D E S I G N E D AN D D E V E L O P E D U N D E R T H E AE G I S O F

NAHEP Component-2 Project “Investments In ICAR Leadership In Agricultural Higher Education”

Division of Computer Applications
ICAR-Indian Agricultural Statistics Research Institute
 Course Details

Course Name Dairy And Food Engineering-I

Unit IV

Basic Principles of Drying

Lesson5

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Created by

Name Role University

Tamil Nadu Veterinary and Animal

DR. KARTHIAYANI ARULSELVAM Content Creator
Sciences University, Chennai

Chhattisgarh Kamdhenu
SUDHIR UPRIT Course Reviewer
Vishvavidyalaya, Durg

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 Objectives
 To understand different drying process and its
significance
 To study different drying phenomenons for drying
of milk and food products
 Introduction
• There are numerous techniques to preserve the foods.
• The drying or dehydrating is the popular method for
preserving food over a longer period of time.
• The purpose of drying the food is to increase the shelf
life of the product with minimized packaging
requirements and reduced shipping weights.
 Drying Process
• In a dairy industry it is common practice to dry the milk and convert it as milk
Powder which is used for numerous applications.
• Drying process had many advantages over the other preservation techniques.
• The quality of a dried food product is judged by the amount of physical and
biochemical degradation occurring during the dehydration process.
• The Product quality and its cost are greatly influenced by the drying operation.
• The drying time, temperature, and water activity influence the final product
quality.
• Low water activity retards or eliminates the growth of microorganisms, but
results in higher lipid oxidation rates.
• Maillard browning reactions peak at intermediate water activities (0.6 to 0.7),
indicating the need for a rapid transition from medium to high water activities.
• Many dried foods are rehydrated before consumption. The structure, density
and particle size of the food plays an important role in reconstitution.
• Processing factors which affect structure, density, and rehydration include
puffing, vacuum, foaming, surface temperature, low temperature processing,
agglomeration, and surface coating.
• Storage stability of a food product increases as the water activity decreases, and
the products that have been dried at lower temperatures exhibit good storage
stability.
• Since lipid-containing foods are susceptible to lipid oxidation at low water
activities, these foods must be stored in oxygen impermeable packages.
• Poor color retention has been a problem in the freeze-drying of coffee because
the number of light-reflecting surfaces is decreased during rapid drying.
• This problem has been improved by slow freezing, partial melting, and
refreezing to insure large ice crystal formation. Other food materials have
different drying problems and specific solutions must be developed.
• Drying should fulfill the following goals:
 Minimal chemical and biochemical degradation reactions
 Selective removal of water over other salts and volatile flavor and aroma substances
 Maintenance of product structure (for a structured food)
 Control of density
 Rapid and simple rehydration or re-dispersion
 Storage stability: less refrigeration and packaging requirements
 Desired color
 Lack of contamination or adulteration
 Minimal product loss
 Rapid rate of water removal (high capacity per unit amount of drying equipment)
 Inexpensive energy source (if phase change is involved)
 Inexpensive regeneration of mass separating agents
 Minimal solids handling problems
 Facility of continuous operation
 Noncomplex apparatus (reliable and minimal labor requirement) (xvi) Minimal
environmental impact
 Drying Fundamentals
• Drying is defined as a process of moisture removal due to simultaneous heat and mass
transfer.
• Heat transfer from the surrounding environment evaporates the surface moisture.
• The moisture can be either transported to the surface of the product and then
evaporated or evaporated internally at a liquid vapor interface and then transported as
vapor to the surface.
• The mechanisms of water transfer in the product during the drying process can be
summarized as follows-:
 water movement due to capillary forces, diffusion of liquid due to concentration
gradients, surface diffusion, water vapor diffusion in pores filled with air, flow due
to pressure gradients, and flow due to water vaporization–condensation.
 In the pores of solids with rigid structure, capillary forces are responsible for the
retention of water, whereas in solids formed by aggregates of fine powders, the
osmotic pressure is responsible for water retention within the solids as well as in
the surface.
• The type of material to be dried is an important factor to consider in all drying
processes, since its physical and chemical properties may changes in the removal of
water from the product.
• The transfer of energy (heat) depends on the air temperature, air humidity, air
flow rate, exposed area of food material, and pressure.
• The physical nature of the food, including temperature, composition, and in
particular moisture content, governs the rate of moisture transfer.
• The dehydration equipment generally utilizes conduction, convection, or
radiation to transfer energy from a heat source to the food material.
• The heat is transferred directly from a hot gas or indirectly through a metal
surface.
• The model equations for dryers cannot be discussed without a thorough
understanding of the basic heat and mass transfer concepts.
• The typical drying cycle consists of three stages: heating the food to the
drying temperature, evaporation of moisture from the product surface
occurring at a rate proportional to the moisture content, and once the critical
moisture point is reached, the falling of the drying rate.
• The critical moisture point depends greatly on the drying rate since high
drying rates will raise the critical point and low drying rates will decrease
them.
 Principle of Drying
• Dehydration involves the simultaneous application of
heat and removal of moisture from foods.
• Factors that control throughout the food processing
are many and it varies for thermal drying.
• Some involve the removal of moisture or volatiles
from various food ingredients or products that differ
in both chemical and physical characteristics.
• Others involve the drying of solutions or liquid
suspensions and different approaches to the problem.
• Three basic methods of heat transfer are used in industrial dryers
in varying degrees of prominence and combinations, specifically,
convection, conduction, and radiation.
• In the food and dairy processing industry, the majority of dryers
employ forced convection with continuous operation.
• With the exception of the indirectly heated rotary dryer and the
film drum dryer, units in which heat is transferred by conduction
are suitable only for batch use.
• This limitation effectively restricts them to applications involving
somewhat modest production runs.
• Radiant, or "infrared," heating is rarely used in drying materials.
• Direct heating is used extensively in industrial drying equipment
where much higher thermal efficiencies are exhibited than with
indirectly heated dryers.
• Direct method is not always acceptable, especially where
product contamination cannot be tolerated, particularly in dairy
industry.
• Indirect heating must be used.
• With forced- convection equipment, indirect heating employs a
condensing vapor such as steam in an extended surface tubular
heat exchanger or in a steam jacket where conduction is the
method of heat transfer.
• Alternative systems that employ proprietary heat-transfer fluids
also can be used. In this method temperature is elevated without
the need for high-pressure operation as may be required with
conventional steam heating.
• This may be reflected in the design and manufacturing cost of
the dryer.
• In addition to the methods listed above, oil- or gas-fired
indirect heat exchangers also can be used.
• Drying of food is a complex phenomenon which
involves momentum, heat and mass transfer, physical
properties of the food, air and water mixtures, and
macro and microstructure of the food.
• There are many possible drying mechanisms, but those
that control the drying of a particle product depend on
its structure and the drying parameters, like drying
conditions, moisture content, dimensions, surface
transfer rates, and equilibrium moisture content.
• These mechanisms falls in to three classes:
(i) evaporation from a free surface,
(ii) flow as a liquid in capillaries, and
(iii) diffusion as a liquid or vapour.
• The first mechanism follows the laws for heat and mass transfer
for a moist product.
• The second mechanism becomes difficult to distinguish from
diffusion when one sets the surface tension potential to be
proportional to the logarithm of the moisture potential (or water
activity).
• The third set of mechanisms follows Fick‘s second law of
diffusion, which is analogous to Fourier‘s law of heat transfer
when the appropriate driving force is used.
 Principle of Drying….

• In convectional drying the heating medium, generally air,

comes into direct contact with the solid.
• Various oven, rotary, fluidized bed, spray, and flash
dryers are typical examples.
• In conduction drying, the heating medium is separated
from the solid by a hot conduction surface.
• Examples are drum, cone and through dryers.
• In radiation dryers, the heat is transmitted as radiant
energy.
• Some dryers also use microwave energy to dry food
materials at atmospheric pressure or at vacuum.
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