-: Presented by :-
Monika K. Tambakhe
Dr. P.A. Pawar
University Department of Chemical Technology, Sant Gadge baba
Amravati University ,Amravati. Maaharashtra,INDIA.444602.
Freeze drying is a relatively recent method of preserving food.
Although it is not widely used in the food industry due to high
operation costs, new improvements such as adsorption,
fluidization, and microwave have been researched in order to
It is the most important technique to dry coffee, enzymes, food
ingredients, high value foods.
This process could be considered as a valuable alternative to
Stabilization of enzymatic systems, new ingredients in the
field of functional foods, and exotic foods are examples of new
applications of this technology.
In a general way, freeze-drying or lyophilization, is the process
of removing water from a product by sublimation and
Sublimation is the transformation of ice directly into a gas
without passing through a liquid phase. Sublimation occurs
when the vapor pressure and the temperature of the ice
surface are below those of the triple point (4.58 mm Hg, 0 ºC),
as shown in the pressure-temperature phase diagram of pure
Figure No.1 - Pressure-temperature phase diagram of pure
water and zones of processes for phase changes
In short, freeze drying is a multiple operation in which the
material to be stabilized is-
1. Frozen hard by low-temperature cooling
2. Dried by direct sublimation of the frozen solvent and by
desorption of the sorbed or bound solvent , generally under
3. Stored in the dry state under controlled conditions.
However, freeze drying is an expensive form of dehydration
for foods because of the slow drying rate and the use of
vacuum. The cost of processing is offset to some extent by
the absence of any need for refrigerated handling and
Increasingly, freeze drying is used for dehydrating foods
otherwise difficult to dry, such as coffee, onions, soups, and
certain sea foods and fruits. Freeze drying is also increasingly
employed in the drying of pharmaceutical products.
Difference between Conventional drying and freeze drying
The history of freeze drying goes back to the ancient Inca’s,
who preserved their food stuff by freezing it in the mountains
in winter time. At the same time the frozen water is removed
through the low vapor pressure of the water in the
surrounding air at those high altitudes.
Of course this process was rather slow, but during the drying
process the quality of the food was maintained due to its
frozen state. Once dried the food materials like potatoes and
vegetables showed a remarkably increased quality over time.
Especially the need for improving shelf life of
pharmaceuticals like penicillin and the preservation of blood
plasma in war time was a drive for the development of the
modern tray-type Lyophilizer as it is still used in many
Another catalyst for the development of the freeze dry
technology was the surplus of coffee in countries like Brazil in
the late 30’s of the previous century.
Also here the tray-type freeze dryer was used for the
sublimation of the water from the frozen coffee granules.
Brand names like Nescafe were founded at that time. Instant
coffee is now the most well-known freeze dried product.
After the war period the technology emerged into its current
popularity for the drying of many different temperature
sensitive materials like pharmaceuticals and food materials.
There exist already 400 different freeze dried food materials
FREEZE DRYING PROCESS -
Freeze drying is a process by which a solvent (usually
water) is removed from a frozen foodstuff or a frozen
solution by sublimation of the solvent and by desorption of
the sorbed solvent (nonfrozen solvent), generally under
reduced pressure. The freeze drying separation method
(process) involves the following three stages:
(a) the freezing stage,
(b) the primary drying stage, and
(c) the secondary drying stage.
FREEZE DRYING PROCESS -
Figure No.2 – Diagram of a material on a tray during freeze
The variable X denotes the position of the sublimation
The freezing stage represents the first separation step
in the freeze drying process, and the performance of the
overall freeze drying process depends significantly on
this stage. The material system to be processed (e.g., gel
suspension, liquid solution, or foodstuff) is cooled down
to a temperature (this temperature depends on the
nature of the product) that is always below the
solidification temperature of the material system.
The objective of the freezing stage is to freeze most of
the water originally present in the product for its
PRIMARY DRYING STAGE
After the freezing stage, the drying chamber where the product
is placed is evacuated and the chamber pressure is reduced to a
value that would allow the sublimation of solvent (water) to take
place in the primary drying stage.
When the water molecules sublime and enter the vapor phase,
they also keep with them a significant amount of the latent heat
of sublimation (2840 kJ/kg ice) and thus the temperature of the
frozen product is again reduced.
If there is no heat supplied to the product by a heat source, then
the vapor pressure of the water at the temperature of the product
reaches the same value as that of the partial pressure of the water
vapor in the drying chamber; therefore, the system reaches
equilibrium and no additional water sublimation from the product
PRIMARY DRYING STAGE
The water vapor produced by the sublimation of the frozen
water in the frozen layer and by the desorption of sorbed (non
frozen) water in the dried layer during the primary drying
stage travels by diffusion and convective flow through the
porous structure of the dried layer and enters the drying
chamber of the freeze dryer. (It should be noted that most of
the water removed during the primary drying stage is
produced by sublimation of the frozen water in the frozen
The time at which there is no more frozen layer is taken to
represent the end of the primary drying stage.
SECONDARY DRYING STAGE
The secondary drying stage involves the removal of water that
did not freeze (sorbed or bound water). In an ideal freeze drying
process, the secondary drying stage starts at the end of the primary
In the secondary drying stage, the bound water is removed by
heating the product under vacuum; the heat is supplied to the
product usually by conduction, convection, or radiation. The
following product temperatures are usually employed: (a) between
10 and 35 C for heat sensitive products and (b) 50 C or more for
The residual moisture content in the dried material at the
end of the secondary drying stage, as well as the temperature
at which the dried material is kept in storage, are critical
factors in determining product stability during its storage life.
HEAT AND MASS TRANSFER IN FREEZE-DRYING -
Rate of heat transfer-
There are three methods of transferring heat to the
1. Heat transfer through the frozen layer
2. Heat transfer through the dried layer
3. Heating by microwaves
Rate of mass transfer-
When heat reaches the sublimation front, it raises the
temperature and the water vapour pressure of the ice.
Vapour then moves through the dried food to a region of
low vapour pressure in the drying chamber.
The factors that control the water vapour pressure gradient
the pressure in the drying chamber
the temperature of the vapour condenser, both of
which should be as low as economically possible.
the temperature of ice at the sublimation front, which
should be as high as possible, without melting.
(a)Heat transfer through frozen layer
(b)Heat transfer from hot surfaces through dry layer
(c)Heat generated in ice by microwaves
COMPONENTS OF A FREEZE-DRYER -
Figure: Components of freeze dryer
COMPONENTS OF A FREEZE-DRYER -
The drying chamber, in which the sample is placed and
heating/cooling takes place, must be vacuum tight and with
temperature controlled shelves.
The condenser must have sufficient condensing surface and
cooling capacity to collect water vapor released by the
product. As vapors contact the condensing surface, they give
up their heat energy and turn into ice crystals that will be
removed from the system. A condenser temperature of -65 ºC
is typical for most commercial freeze dryers.
The vacuum pump removes non-condensable gases to achieve
high vacuum levels (below 4 mm Hg) in the chamber and
The heat source provides the latent heat of sublimation, and
its temperature may vary from –30 to 150 ºC .
In the freeze drying plant, three process sections are
especially energy consuming.
Process section 1 involves the freezing of the wet product. As
this is normally considered one of the preparatory steps
before the freeze drying proper, we will concentrate on the
other two that take place in the freeze drying cabinet .
Process section 2 involves the controlled supply of heat to
the product to cover requirements for the sublimation and
Process section 3 involves the removal from the freeze
drying chamber of the vast volumes of water vapor released
drying the sublimation and desorption processes. Of these
three process sections, removal of the water vapour always
consumes the largest amount of energy. The efficiency of
water vapor removal, the vapor trap system, therefore has a
decisive effect on the total energy consumption of the freeze
FREEZE DRYING PLANTS AND EQUIPMENT
1. Pilot freeze drying
2. Industrial freeze drying
a. Tray and Pharmaceutical Freeze Dryers
b. Multibatch Freeze Dryers
c. Continuous Freeze Dryers
d. Tunnel Freeze Dryers
e. Vacuum-Spray Freeze Dryers
FREEZE DRYING PLANTS AND
1. PILOT FREEZE DRYING
Freeze drying pilot units appropriate for use in the
pharmaceutical and food industries, as well as in the
laboratory, are in high demand because they are used to
explore possibilities for the preservation of labile products,
especially with those of biological origin.
They can freeze-dry batches consisting of from 2 to 20 kg of
The unit consists of
(a) a freezing fluid system that can be sent to the heat
exchanger in the section of the condenser or into the
refrigeration coils for product freezing,
(b)a heating circuit (silicon oil is the heating fluid) for plate
heating and defrosting of the condenser, and
(c) a vacuum system for evacuating air from the apparatus
before and during drying.
1. PILOT FREEZE DRYING
Figure : Pilot Freeze dryer
2. INDUSTRIAL FREEZE DRYING -
(a)Tray and Pharmaceutical Freeze Dryers:
By far the largest number of the industrial freeze dryers
in operation is of the vacuum batch type with freeze drying of
the product in trays. There are two main types, depending on
the type of condenser used. In the first type, the condenser
plates are alongside the tray heater assembly and in the same
chamber; in the second type the condenser is in a separate
chamber joined to the first by a wide, in general, butterfly
valve. This latter type of plant is always used in pharmaceutical
industries, but it can also be used for the freeze drying of foods.
(b)Multibatch Freeze Dryers :
The freeze drying process in a batch plant is normally
program controlled to minimize the drying time and to
maximize the production of the plant. With a single-batch
plant the load on the various systems will be very variable
throughout the drying cycle.
The optimal utilization of resources will not be possible in a
single-cabinet batch plant.
The extent of this disadvantage can be eliminated when an
industrial freeze drying plant is built with a number of batch
cabinets. This makes possible the simultaneous production of
different products, which increases the operation flexibility of
the plant .
A large number of industrial freeze drying plants operate
today in this way as multi-cabinet batch plants .
(c)Continuous Freeze Dryers :
Recent years have shown a growing interest in freeze
drying plants operating with a continuous flow of material
through the process. Particularly in industries working with a
single standardized product and the preparation of the
product is by a continuous process, such plants are really
They give continuity in processing throughout and constant
operating conditions that are easily controlled, and they
require less manual operation and supervision.
Continuous freeze dryers are used for freeze drying of
product in trays and for freeze drying of agitated bulk
(d)Tunnel Freeze Dryers :
Large commercial plants for processing cottage cheese and coffee
have been built up in this way. The tunnel freeze dryers have the
same advantages of plant capacity utilization that can be achieved as
in multi-batch plants, but the flexibility for simultaneous production
of different products or in switching from one product to another is
In the tunnel type of freeze dryer,
the process takes place in a large
vacuum cabinet into which the
tray-carrying trolleys are loaded at
intervals through a large vacuum
lock at one end of the tunnel and
discharged similarly at the other
(e)Vacuum-Spray Freeze Dryers:
This dryer has been developed for coffee extract, tea
infusion, or milk. The product is sprayed from a single jet
upward or downward in a cylindrical tower of 3.7m diameter
by 5.5m high. The liquids solidify into small particles by
evaporative freezing The whole plant operates under a
vacuum of about 67 Pa. Frozen particles obtained by spraying
into a vacuum are about 150 mm in diameter and lose about
15% moisture in the initial evaporation. There is no sticking of
THE MANUFACTURING PROCES -
Raw Materials :
• Some foods are extremely well-suited to the freeze-
drying process; others do not fare so well. Liquids, thin
portions of meat, and small fruits and vegetables can be
freeze-dried easily. Coffee is the most common freeze-
• Thick portions of meat and larger, whole vegetables
and fruits cannot be freeze dried with any success
Testing and preparation:
• The food is first checked for contamination and purity.
• Some kinds of food, like seafood and meats, must be
cooked before freeze drying.
THE MANUFACTURING PROCES -
Freezing : Temperature is as low as -40 F (-40 C). In this
extremely cold temperature, the food is quickly frozen.
Drying : The carts are wheeled out of the cold room and into a
vacuum drying chamber. In a large plant, there may be 20 to 30
drying chambers in operation at any time. The drying procedure
involves a sublimation process.
Sizing and blending :Some food pieces may be ground to a
smaller size or may be reduced to a powder. Others may be
screened to separate them by size. Two or more different
products may also be blended together to meet a customer's
Packing :Freeze-dried foods must be sealed in airtight
containers to prevent them from absorbing moisture from the
air. Several types of containers may be used: plastic laminated
foil pouches, metal and plastic cans, or metal and fiber drums for
bulk packaging. Some freeze-dried food is vacuum packed, in
which the air is evacuated from the container before sealing.
QUALITY CONTROL -
Each food has different processing, storage, and
At large freeze-drying facilities, electronic
microprocessors regulate the times, temperatures, and
pressures throughout each step of the process.
A central computer collects this data, analyzes it using
statistical quality control methods, and stores it for
This assures that the food sent out to the public for
consumption has been through a strictly controlled
process that meets government guidelines and varies
only slightly from batch to batch
Figure :Effect of freezing-rate and heating temperature on freeze-drying
rate of “guacamole”
INFLUENCE OF PARAMETERS -
INFLUENCE OF PARAMETERS -
1. Freezing rate : Influence on the ice configuration and thus on
the final structure of the freeze-dried product
2. Heat flux : to reduce drying rate. However, if the drying
proceeds too rapidly (high heat flux), the product may melt,
collapse or can be blown out of the container Excessive heat
may cause the dry cake to char or shrink.
3. Chamber pressure: Chamber pressure affects the transport
properties, thermal conductivity and water vapor diffusivity.
4. Temperature: affecting the quality of freeze-dried products .
Maintaining low temperatures during primary drying will
reduce aroma losses. If the temperature of ice in the condenser
is higher than product’s temperature, water vapor will tend to
move toward the product, and drying will stop.
Freeze drying is costly because of the long drying times
involved; this factor has hindered the application of the
technique to drying of materials in bulk.
Fixed cost -The average fixed cost is given by the ratio of
the annual fixed cost to the annual capacity.
Running cost - The running cost consists of labor and
utilities costs. The utilities cost can be estimated from a
heat and energy balance for 1-kg water undergoing
freezing, sublimation, condensation, and melting.
FREEZE DRYING COSTS -
Freeze dried Garlic- Freeze Dried Turmeric-
Freeze Dried Red Chilly and Green Chilly-
APPLICATIONS OF FREEZE DRYING-
Biggest market for freeze-drying is the food industry.
Freeze-dried food is used by hikers, hunters, astronauts, the
military, as well as being used in the food industry for
dehydrated soups and meals for consumers in the
supermarket. The largest application is freeze-dried coffee.
freeze-dried microorganisms- frequently used for
fermentation reactions , used in bioconversion reactions and
stored for research.
In the pharmaceutical industry – For saving
many medicines lives and for life saving substance that is
Freeze dried fruit powders/ slices dices are often
for making many delicious food products like Fruit
drinks, ice creams, thick shakes, yogurt and other
desserts when fresh fruits are not available.
used to make soft candy, toffee and hard candy.
making instant juice mixes, flavoring baby / infant
foods, preparing food premixes, and range of bakery
use in rich cream fillings, chocolate products and
baked goods as well as in cereals and fruit bars.
FREEZE DRIED FOOD PRODUCT APPLICATIONS -
FREEZE DRIED FOOD PRODUCT APPLICATIONS
Fruit granules are used in products where the fruit content
desired also texture and crunchiness is required.
Freeze dried spices and vegetable used in manufacturing of
instant vegetable noodles, soups, snacks and different kinds of
Freeze dried food is use for making pet foods.
freeze dried soup
ADVANTAGES OF FREEZE DRYING-
1. Retain taste, smell and texture –
2. Fewer loss of Nutrient –
3. Reduce weight –
4. Fewer Preservatives –
5. Extended Shelf Life-
6. The Safety Advantage-
7. Rapid Rehydration Advantage-
convenient method for drying those decomposable
products (mostly pharmaceuticals, e.g., plasma, vaccines,
antibiotics, sera, and growth hormones) that cannot be
stabilized in any other way or that show markedly improved
quality for a rather high average cost (coffee, mushroom s,
diced chicken, and others )
suitable for high-value products with specific biological or