Freezing helps to Inhibit the growth of microorganisms hence help in preservation of foods. So, freezing is a very easy and effective method for the preservation of fruits and vegetables and to retain them for longer duration.

1. FREEZING OF FRUITS
AND VEGETABLES
Presented To:
Dr. Somesh Sharma
Presented By:
Abhishek Thakur
B. Tech(FT) SU13540

2. WHAT IS FREEZING??
It is a unit operation.
Temparature of the food is reduced below its freezing
point generally to -18 °C or below..
Change in state.
Allows preservation of taste, texture, and nutritional
value in foods.
Provide a significant extended shelf life.
Retards growth of microorganisms.

3. FREEZING PROCESS
The material to be
frozen first cools
down to the
temperature at which
nucleation starts.
Before ice can form, a
nucleus, is required
upon which the
crystal can grow; the
process of producing
this seed is defined as
nucleation.
Once the first crystal
appears in the
solution, a phase
change occurs from
liquid to solid with
further crystal
growth.

4. CONT...
 Freezing point is defined as the temperature at which
the first ice crystal appears and the liquid at that
temperature is in equilibrium with the solid.
 Freezing time is defined as time required to lower
product temperature from its initial temperature to a
given temperature at its thermal center.
 Freezing rate is defined as the ratio of difference
between initial and final temperature of product to
freezing time.

5. GRAPH SHOWING FREEZING PROCESS FOR FOODS

6. TYPES OF FREEZING
FAST FREEZING SLOW FREEZING
 Quick or fast freezing occurs
at –25ºC or less. Ice crystals
are small and do not damage
food cells.
 Slow freezing occurs at -24
ºC or above. Ice crystals are
big and damage the food cells
causing loss of texture,
nutrients, colour & flavour on
thawing.

7. Water contents and freezing points of selected foods

8. TYPES OF FREEZERS

9. AIR BLAST FREEZER
 Either still air or forced air is used.
 Air is recirculated over food at between 30ºC and 40ºC at a velocity of
1.5–6.0 m/s.
 In batch equipment, food is stacked on trays in rooms or cabinets.
 Continuous equipment consists of trolleys stacked with trays of food or
on conveyor belts which carry the food through an insulated tunnel.

10. BELT FREEZER
 Have a continuous flexible mesh belt which is formed into
spiral tiers and carries food up through a refrigerated
chamber.
 Cold air or sprays of liquid nitrogen are directed down
through the belt stack in a counter current flow, which
reduces weight losses due to evaporation of moisture.

11. PLATE FREEZER
 It consist of a vertical or horizontal stack of hollow plates,
through which refrigerant is pumped at 40ºC.
 Slight pressure is applied to improves the contact between
surfaces of the food and the plates and thereby increases
the rate of heat transfer.

12. CRYOGENIC FREEZER
 Here the food is exposed to an atmosphere below -
60°C through direct contact with liquefied gases such
as nitrogen or CO2.

13. LIQUID NITROGEN FREEZER
LIQUID CARBONDIOXIDE FREEZER
 When the CO2 gas is released to the atmosphere at -70 °C, half of the gas becomes
dry-ice snow and the other half stays in the form of vapor.
 This unusual property of liquid carbon dioxide is used in a variety of freezing
systems, one of which is a prefreezing treatment before the product is exposed to
nitrogen spray

14. IMMERSION FREEZER
 The immersion freezer consists of a tank with a cooled freezing media,
such as glycol,glycerol, sodium chloride, calcium chloride, and
mixtures of salt and sugar.
 The product is immersed in this solution or sprayed while being
conveyed through the freezer, resulting in fast temperature reduction
through direct heat exchange.

16. A general flow chart of
frozen fruits and
vegetables

17. Flow diagram of freezing
process for fruit-based
product.

18. Flow diagram of freezing
process of vegetable-based
product.

19. EFFECTS OF FREEZING
 Damage caused to cells by ice crystal growth.
 Freezing causes negligible changes to pigments, flavours or
nutritionally important components.
 When water in the cells freezes, an expansion occurs and ice crystals
cause the cell walls to rupture . Consequently, the texture of the
produce is generally much softer after thawing when compared to non-
frozen produce.
 Chemical changes that can cause spoilage and deterioration of fresh
fruits and vegetables will continue after harvesting.
 Development of rancid oxidative flavours through contact of the frozen
product with air.