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1. Technology of frozen
foods
Aakash Gill
M.Tech. Dairy Technology
ICAR – National Dairy Research
Institute, Karnal

2. The effect of refrigeration on
foods is two folds :
 A decrease in temperature results in a
slowing down of chemical, microbiological
and biochemical processes.
 At temperature below 0oC water freezes
out of solution as ice, which is equivalent
in terms of water availability to
dehydration or a reduction in aw

3. Effect of freezing on tissues
 Foods do not have sharp freezing
points, but freeze over a range of
temperature depending on the water
content and cell composition.
 Rapid freezing, and storage without
wide fluctuations in temperature, lead
to small intracellular ice crystals and
maintenance tissues with minimum
damage to cell membranes.

4. Effect on freezing on
microorganisms
 The growth of microorganisms in foods at
temperatures below about –12oC has been
confirmed. Thus storage of frozen foods at about
–18oC and below prevents microbiological
spoilage.
 Although microbial numbers are usually reduced
during freezing and frozen storage (except for
spores), frozen foods are not sterile and can spoil
as rapidly as the unfrozen product if temperature
are sufficiently high and storage times at these
temperatures are excessive.

5. Methods of freezing
Freezing techniques include :
- the use of cold air blasts or other low
temperature gases coming in contact with the
food, e.g. blasts, tunnel, fluidized bed, spiral,
belt freezers.
- Indirect contact freezing, e.g. plate freezers,
where packaged foods or liquids are brought into
contact with metal surfaces (plate, cylinders)
cooled by circulating refrigerant (multi-plate
freezers).
- Direct immersion of the food into a liquid
refrigerant, or spraying liquid refrigerant over the
food (e.g. liquid nitrogen, and freon, sugar or
salt solutions).

6. The freezing method chosen for each
product will depend on :
- product quality and freezing rate desired
- Type and shape of product, package, etc.
- Flexibility required in freezing operations.
- Costs of freezing for alternative
techniques.

7. Freezing of fruits and
vegetables
 Providing storage temperatures do not
exceed the minimum for microbial growth
for extended periods of time, frozen food
quality deteriorates principally as a result
of physical, chemical, and biochemical
changes.

8. Treatment prior to freezing aimed at
reducing deteriorative change during
freezing and frozen storage include :
- Blanching of some fruits and most
vegetables to inactivate peroxidase,
catalase and brown enzymes, reduce
cellular oxygen, reduce microbial
numbers and improve colour
- Addition of or dipping into ascorbic acid
or sulphur dioxide solutions to retain
colour and reduce browning

9. - packing of fruits in dry sugar or sugar
syrups to increase freezing rate and
reduce browning, by reducing access of
oxygen to the fruit.
- Changing the pH of some fruits to
decrease browning reactions.

10.  Enzymic change are particularly important
causes of quality changes in fruits, and these
enzymes must be inactivated or inhibited if
satisfactory quality is to be retained.
 During freezing and frozen storage the
concentration of cellular constituents
including enzymes and their substrate
increase, hence rates of enzymic activity in
frozen tissues can be appreciable, despite the
low temperatures

11. Quality retention in frozen foods
The principal factors that effect the rentention
of quality in frozen foods are :
- the quality of the raw material used (variety,
maturity, suitability for freezing and frozen
storage).
- The treatment given prior to freezing
(blanching, SO2, ascorbic acid).
- The freezing method and freezing rate.
- The storage temperature, and temperature
fluctuations.
- The storage time
- The humidity of the storage environment,
especially food is unpackaged
- The nature of packaging materials.

12. Quality deterioration occurs principally as
a result of :
- Changes in colour (loss of natural colour
constituents, e.g. chlorophil pigments,
development of off colour)
- Changes in tecture (loss of cloud destruction of
gels, protein denaturation, toughening)
- Changes in flavour (loss of natural flavour,
development of off-flavour, rancidity)
- Changes in nutrients, such as ascorbic acid in
fruits and vegetables, unsaturated lipids,
essential amino acids.