Non-Thermal food technolgy. Type of Freezing. Preservation Technique.

1. Freeze-drying on Food
Preservation technique by low temperature
Chetna
M.Sc. FN
Semester 4

2. Introduction
● Freeze drying is a process in which water is frozen, followed by its removal from
the sample, initially by sublimation (primary drying) and then by desorption
(secondary drying).
● Freeze- drying is a process of drying in which water is sublimed from the product
after it is frozen.
● Freeze drying also known as lyophilization. The term “lyophilization” describes a
process to produce a product that “loves the dry state”.
● It is a drying process which is applicable to manufacture of certain pharmaceuticals
and biologicals that are thermolabile or otherwise unstable in aqueous solutions for
prolonged storage periods, but that are stable in the dry state.
● The equipments used for freeze-drying processing are called Freeze dryer.

3. Freeze-drying processing
● Freeze drying is mainly used to remove the water from sensitive products, mostly of
biological origin, without damaging them, so they can be preserved easily, in a
permanently storable state and be reconstituted simply by adding water.
● Examples of freeze dried products are: antibiotics, bacteria, sera, vaccines,
Diagnostic, medications, protein-containing and biotechnological products, cells and
tissues, chemicals, and Foods for special purpose.
● The product to be dried is frozen under atmospheric pressure.
● Then, in an initial drying phase referred to as primary drying, the water (in form of
ice) is removed by sublimation
● In the second phase, called secondary drying, it is removed by desorption. Freeze
drying is carried out under vacuum.

4. Principle
● The main principle involved in freeze drying is a phenomenon called
sublimation, where water passes directly from solid state (ice) to the
vapor state without passing through the liquid state.
● The material to be dried is first frozen and then subjected under a
high vacuum to heat (by conduction or radiation or by both) so that
frozen liquid sublimes leaving only solid (dried components of the
original liquid).
Video link: https://youtu.be/eE993Vs7j8s

5. Steps involved in freeze-drying processing

6. ● Pretreatment includes any method of treating the product prior to freezing.
● This may include concentrating the product, formulation revision (i.e., addition
of components to increase stability and/or improve processing), decreasing a high
vapor pressure solvent or increasing the surface area.
● The purpose of pretreatments is to select high-quality raw materials, to reduce the
loss of color, smell, flavor and nutrients of foods during the processing, storage
and transportation processes, and to improve the heat and mass transfer
efficiency in freeze-drying process.
Step 1: Pretreatment

7. Step 2: Freezing
● The product must be frozen to a low enough temperature to be completely solidify.
Since freeze drying is a change in state from the solid phase to the gaseous phase,
material to be freeze-dried must first be adequately Pre-frozen.
● The method of prefreezing and the final temperature of the frozen product can affect
the ability to successfully freeze dry the material.
● Rapid cooling results in small ice crystals, useful in preserving structures to be
examined microscopically, but resulting in a product that is, more difficult to freeze
dry.
● Slower cooling results in large ice crystals and less restrictive channel in the matrix
during the drying process.

8. ● Products freeze in two ways,
○ the majority of products that are subjected to freeze-drying consists
primarily of water.
○ The second type of frozen product is a suspension that undergoes glass
formation during the freezing process.
● It is very important in freeze-drying to pre freeze the product to below the
eutectic temperature before beginning the freeze-drying process.
Continued…..

9. Step 3: Primary drying
● After pre-freezing the product, conditions must be established in which ice can
be removed from the frozen product via sublimation, resulting in a dry,
structurally intact product. This requires very carefully control of the two
parameters.
○ Temperature and
○ Pressure involved in freeze-drying system
● The rate of sublimation of ice from a frozen product depends upon the difference
in vapor pressure of the product compared to the vapor pressure of the ice
collector.

10. ● Molecules migrate from the high- pressure sample to a lower pressure area. Since
vapor pressure is related to temperature, it is necessary that the product
temperature is warmer than the cold trap (ice collector) temperature.
● Temperature at which a product is freeze dried is balanced between the
temperature that maintains the frozen integrity of the product and the temperature
that maximizes the vapor pressure of the product.
Continued...

11. Step 4: Secondary Drying
● After primary freeze-drying is complete, and all ice has sublimed, bound
moisture is still present in the product.
● The product appears dry, but the residual moisture content may be as high as
7-8% continued drying is necessary at warmer temperature to reduce the
residual moisture content to optimum values. This process is called
‘Isothermal Desorption’ as the bound water is desorbed from the product.
● Secondary drying is normally continued at a product temperature higher than
ambient but compatible with the sensitivity of the product.

12. ● In contrast to processing conditions for primary drying which use low shelf
temperature and a moderate vacuum, desorption drying is facilitated by raising shelf
temperature and reducing chamber pressure to a minimum.
● Care should be exercised in raising shelf temperature too highly; since, protein
polymerization or biodegradation may result from using high processing temperature
during secondary drying.
● Secondary drying is usually carried out for approximately ⅓ or ½ the time required
for primary drying.
● The general practice in freeze-drying is to increase the shelf temperature during
secondary drying and to decrease chamber pressure to the lowest attainable level.
Continued...

13. Freeze drying equipment: Freeze dryers
❖ Three categories of freeze dryers
➢ Manifold freeze dryers
➢ Rotary freeze dryers
➢ Tray style freeze dryers
❖ 2 components are components in all types
➢ Vacuum pump
➢ Condenser
❖ The manifold, rotary and tray type freeze-dryers differ in the method
by which the dried substance is interfaced with a condenser.

14. Manifold Freeze Dryer

15. Tray Style Freeze Dryer

16. Rotary Freeze Dryer

17. Components of Freeze dryers
● Vacuum system: consists of a separate vacuum pump
connected to an airtight condenser and attached product
chamber.
● Refrigeration system: cools the condenser located inside
the freeze dryer. It also cool shelves in the product
chamber for the freezing of the product.
● Control system: include temperature and pressure sensing
ability. Advanced controllers use the programming of a
complete “recipe” and monitor freeze drying process. It
depends on the application and use.
● Product chambers: a larger chamber with a system of
shelves on which to place the product.
● Condenser: It is to attract the vapors being sublimed off of
the product. The sublimated ice accumulates in the
condenser. temperature of the condenser substantially
below the temperature of the product.

18. Fundamental processes in freeze drying
1. Freezing
● The product is frozen.
● This provides a necessary condition for low temperature drying.
1. Vacuum
● After freezing, the product is placed under vacuum. This enables the frozen solvent in
the product to vaporize without passing through the liquid phase, a process known as
sublimation.
1. Heat
● Heat is applied to frozen product to accelerate sublimation.
1. Condensation
● Low temperature condenser plates remove the vaporized solvent from the vacuum
chamber by converting it back to a solid. This completes the separation process.
● Resulting product has a very large surface area thus promoting rapid dissolution of
dried product.

19. Heat and Mass
Transfer
(a) Heat thru frozen layer
(b) Heat thru dried layer
(c) Heat by microwaves
Mass
Transfer
Heat
Transfer Dry layer
Frozen/Ice layer or
Ice front

20. Heat and Mass Transfer
● Freeze-drying is a complex operation where both heat and mass transfer processes occur in
the interior of the food product.
● To generate vapor from the ice, energy in the form of heat is transferred to the sublimation
zone. Unlike the mass transfer process, which is constantly taking place through the dried
layer (mass flux), heat transfer (heat flux) can occur
○ through the frozen layer (Figure a) – in this case, the frozen product is in direct
contact with the heat source, and the heat transfer is by conduction;
○ by conduction through the dried layer (Figure b) and in the opposite direction of the
mass flux – in this case, near the food product is the heat source that irradiates heat to
the dried layer;
○ within the frozen layer (Figure c) by heat generation using microwaves – in fact,
microwaves are currently preferred as a heat source in freeze-drying due to their
capability to infiltrate profoundly into the food product, resulting in a uniform and
effective heating.

21. Applications of freeze drying
❖ Bioproducts
➢ Freeze-drying method has its application in storing intact RNA in the freeze-dried
tissues with no degradation
➢ Freeze-drying is also commonly used for culture conservation and the production of
concentrated starter cultures.
❖ Pharmaceuticals
➢ The freeze-drying process is used for the preparation of lyophilized dry emulsion
powder for the delivery of poorly soluble drugs.
➢ With the application of freeze-drying, a sterile and freeze-dried nanoparticle of
required size can be prepared.

22. Applications of freeze drying in food industry
● Freeze-drying process makes food products lightweight and hence transportation is
easy and convenient.
● A product with marvellous quality is obtained as the non-availability of liquid water
and low temperature requirement helps in cessation of deterioration and
microbiological reactions.
● The freeze-dried food items can be preserved for a long time without any change in
physical and chemical characteristics. It is also a huge advantage for astronauts,
hikers, and campers; they don't have to carry excess loads or worry about food
spoiling.
● It has also helped to preserve certain seasonal fruits, and make them available all
year around.

23. Advantages of freeze drying on food
(1) An effective way to keep the color, smell, flavor and heat-sensitive nutrients of
food and also eliminates the surface hardening of food.
(2) Freeze-dried food is porous and easy to be rehydrated and instantly dissolved. It can
be consumed directly or after rehydration.
(3) Freeze-dried food contains very low moisture content, it has relatively small density
and is easy to be transported.
(4)The freeze-dried food can be preserved at room temperature for a long time, while
the cost of transportation is much lower than that of frozen food.
(5) No additives are added into the food during the freeze drying process.

24. Disadvantages of freeze drying on food
(1) If exposed directly to air, freeze-dried food will be rehydrated quickly and a series
of chemical reactions will happen, resulting in the deterioration of food.
(2) During transportation and sale process, freeze-dried food is easy to be powdered or
cracked for its loose porous structure.
(3) Time-consuming and energy-consuming process and use of vacuum and
refrigeration equipments due to initial costs are relatively high , which lead to higher
product costs of freeze-dried food.
(4) Freeze drying of food demands higher technological requirements and the
techniques are relevant to the types of foods.

25. Other video link:
● https://youtu.be/TCNTpf98_ak
● https://youtu.be/PxwgB-uFiR8
● https://youtu.be/NIoP2N4TeqQ
Thank you