FST 151
FOOD FREEZING
FOOD SCIENCE AND TECHNOLOGY 151
Special topics: Freeze Drying
Lecture Notes
Prof. Vinod K. Jindal
(Formerly Professor, Asian Institute of Technology)
Visiting Professor
Chemical Engineering Department
Mahidol University
Salaya, Nakornpathom
Thailand
Freeze Drying - Introduction
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 0C
for long time. Freeze-drying has become a most
important technique for the preservation of heatsensitive pharmaceuticals and foods.
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.
Several typical modes of heat transfer and
mass transfer in the drying process
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heat transfer
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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.
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.
Freeze-drying
Normal (right) and freeze-dried (left) spaghetti
•
Completely remove water from some material, such as food, while leaving the basic structure and
composition of the material intact
•
Two reasons
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Keeps food from spoiling for a long period of time
Significantly reduces the total weight of the food
How?
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Freeze the material
Lower the pressure (<0.006 atm)
Increase the temperature slightly
Freeze-drying
Normal (right) and freeze-dried (left) spaghetti
•
How?
– Freeze the material
– Lower the pressure
– Increase the temperature slightly
A commercial freeze dryer
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
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.
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).
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.
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.)
• 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).
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.
• Disadvantage:
• The open porous structure of the food may allow oxygen to enter and
cause oxidative degradation of lipids (packaging in inert gas necessary).
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.
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
food, leisure food. exported vegetables and fruits, or in
travelling, exploration, navigation and so on.
Problems and difficulties: Freeze-drying process is a
time-consuming and energy consuming process; the
parameters of the freeze-drying process have decisive
impact on the quality of freeze-dried food .
Freeze-dried carrot, Freeze-dried garden radish, Freeze-dried parsley
Freeze-dried broccoli Freeze-dried meat
Freeze-dried onion
Freeze-dried chestnut
Freeze-dried ginger slices
Freeze-dried pumpkin
Freeze-dried Potato wedges