
and problems solutions
The syllebus

34. 1. Introduction to Food Engineering by R. Paul Sing
2. Food Process Engineering by Heldman
3. Hand book of Food Engineering By Dennis R.
Heldman
4. Introduction to Food Process Engineering by
P.G.Smith
5. Food Engineering Operation by J.G. Brennan
6. Unit operation in Food Engineering by Gustavo V.
Barbosa-Cánovas
7. Fundamentals of Food Process Engineering by
Romeo T. Toledo
Books For Food EngineeringBooks For Food Engineering