Rate of drying for food products

1. RATE OF DRYING FOR FOOD
PRODUCTS
OBT554
Unit 4
Dr K.Geetha
Associate Professor, Dept of Biotechnology,
Kamaraj College of Engg &Tech, Madurai

2. Introduction
 Drying is one of the oldest and most important
preservation techniques
 Involves reduction of moisture content of the
food to the level where microbial growth is
inhibited and rate of deteriorative chemical
reactions is minimized.
 Dryer and drying process selection for a specific
operation is a complex process and various
factors have to be considered while making the
selection.

3. Principle of Drying
 Drying process is governed by principle of
heat and mass transfer.
 The removal of moisture from the food
involves two basic phenomenon:
a) Vaporization of moisture from the surface of
the material
b) Movement of moisture from the interior of
the food to its surface as a result of
diffusion, cell contraction and vapor
pressure gradient.

4. Types of Drying
 Three categories depending on the mode of heat
transfer and water vapor removal:
a) Contact drying or convective drying: Material is in
direct contact with the drying air and moisture from
the material directly evaporates to the surrounding air.
Over 85% of the industrial dryers are of convective
type.
b) Vacuum drying: Material is dried under vacuum at
much lower temperatures compared to contact drying.
Heat is added indirectly either by conduction or by
radiation.
c) Freeze drying: In this method of drying, water is
removed by process of sublimation. Suitable
temperature and pressure conditions are maintained in
the dryer to ensure sublimation to occur.

5. Basic Terminologies
 Moisture content - amount of water present in the food material and is
usually expressed in percent.
a) Moisture Content on wet basis (Xwb): It is expressed as ratio of the weight
of water present in food to the total weight of the food product.
b) Moisture Content on dry basis (Xdb): It is expressed as ratio of the weight
of water present in the food product to the weight of dry matter.
 Dry bulb temperature (Tdb): The temperature of air determined using
thermometer is known as dry bulb temperature.
 Wet bulb temperature (Twb): It is the temperature measured by
thermometer with its bulb wrapped in a wet cloth.

6. Rate of Drying
 Drying rate is function of moisture content of
product.
 The drying process based on drying rate can
be divided into three periods:
 constant rate drying period,
 first falling rate period and
 second falling rate period

8. Constant rate period
 The process AB in drying rate curve represents the constant rate
drying period.
 In constant rate drying, the surface of the food behaves as free
water surface.
 The rate of evaporation of water from the surface is equal to the
moisture migration from the interior of the product to its surface.
 This period continues till critical moisture content* is reached.
 The constant drying period is governed fully by external heat and
mass transfer as the surface of the product is always covered with
the film of free water.
 The product temperature is usually at the wet bulb temperature
of the drying air.
 Example: Drying of washed seeds takes place in constant rate
period.
* The critical moisture content is the average material moisture
content at which the drying rate begins to decline.

9. Constant rate period
 During constant rate period, the drying rate
depends on
i) Difference in temperature of air and wetted
surface at constant air velocity and relative
humidity,
ii) Difference in humidity between the air
stream and the wet surface at constant air
velocity and temperature, and
iii) Air velocity at constant temperature and
relative humidity

10. Constant rate period
 Critical moisture content
(CMC):
 Point B on plot represents
critical moisture content.
 It is the moisture content of
the product at which
constant rate drying period
ceases and falling rate
period begins.
 The critical moisture
content depends on type of
product and its thickness.

11. Falling rate period
 Most of the agricultural
products are dried in falling
rate drying period.
 The rate of drying is
controlled by moisture
diffusion towards the surface
and further removal of
moisture from the free
surface.
 Falling rate period is divided
into two zones: first falling
rate free period and second
falling rate period as shown
by process BC and CD in
drying curve respectively

12. a) First falling rate period
 First falling rate period is unsaturated surface
drying.
 As drying proceeds, the wet surface area
decreases as a result of which drying rate also
decreases.
 Explanation: The rate at which surface moisture
is evaporated from the drying surface is more
than the rate at which moisture is migrated from
the interior to the surface.
 The reduction of wet surface to zero is end of the
first falling rate period.

13. b) Second falling rate period
 In this period, sub surface evaporation takes place and
drying continues till equilibrium moisture content is
reached.
 Different mechanisms have been proposed for describing
drying in capillary porous products.
i) Capillary flow: Liquid diffusion due to surface forces
ii) Liquid diffusion: Liquid movement due to difference in
moisture concentration
iii) Surface diffusion: Liquid movement due to moisture
diffusion on the pores surfaces
iv) Vapor diffusion: Vapor movement due to vapor
pressure difference
v) Thermal diffusion: Vapor movement due to
temperature difference
vi) Hydrodynamic flow: Liquid and vapor movement due
to total pressure difference

14. Drying time calculation
 Drying time is the time required to reduce the
moisture content of food product from initial to
desired moisture content level.
 It is possible to estimate drying time if values of
both initial moisture content and critical moisture
content are known.

15. By definition, rate of drying is given as

19. The choice of the best type of dryer to use for
a particular application is generally dictated
by the following factors:
a. the nature of the product, both physical and
chemical;
b. the value of the product;
c. the scale of production;
d. the available heating media;
e. the product quality consideration;
f. space requirements;
g. the nature of the vapor, (toxicity, flammability);
h. the nature of the solid, (flammability, dust
explosion hazard, toxicity)

20. Conclusion
 Drying is one of the most important unit
operations in food processing industry.
 Inappropriate application of heat and mass
transfer rates lead to under or over drying of
the food resulting in quality problems without
major increase in the drying kinetics.
 Understanding the principles of drying
enables one to make appropriate decision in
selection of drying process.

21. THANK YOU