This is about dehydration process.

1. 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.
Dehydration
Principle
Dehydration or drying of food is a complex phenomenon involving 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.

2. I. Liquid diffusion: if the wet solid is at a temperature below the boiling
point of the liquid
II. Vapor diffusion: if the liquid vaporizes within material
III. Condensation diffusion : if drying takes place at very low
temperatures and pressures, e.g., in freeze drying
IV. Surface diffusion (possible although not proven)
V. Hydrostatic pressure differences: when internal vaporization rates
exceed the rate of vapor transport through the solid to the
surroundings
VI. Combinations of the above mechanisms
MECHANISM OF DRYING

3.  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)
 Minimal environmental impact
Drying should fulfill the following goals

4. I. Convective drying - by direct contact of the material dried with a
drying agent, usually being heated air or flue gas (usually mixed with
air);
II. Contact drying - by transferring heat from the heat carrier to the
material through the wall separating them;
III. Radiation drying – by transferring heat by infrared rays;
IV. Dielectric drying - by heating in the field of high-frequency currents;
V. Freeze-drying is drying in a frozen state under high vacuum. The
water is sublimed off from frozen food, structure of the food product
is maintained
The last three types of drying are used relatively rarely and are usually
called special types of drying.
Types of drying

5. 1. Drying Temperature- This varies with food and the method
of drying, greater the temperature difference between
heating medium and food, greater the rate of heat transfer
2. Relative Humidity of air- Sorption characteristic of food to
be dried should be known, as equilibrium moisture content
(EMC) is the lowest moisture content that can be achieved
under given set of temperature and humidity conditions.
3. Velocity of air- Higher the velocity of air, more efficient the
process of drying
4. Drying Time- Drying time depends upon the type of food
and its moisture content and temperature
FACTORS THAT INFLUENCE DRYING OF FOOD

6.  Preservation of foods
 Decreasing the weight and bulk of food to economize shipping
and canning costs
 Production of convenience products such as instant coffee, milk
powder, instant mash potatoes
PURPOSE OF DRYING
1. Sun drying
2. Mechanical driers
a) by heat
b) direct contact with heated surface
c) application of enrgy from radiating microwave or dielectric
source
3. Freeze drying or lyophilization
METHODS OF DRYING

7.  These types of dryers use trays or similar product holders to expose
the product to heated air in an enclosed space.
 The trays holding the product inside a cabinet or similar enclosure are
exposed to heated air so that dehydration will proceed.
 Air movement over the product surface is at relatively high velocities to
ensure that heat and mass transfer will proceed in an efficient manner.
 In most cases, cabinet dryers are operated as batch systems and have
the disadvantage of non-uniform drying of a product at different
locations within the system. Normally, the product trays must be rotated
to improve uniformity of drying.
TRAY DRYING

8.  Non-uniform drying of a product at different locations within
the system.
 Time required for drying is more
 The major disadvantages of this type of dryers are the high
labor cost involved during the loading and unloading of the
drying materials and the low capacities of the units.
ADVANTAGES
 Simple in handling
 Lesser capital cost
DISADVANTAGES