drying of food

1. DRYING OF FOOD
BY
Pintu Choudhary
Master of Technology
( Food Science and Technology)
Indian Institute of Crop Processing Technology
Ministry of Food Processing Industries
Government of India
Thanjavur - 613005

2. DRYING
 Thermal drying: Drying commonly describes
the process of thermally removing volatile
substances (moisture) to yield a solid product.
 Non-thermal drying
1. As Squeezing wetted sponge
2. Adsorption by desiccant (desiccation)
3. Extraction.

3. PURPOSES OF DRYING
In food technology, drying is carried out for one or more
of the following reasons:
1. To avoid or eliminate moisture which may lead to
corrosion and decrease the product stability.
2. To improve or keep the good properties of a material,
e.g. flow ability, compressibility.
3. To reduce the cost of transportation of large volume
materials ( liquids)
4. To make the material easy or more suitable for
handling.
5. Preservative.
6. The final step in: Evaporation, Filtration,
Crystallization

4. TYPES OF DRYING
 Direct :Convective Dying
Drying is established through direct contact between the
product and the gas heating medium .Material reaches
steady state temperature near gas wet temperature
 Indirect Drying:
Established from heated surface in contact with the product.
The heating medium and product are separated by wall.
Material reaches steady state temperature near liquid
boiling point for contact rate drying .
 Radiation :
Heat transfer establishes by radiation from energy source.
This is no contact from heated surface or medium and
product

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

6. TRAY DRYING
 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.

7. TRAY DRYING
Cabinet Type Tray Drier

8. ADVANTAGES OF TRAY
DRYING
 Simple in handling
 Lesser capital cost
DISADVANTAGES
 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.

9. FLASH DRYER

10. WORKING OF FLASH DRYER
 Flash dryers utilizing superheated steam as the
drying medium have some unique quality and
energy advantages over air drying systems.
 Flash dryers consisting of inert media have been
employed at pilot scales to dry slurries and
suspensions, which are sprayed onto them.
 The particles are coated thinly by the slurry and
dried rapidly as a thin film.
 Attrition due to inter-particle collisions and
shrinkage induced breakage of the dried film
allows entrainment of the dry powder into the
drying gas for collection in a cyclone or bughouse.

11. ADVANTAGES
 Short contact time and parallel flow make possible to
dry thermo labile materials.
 The dryer needs only a very small area and can be
installed outside a building.
 The dryer is easy to control. The low material content
in the dryer enables equilibrium conditions to be
reached very quickly.
 Due to small number of moving parts the maintenance
cost is low.
 The capital costs are low in comparison with other
types of dryers.
 Simultaneous drying and transportation is useful for
materials handling process.

12. DISADVANTAGES
 High efficiency of gas cleaning system is required.
Because of powder emission, the dryer cannot be used
for toxic materials.
 In some cases this disadvantage can be avoided using
superheated steam as a drying agent.
 For lumped materials difficult to disperse, drying is
impossible to carry out in this apparatus.
 There is a risk of fire and explosion, so care must be
taken to avoid flammability limits in the dryer.
 In general, especially when recirculation is applied,
not all material particles have the same residence time
in the dryer .

13. APPLICATION OF FLASH
DRYING
 Drying of heat-sensitive products in this type of dryers is
very useful.
 Flash dryers are successfully used in the chemical, food,
pharmaceutical, mining, ceramic, and wood industries.
 Magnesium sulfate, magnesium carbonate, copper sulfate,
di-calcium phosphate, ammonium sulfate and phosphate,
calcium carbonate and phosphate, and boric and adipic
acids are common examples of chemicals and by-products.
 Cement, aniline dyes, blowing agents, chlorinated rubber,
coal dust, copper oxide, gypsum, iron oxide, and silica gel
catalyst are typical by-products and minerals that can be
dried in a very efficient way in flash dryers.
 Flash dryers are widely used in the plastic and polymer
industries.

14. EXPECTED NEW
DEVELOPMENTS IN FLASH
DRYER
 Enhancement of product quality
 Increased efficiency by heat recovery
 Applications of heat pumps in drying systems
 Efficient combined processes
 Development of models for overall systems that
would include mechanical dewatering, drying,
heat recovery, powder collection, cooling stages,
etc. in order to study interactions between
various parts of the system and develop improved
design procedures

15. DRUM DRYER
 It consists of a drum of about 0.75-1.5 m in diameter
and 2-4 m in length, heated internally, usually by
steam, and rotated on its longitudinal axis.
 Operation: The liquid is applied to the surface and
spread to a film, this may be done in various ways, but
the simplest method is that shown in the diagram,
where the drum dips into a feed pan. Drying rate is
controlled by using a suitable speed of rotation and the
drum temperature. The product is scraped from the
surface of the drum by means of a doctor knife.

16. FIG. DRUM DRYER

17. ADVANTAGES OF THE DRUM
DRYER
1. The method gives rapid drying, the thin film
spread over a large area resulting in rapid heat
and mass transfer.
2. The equipment is compact, occupying much less
space than other dryers.
3. Heating time is short, being only a few seconds.
4. The drum can be enclosed in a vacuum jacket,
enabling the temperature of drying to be reduced.
5. The product is obtained in flake form, which is
convenient for many purposes.

18.  Operating conditions are critical and it is necessary to
introduce careful control on feed rate, film thickness,
speed of drum rotation and drum temperature.
The uneven scrapping of the doctor’s blade at the
rubbery and glassy parts of the sheet forms wrinkles in
the sheet which eventually become ‘‘sticks’’ in the final
product. The ‘‘sticks’’ reduce the quality of the product
by making it very hard to disperse and physically
unsatisfactory in appearance
DISADVANTAGES OF DRUM
DRYER

19. APPLICATIONS:
 The drum dryer has been used extensively to dry chemicals
and food products. Examples of Chemicals are
polyacrylamides, and various salts such as silicate,
benzoate, propionate, and acetate salts.
 Drum dryers have been successfully used in drying sludge
 The drum dryer is also extensively used to dry and
gelatinize or ‘‘cook’’ starch slurries, such as potato,
rice,wheat,maize,corn, soybean,banana,and cowpea slurries
to produce pregelatinized starch for instant foods.
 Non-starch, low-sugar foods, such as tomato puree, milk,
skim milk, whey, beef broth, yeast, coffee, and malt extract,
have also been successfully dried on a drum dryer.
 Heat-sensitive products such as pharmaceuticals30 and
vitamin-containing products can be dried in a vacuum drum
dryer.

20. FOAM MAT DRYING
 Foam-mat drying is one of the simple methods of
drying in which a liquid food concentrate along with a
suitable foaming agent is whipped to form a stable
foam and is subjected to dehydration in the form of a
mat of foam at relatively low temperature .
 Rate of drying in this process is comparatively very
high because of an enormous increase in the liquid-gas
interface, in spite of the fact that the heat transfer is
impeded by a large volume of gas present in the
foamed mass.

21. FOAMING AGENT USE FOR
MAKING FOAM
1.Glycerol
2.Monostearate ,
3.Egg albumin,
4.Ground nut protein isolate,
5.Gur gum and
6.Carboxy methyl cellulose (CMC)
Fig. foam mat dryer

22. ADVANTAGES
 Reduction in thermal exposure to heat
sensitive foods, dried foams retain a porous
structure, allowing rapid rehydration
characteristics.
 More retention of nutrition .
APPLICATIONS
 Several applications of foam drying have been
developed, including vacuum puff drying, foam
spray drying, and foam-mat drying

23. FREEZE DRYING
 Freeze drying is a process used to dry extremely heat –
sensitive materials. It allows the drying , without
excessive damage, of proteins, blood products and even
microorganisms, which retain a small but significant
viability.
 In this process the initial liquid solution or suspension
is frozen, the pressure above the frozen state is
reduced and the water removed by sublimation.
 Thus a liquid –to-vapour transition takes place, but
here three states of matter involved: liquid to solid,
then solid to vapour

24. THE PHASE DIAGRAM FOR WATER
 The diagram consists of three
separate areas representing the
phases of water, solid, liquid, and
vapour . The point O is the only
point where all the three
phases can coexist, and is known
as the triple point .
 On heating at constant atmospheric pressure ice will melt when
the temperature rises to 0 C . At this constant temperature and
pressure it will then change to water. Continued heating will
raise the temperature of the water to 100 C where, if heat
addition is continued, the liquid water will be converted into
water vapour at 100 C.

25.  If , however, solid ice is maintained at a pressure
below the triple point then on heating the ice will
sublime and pass directly to water vapour without
passing through the liquid phase.
 This sublimation, and therefore drying, can occur
at a temperature below 0 C.
 This will only happen if the pressure is prevented
from rising above the triple point pressure .
 It may be thought that as the process takes place
at a low temperature the heat required to sublime
the ice will be small.

26. FREEZE DRYER

27. STAGES OF THE FREEZE
DRYING PROCESS
1. Freezing stage
Shell freezing
Centrifugal evaporative freezing
2. Vacuum application stage
3. Sublimation stage
Primary drying
Heat transfer
Vapour removal
Rate of drying
4. Secondary drying and
5. Packaging

28. ADVANTAGES OF FREEZE
DRYING
1. Drying takes place at very low temperatures, so the
chemical decomposition, particularly hydrolysis is
minimized.
2. The solution is frozen occupying the same volume as
the original solution, thus , the product is light and
porous.
3. The porous form of the product gives ready solubility.
4. There is no concentration of solution prior to drying.
Hence, salts do not concentrate and denature
proteins, as occurs with other drying methods.
5. As the process takes place under high vacuum there
is little contact with air, and oxidation is minimized.

29. INDUSTRIAL FREEZE
DRYERS
 Tray and Pharmaceutical Freeze Dry
 Multi-batch Freeze Dryer
 Tunnel Freeze Dryers
 Vacuum-Spray Freeze Dryers
 Continuous Freeze Dryers

30. DISADVANTAGES & USES OF
FREEZE DRYING
Disadvantages:
There are two main disadvantages:
1.The porosity, ready solubility and complete dryness yield a
very hygroscopic product. Unless products are dried in their
final container and sealed in situ, packing require special
conditions.
2.The process is very slow and uses complicated plant, which
is very expensive .It is not a general method of drying but
limited to certain types of valuable products.
Uses of freeze drying :
The method is used for products that can not be dried by any
other heat method. These include biological products, e.g.
antibiotics, blood products, vaccines, enzyme preparations
and microbiological cultures.

31. LIMITATIONS OF THE FREEZE DRYING
PROCESS.
1. The freeze drying of products such as blood plasma, although simple
in theory, presents a number of practical problems.
2. The depression of the freezing point caused by the presence of
dissolved solutes means that the solution must be cooled below the
normal freezing temperature for pure water (-10-30).
3. Sublimation can only occur at the frozen surface and is slow process
(1mm thickness of ice per hour). So, the surface area must therefore
be increased .
4. the liquid thickness prior to freezing be reduced in order to reduce
the thickness of ice to be sublimated.
5. At low pressure large volumes of water vapour are produced which
must be removed to prevent the pressure rising above the triple point
pressure.
6. The dry material often needs to be sterile, and it must also be
prevented from regaining moisture prior to the final packaging.

32. VACUUM DRYING
This equipment is a good example of conduction drier.
The vacuum oven consists of a jacketed vessel to
withstand vacuum within the oven. There are supports
for the shelves giving a larger area for conduction heat
transfer. The oven can be closed by a door. The oven is
connected through a condenser and liquid receiver to a
vacuum pump.
Operating pressure can be as low as 0.03-0.03 bar, at
which pressures water boils at 25-35 C.

33. FIG. VACUUM DRYER

34. ADVANTAGES OF VACUUM
OVEN:
 Drying takes place at a low temperature.
 Maximum retention of nutrition
 There is little air present, so there is minimum risk of
oxidation.
LIMITATIONS:
 Sometimes burning onto trays in vacuum shelf driers,
 Shrinkage in food which reduces the contact between
the food and heated surfaces of both types of
equipment. They have relatively high capital and
operating costs and low production rates and
 Used mainly to produce puff-dried foods.

35. FLUIDIZED BED DRYING
 The product pieces are suspended in the heated air
throughout the time required for drying. The
movement of product through the system is enhanced
by the change in mass of individual particles as
moisture is evaporated.
 The movement of the product created by fluidized
particles results in equal drying from all product
surfaces.

36. FLUIDIZED BED DYING

37. WORKING PRINCIPLE
Good contact between the warm drying air and wet
particles is found in the fluidized – bed drier.
Principles of fluidization: The particulate
matter is contained in a vessel, of which is
perforated, enabling a fluid to pass through the bed
of solids from below.
 If the air velocity through the bed is increased
gradually and the pressure drop through the bed is
measured, a graph of the operation shows several
distinct regions,
As indicated in the fig. Effect of air velocity on
pressure drop through a fluidized bed........

38. EFFECT OF AIR VELOCITY ON
PRESSURE DROP THROUGH A
FLUIDIZED BED

39. ADVANTAGES OF FLUIDIZED-BED
DRYING
Efficient heat and mass transfer give high drying rates,
so that drying times are shorter than with static-bed
convection driers. Economic, heat challenge to thermo-
labile materials is minimized.
1. The fluidized state of the bed ensures that drying
occurs from the surface of all the individual particles
and not just from the surface of the bed. Hence, most of
the drying will be at constant rate and the falling –rate
period is very short.
2. The temperature of a fluidized bed is uniform and can
be controlled precisely.
3. The turbulence in a fluidized bed causes some
attrition to the surface of the granule. This produces a
more spherical free-flowing product.

40. DISADVANTAGES OF FLUIDIZED-BED
DRYING
1. The turbulence of the fluidized state may cause
excessive attrition of some materials, with damage to
some granules and the production of too much dust.
2. Fine particles may become entrained in the
fluidizing air and must be collected by bag filters,
leading to segregation and loss of fines.
3. The vigorous movement of particles in hot dry air can
lead to the generation of static electricity charges.
The danger is increased if the fluidized material
contains a volatile solvent such as isopropanol.
Adequate electrical earthling is essential.
4. The free movement of individual particles
eliminates the risk of soluble materials migrating, as
may occur in static beds
Cond…

41. NEW DEVELOPMENT IN
FLUIDIZED BED DRYER
 In fluidized bed drying is the idea of applying
microwave energy field continuously or
intermittently in a fluidized or spouted bed.
 Use of superheated steam will probably become
more popular in some applications in the future

42. SPRAY DRYER
 The spray dryer provides a large surface area
for heat and mass transfer by atomizing the
liquid to small droplets. These are sprayed into
a stream of hot air, so that each droplet dries
to a solid particle.
 The drying chamber resembles the cyclone
ensuring good circulation of air, to facilitate
heat and mass transfer, and that dried
particles are separated by the centrifugal
action.....

43. SPRAY DRYER

44. Working Mechanism of spray dryer

45. COMPONENT OF SPRAY DRYER
• Centrifugal atomizer. Liquid is fed to the center of a
rotating disc or bowl having a peripheral velocity of 90–
200ms1. Droplets, 50–60 m in diameter, are flung from the
edge to form a uniform spray.
• Pressure nozzle atomizer. Liquid is forced at a high
pressure (700–2000103 Pa) through a small aperture to form
droplet sizes of 180–250 m. Grooves on the inside of the
nozzle cause the spray to form into a cone shape and therefore
to use the full volume of the drying chamber.
• Two-fluid nozzle atomizer. Compressed air creates
turbulence which atomizes the liquid. The operating pressure
is lower than the pressure nozzle, but a wider range of droplet
sizes is produced.
• Ultrasonic nozzle atomizer. A two-stage atomizer in
which liquid is first atomized by a nozzle atomizer and then
using ultrasonic energy to induce further cavitation

46. CHARACTERIZATION OF SPRAY
DRIED PRODUCTS
 The products are uniform in appearance and have
characteristic shape, in the form of hollow spheres
with a small hole. This arises from the drying process,
since the droplet enters the hot air stream, and dries
on the outside to form an outer crust with liquid still
in the center. This liquid then vaporizes, the vapour
escaping by blowing a hole in the sphere.
 This method of drying allows a dry product to retain
some properties of feed , e.g., a drop from an emulsion
dries with continuous phase on the outside. When
reconstituted, the emulsion is easily re-formed.

47. ADVANTAGES OF THE SPRAY
DRYING PROCESS
1. Product properties and quality are more effectively
controlled.
2. Heat-sensitive foods, biological products, pharmaceuticals
can be dried at atmospheric pressure and low
temperatures. Sometimes inert atmosphere is employed.
3. Spray drying permits high-tonnage production in
continuous operation and relatively simple equipment .
4. The product comes into contact with the equipment
surfaces in an anhydrous condition, thus simplifying
corrosion problems and selection of materials of
construction.
5. Spray drying produces relatively uniform, spherical
particles with nearly the same proportion of nonvolatile
compounds as in the liquid feed.
6. As the operating gas temperature may range from 150 to
600C, the efficiency is comparable to that of other types of
direct dryers .

48. DISADVANTAGES OF SPRAY
DRYING
1. Spray drying fails if a high bulk density product
is required.
2. In general it is not flexible. A unit designed for
fine atomization may not be able to produce a
coarse pro duct, and vice versa.
3. For a given capacity larger evaporation rates
are generally required than with other types of
dryers. The feed must be pump able.
4. There is a high initial investment compared to
other types of continuous dryers.
5. Product recovery and dust collection increase
the cost of drying

49. APPLICATIONS OF SPRAY
DRYER
1. Drying of any substance in solution or in suspension form.
2. It is most useful for drying of thermo labile materials e.g.
antibiotics.
3. Suitable for large quantities solution.
4. Suitable for both soluble and insoluble substances e.g.
citric acid, gelatin, starch.
5. It can produce spherical particles in the respiratory range
e.g. dry powder inhalers.
6. Drying of milk, soap and detergents which is
pharmaceutically related compounds.

50. NEW DEVELOPMENTS IN SPRAY
DRYING
 Superheated steam spray drying
 Two-stage horizontal spray dryer
 Low humidity spray drying
 Spray freeze drying
 Encapsulation
 Energy efficiency enhancement

51. THE GENERAL PRINCIPLES FOR
EFFICIENT DRYING
1. Large surface area for heat transfer.
2. Efficient heat transfer per unit area (to supply sufficient
latent heat of vaporization or heat of sublimation in case of
freeze-drying)
3. Efficient mass transfer of evaporated water through any
surrounding boundary layers, i.e. sufficient turbulence to
minimize boundary layer thickness.
4. Efficient vapour removal , i.e. low relative humidity air at
adequate velocity.
5. It is convenient to categorize food driers according to the
heat transfer method they use, i.e. convective, conductive or
radiant.

52. NEW DEVELOPMENT AND
EMERGING DRYING
TECHNOLOGY
1. New technologies are needed for:
2. Drying of new products and/or processes
3. Higher capacities than current technology permits
4. Better quality and quality control than currently
feasible
5. Reduced environmental impact, use of renewable
energy
6. Reduced fire, explosion, toxic hazards, safer
operation
7. Better efficiency (resulting in lower cost)
8. Lower cost (operating, maintenance cost and capital)
9. Shorter processing time while maintaining high
product quality