The process of freeze drying with greater emphasis on the uses in the fisheries food processing sector. The presentation shows the process involved and the different steps involved and the effect of the process on the food material.

1. By: Siddhartha RoyBy: Siddhartha Roy

2. Freeze-drying is a kind of preservation technology, by
which the material is cooled below its eutectic
temperature or glass transition temperature firstly to
be solidified completely, then dried in vacuum space at
low temperature by sublimation drying and desorption
drying till 95-99% of moisture is removed.
The product can be stored at room temperature or 4 0
C
for long time. Freeze-drying has become a most
important technique for the preservation of heat-
sensitive pharmaceuticals and foods.
Freeze Drying - Introduction

3. Sublimation Drying Process
Sublimation drying, also called primary drying, is
heating frozen material inside the drying chamber; and
the chamber is kept under low pressure by a vacuum
pump. During this step, the frozen water inside the
material is sublimated directly to water vapor; and the
material is dried.
The sublimation starts from the outside surface and
passes inward gradually; the porous dried layer is formed
in the same way simultaneously. The interface of frozen
layer and the dried layer is called the sublimation front or
the ice front.

4. Mass transfer
During sublimation drying, there is massive vapor
to be produced from the material. The vapor must be
moved away promptly, otherwise the pressure in the
drying chamber will be elevated, the sublimation
surface temperature will increase, and finally the
frozen layer will melt.
The water vapor escaped from the sublimation
surface, passing through the dried layer, flows to the
cold trap and forms frost on its surface.

5. Differences between conventional drying and freeze drying

6. Steps in Freeze drying
• 1) freeze food in the conventional freezing
equipment
• 2) remove water during subsequent drying
• (If the water vapor pressure of a food is below 4.58 Torr (610.05
Pa) and the water is frozen, when the food is heated the solid
ice sublimes directly to vapor without melting.
• The water vapor is continuously removed from the food by
keeping the pressure in the freeze drier cabinet below the
vapor pressure at the surface of the ice, removing vapor with a
vacuum pump and condensing it on refrigeration coils.
• As drying proceeds a sublimation front moves into the
frozen food leaving partly dried food behind it.

8. Freeze-drying
• How?
– Freeze the material
– Lower the pressure
– Increase the temperature slightly
Normal (right) and freeze-dried (left) spaghetti

9. Phase diagram

10. Schematic diagram of a typical tunnel freeze dryer

11. A commercial freeze dryer

12. A commercial freeze dryer

13. Methods for transferring heat to the
sublimation front
• 1 Heat transfer through the frozen layer
• 2 Heat transfer through the dried layer
• 3 Heating by microwave

14. Mass transfer during freeze drying
• When heat reaches the sublimation front, it raises the
temperature and the water vapor pressure of the ice. Vapor
then move through the dried food to a region of low vapor
pressure in the drying chamber.
• 1 g of ice forms 2 m3
of vapor at 67 Pa.

16. Porous structure of freeze-dried Food

17. Collapse temperatures* for selected foods in freeze drying
* Above Collapse temperatures there is an instantaneous
irreversible collapse of the food structure, which
restricts the rate of vapour transfer and effectively ends
the drying operation. Usually lower than freezing point.

18. Factors that control the water vapor
gradient:
• The pressure in the drying chamber
• The temperature of the vapor condenser.
• The temperature of ice at the sublimation
front, which should be as high as possible,
without melting
• (the lowest economical chamber pressure is approximately
13 Pa and the lowest condenser temperature of ice should
be -35 ºC).

19. Freeze drying equipment
• Freeze dryer consists of:
– Vacuum chamber (with trays to hold the food
during drying)
– Heater to supply latent heat of sublimation.
– Refrigerator coil for condense the vapors directly
to ice
– Vacuum pump removes non-condensable vapors.

20. A Freeze drier

22. Types of freeze dryers
• Conduction type
• Irradiation type
(Convection heating is not important in the partial
vacuum of the freeze drier cabinet.)
(The surface temperature of the food does not
exceed 60ºC.)

23. • Contact ( or conduction) freeze dryers:
• Foods is placed onto trays which rest on heater plates.
• Slow drying process ( heat transferred by conduction to only one
side of the food).
• Accelerated freeze dryers:
• Food is held between two layers of expanded metal mesh and
subjected to a slight pressure on both sides.
• Heating is by conduction, but heat is transferred more rapidly
into food by the mesh than by solid plates, and vapor escapes
more easily from the surface of the food.
• Radiation freeze dryers:
• Infrared radiation from radiant heater is used to heat shallow
layer of food on flat trays.
• Heating is more uniform than in conduction types.
• Microwave and dielectric freeze dryers:
• Radio frequency heater have potential use in freeze drying but
not widely used (Difficult to control because water heated more
than ice).

24. Accelerated Freeze Drying
In Accelerated Freeze-drying, food is held between
two layers of expanded metal mesh and subjected
to a slight pressure on both sides. Heating is by
conduction, but heat is transferred more rapidly into
food by the mesh than by solid plates, and vapour
escapes more easily from the surface of the food.
Both mechanisms cause a reduction in drying times
compared with contact methods.

26. Atmospheric pressure freeze drying
• Atmospheric pressure freeze drying is accomplished by
rapidly circulating very dry air over a frozen product.
• In comparison with the classical freeze-drying process,
freeze-drying without a vacuum has a number of
advantages.
• The circulating dry gas provides improved heat and mass
transfer from the frozen sample, in the same way as
washing dries quicker on a windy day.
• By using atmospheric spray drying processes instead of
a cake, a fine, free-flowing powder is obtained

27. Continued…
• Particles can be obtained which have submicron
diameters, this is tenfold smaller than can be
generally obtained by milling.
• The particulate nature, with its high surface area
results in an easily rehydratable product.

28. Comparison between VFD and AFD
( vacuum Vs atmospheric)

29. Effects of freeze drying on foods
• Advantages:
• Very high retention of sensory characteristics and
nutritional qualities and a shelf-life longer than 12
months when correctly packaged.
• Aroma retention up to 80 – 100 %
• The texture of freeze-dried foods is well maintained;
little shrinkage and no case hardening.
• The open porous structure allow a rapid and full re-
hydration, but it is fragile and requires protection
from mechanical damage.
• Only minor changes to proteins, starches or other
carbohydrates.

30. Disadvantages:
• The open porous structure of the food may
allow oxygen to enter and cause oxidative
degradation of lipids (packaging in inert gas
necessary).

31. Storage of the Freeze Drying Products
The main purpose of freeze-drying is to strengthen the
stability of products, i.e., to reduce the chemical and
biological reactions to lengthen the shelf life of products
from hours to days even years. However, even if the
products are stable at the conclusion of freeze drying,
they may also lose activeness in long-term storage.
The quality changes of products include physical,
chemical, microbiological natures protein denaturation,
browning ( Maillard reaction), oxidation, hydrolysis and
others.

32. Freeze drying of foods
The freeze drying of fruits and vegetables is emerging
and high value-added technology.
Application: Freeze-dried foods are high value-added
products, regarded as high-quality food, convenience foo
d, leisure food. exported vegetables and fruits, or in trave
lling, exploration, navigation and so on.
Problems and difficulties: Freeze-drying process is
a time-consuming and energy consuming process; the p
arameters of the freeze-drying process have decisive imp
act on the quality of freeze-dried food .

33. Freeze-dried carrot, Freeze-dried garden radish, Freeze-dried parsley
Freeze-dried broccoli Freeze-dried meat Freeze-dried onion

34. Freeze dried fruits

36. Freeze-dried chestnut Freeze-dried ginger slices
Freeze-dried pumpkin Freeze-dried Potato wedges

38. Freeze dried fish foods

39. Thank You