The Processing and preservation of food is important to make the food fit for human consumption in which drying is the oldest method

1. BANARAS HINDU UNIVERSITY
ADVANCES IN FOOD PROCESSING ANDPRESERVATION
BY DRYING
SUBMITTED BY-:
ANSHIKA AGARWAL
Msc Food technology
22412FST006

2. TOPICS TO BE COVERED
 INTRODUCTION
 DIFFERENCE BETWEEN
DRYING AND DEHYDRATION
 BENEFITS OF DRYING
 FACTORS AFFECTING RATE
OF DRYING
 MECHANISM OF DRYING
 DRYING CURVE
 DIFFERENT DRYERS USED
IN FOOD INDUSTRY
 RECENT ADVANCES
 CONCLUSION
 REFERENCES

3. INTRODUCTION
 Foods are complex substances because of their biological
origin and are mainly derived from animal or plant sources.
 Most foods are highly perishable because of high moisture
contents which make them susceptible to biochemical
reactions and microb ial spoilage, there by requiring
preservation in one way or the other. Among the common
methods used in preserving food products are minimal
processing, refrigeration, smoking and dehydration or
drying(Pinheiro et al., 2010, Damiani et al., 2010,
Fernandes et al.,2010; Corrêa et al., 2010)
 Drying has been used as a method of reducing post-harvest
losses in many agricultural produce for a long time (Gatea,
2011) and as well as improving their commercial value.

4. DIFFERENCE BETWEEN
DRYING AND DEHYDRATION
Drying and dehydration are inter-related terms and are used
interchangeably to describe the unit operations involved in removal of
water by evaporation or sublimation. Drying generally refers to the
method of removal of moisture from the food under natural condition such
as sunlight and wind such as open sun drying, shade drying etc.
Whereas, dehydration refers to a process of removal of moisture by
application of artificial heat under controlled conditions of temperatures,
humidity and air flow
SOURCE- ecoursesonline.iari.res.in

5. BENEFITS OF DRYING
DRYING
EASE OF
HANDLING
REDUCTIO
N
MICROBIO
LOGICAL
DEGRADA
TION
REDUCED
STORAGE
COST
REDUCED
PACAGING
COST
REDUCED
TRANSPO
RTATION
COST
EXTENDED
SHELF LIFE

6. FACTORS AFFECTING RATE
OF DRYING
 Particle size
 Nature of material
 Nature of moisture present in food
material(bound/unbound)
 Surface area
 Initial and final moisture content
 Thickness of food material
 Temperature
 Nature of product
 Relative humidity

7. MECHANISM OF DRYING
Drying involves the simultaneous application of heat
and removal of water from the food. When hot air is
blown over a wet food, the water vapor diffuses
through a boundary film of air surrounding the food
and is carried away by the moving air.
A water vapor pressure gradient is established from the moist
interior of the food to the dry air which provides the driving
force for water removal from the food. The boundary film acts as
a barrier to both heat transfer and water vapor removal during
drying. The moving air velocity determines the thickness of the
boundary film
Water vapour leaves the surface of the food and
increase the humidity of the surrounding air, to cause
reduction in the water vapour pressure gradient which
reduces the rate of drying.
Therefore, the moving air should be faster to reduce
the thickness of boundary film and hence to achieve
faster rate of drying.
Hence, for drying of moist horticultural produce the
parameters taken into consideration are moderately
high dry bulb temperature(ambient air temperature),
low relative humidity and high air velocity.

8. DRYING CURVE
 For each and every product, there is a
representative curve that describes the
drying characteristics for that product at
specific temperature, velocity and
pressure conditions. This curve is
referred to as the drying curve for a
specific product.
 Drying occurs in three different periods
or phases like the first phase or initial
period, the second phase or constant
rate period and the third phase or falling
rate period.

9. of material contain much of water
that liquid surface exists which
dries in a similar fashion to an
open-faced body of water. The
graph is linear at first, then curves
and eventually levels off. Constant
rate drying period (B-C) will
proceed until free moisture appears
from the surface, the moisture
removal rate will then become
progressively less. At CMC the
drying rate ceases and remains
constant. During the constant rate
period, the moisture from the
interior migrates to the surface by
various means and is vaporized.
B)FALLING RATE PERIODS-The
constant rate period ends when the
migration rate of water from the
interior of the surface becomes less
than the rate of evaporation from
the surface. The period subsequent
to the critical point is called ‘the
falling rate period’. Following this
point, the surface temperature
rises, and the drying rate falls off
rapidly. The falling rate period takes
a far longer time than the constant
rate period, even though the
moisture removal may be much
less.
DRYING CURVE

10.  (i) First falling period
The moisture content at the end of the constant
rate period (point c), is the ‘critical moisture
content’. At this point, the surface of the solid is
no longer saturated, and the rate of drying
decreases with the decrease in moisture content.
At point C, the surface moisture film evaporates
fully, and with the further decrease in moisture
content, the drying rate is
controlled by the rate of moisture movement
through the solid.
 (ii) Second falling period:
 Period C to D represents conditions when the
drying rate is largely independent of conditions
outside the solid. The moisture transfer may be
due to any combination of liquid diffusion,
capillary movement, and vapor diffusion.

11. DIFFERENT TYPES OF
DRYING EQUIPMENT USED IN
FOOD INDUSTRY

12. Direct solar dryer
Tunnel
drier
Fluidized
bed drier

13. Conveyor
drier
Cabinet
drier
Drum drier Spray drier

14. WHY DRYING IS PRIMIARILY
USED FOR FOOD
PRESERVATION AND
PROCESSING ??
 The preservation of foods by drying is the
time-honored and most common method used
by humans and the food processing industry.
 It is the oldest method

15. RECENT ADVANCES
IN FOOD
PROCESSING AND
PRESERVATION BY
DRYING
Why do we need advances in regular
drying techniques??

16. Heat Pump Drying (HPD)
In conventional dryers, high-quality energy (such as
electricity or fuel) would be used to heat the air and
a stream of moist, hot air would be expelled at the
exhaust. This represented a significant amount of
low-grade energy that was being lost during the
process. In order to reduce this loss, heat pumps
were introduced to the systems to frorecover the
latent heat of evaporation of water lost in the
exhaust m the dryer. With the heat pump
evaporator placed in the exhaust stream of the
dryer, the air is cooled (to recover the sensible heat
component) and then dehumidified (to recover
latent heat) by the refrigerant. The heat added to
the refrigerant is then transferred to the air stream
entering the dryer at the condenser of the heat
pump, thus raising its temperature. The added
benefit of dehumidifying the drying air is also
realized when the air leaves the dryer and is
recirculated, increasing its capacity to achieve
better drying (Alishah et al., 2018; Patel & Kar,
2012; Sosle et al., 2003).
Microwave Drying (MD)
Microwave drying is used in food drying in which
the microwaves penetrate the material and
converted to heat that allows the removal of
the moisture (Drouzas and Schubert,
1996). This technology has many features,

17. Refractance Window Dryer (RWD)
During the Refractance window drying process,
circulating water at 95-97°C transfers thermal
energy to the materials being dried. An evenly
distributed plastic conveyer belt passes over a
hot water trough as the pureed products are
distributed. When the dried product reaches the
cold-water section, it hardens, making it easier
to separate from the belt using a scraper.
Ideally, this technology is suitable for products
with a distinct aroma and vibrant colour that are
pureed or semi-solid. Using this method, fruits,
vegetables and herbs with high moisture
content can be dried in 3-5 minutes, while
retaining their colour, vitamins and
antioxidants.
Radio-frequency (RF) Drying
he RF method of heating food is faster and
more efficient, because internal heat is
generated in the treated food, due to ionic
conductance and dipole rotation of molecules.
Hence, food quality could be preserved by
evaporating only the water and heating it only
minimally. RF heaters are used frequently in the
final stages of drying to
improve energy efficiency and product quality.
Conventional hot-air drying methods for solid or
semi-solid foods are inefficient at removing
moisture during the falling rate period.

18. Osmotic drying
In osmotic drying, foods to be dried is placed in a hypertonic solution which
causes a difference in concentration and causes the water content of foods to
be driven out from the sample to the solution. Diffusion of the solutes from the
solution into the tissue of the fruits and vegetables also takes place.
Freeze drying
Freeze drying is the process of drying a substance through freezing and
removal of solvents associated with direct sublimation. Unavailability of liquid
water during freezing and low temperature results in the production of a
superior quality end product and most of the reactions involving the microbes
are completely stopped
Hybrid drying /combination drying
Chou and Chua (2001) reviewed new hybrid drying technologies for heat
sensitive food. Donsi et al. (1998) showed the combination of hot air drying and
freeze drying increased the quality of dehydrated fruits and vegetables.
Superheated steam drying
Drying with superheated steam (SS) in the absence of air in a medium
composed entirely of steam. The ability of SS to dry food material is due to the
addition of sensible heat to raise its temperature above the corresponding
saturation temperature at a given pressure. It is not necessary to exhaust the
evaporated water from the produce until the pressure develops beyond certain
limit. After that excess steam will be released. The great advantage is that
recycling of drying method is possible, provided additional sensible heat is

19. CONCLUSION
 Drying is the perhaps the oldest method
of food processing and preservation.
 Convectional methods of drying is being
for the preservation of various food
products but the advances have been
done to them for various purposes like –
decrease the drying time, preserve the
valuable food commodities, make the
whole process more energy intensive ,
reduces the losses that occur during the
radiation drying etc.
 Quality is thus improved by the various
recent advances in drying techniques

20. REFERENCES
1. www.ecourseonline.iari.res.in – processing
of horticultural crops
2. Chapter 18- Handbook of food preservation,
second edition by Mohammad Shafiur
Rahman and Conrad O. Perera
3. https://doi.org/10.1016/B978-1-78242-284-
6.00004-0
4. Research article-Recent advances in
conventional drying of foods by Prasad
Rasane et. al
5. Review article - Recent advances in drying
and dehydration of fruits and vegetables by
Sagar V. R. . Suresh Kumar P.