Freezing is a method of food preservation where heat is removed from food, lowering its temperature below its freezing point to solidify water inside it. There are two main types of freezing - slow freezing, which forms large ice crystals and damages cells, and fast freezing, which forms small ice crystals with minimal damage. Freezing aims to preserve taste, texture, nutrition and extend shelf life by slowing microbial growth and chemical reactions. Common freezing methods include air blast freezing, belt freezing, plate freezing and cryogenic freezing using liquid nitrogen. While freezing prevents spoilage, it can cause some quality losses through changes in volume, concentration, protein denaturation and vitamin loss.

1. Freezing
Submitted To. Mr. shubham Rohilla
Submitted By. Rahul
Roll No. 2124667
B.S.C Food Science And Tech.
4th sem.

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What is freezing ?
A method of food preservation whereby:
• The heat is removed .
• where a portion of liquid turns into a solid.
• Temperature of the food is reduced below its freezing point (T<Tf).
• Generally -18°C.
Principle : Freezing is a phase transition phenomenon in which a substance transforms
its state from liquid to solid below its freezing point. Principle of Freezing: When a
substance changes its state from liquid to solid, the evolution of heat takes place from
the body to its surroundings.
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Goals of Preservation
The basic goal is to :
• Preserve the taste, texture, and nutritional value in foods.
• Reducing water activity.
• Provide a significant extended shelf life.
• Retards growth of microorganisms.
• To lower temperature enough to slow down chemical reactions – (every
10°C decrease in temperature that lead to half the reaction rate).
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4. Important terminologies
• Sensible heat: The increase/decrease in temperature on addition/removal
of heat in object.
• Latent heat : The heat is given out during the solidification of a unit
crystallization mass of a liquid.
• Supercoiling: Going below the freezing temperature without formation of
crystals.
• Nucleation. Nucleation is simply defined as the first random formation of
a distinct thermodynamic new phase (daughter phase) that
have the ability to irreversibly grow into larger sized nucleus
within the body of a metastable parent phase
Or the first appearance of ice crystals.
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Latent heat of
crystallization

5. Freezing Curve
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6. Freezing curve:
Section AS
• The food is cooled to below its freezing point (=sensible heat).
• At point S the water remains liquid, although the temperature is
below the freezing point.
• Phenomenon is called supercooling and partly determines the
crystal size.
• Section SB
• The temperature rises rapidly to the freezing point as ice crystals
begin to form and latent heat of crystallization is released.
• Crystallization occurs at point B of freezing curve.
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7. • Section BC
• Heat is removed from the food at the same rate as before
• Latent heat is removed and ice forms, but temperature almost
constant.
• The freezing point is depressed by the increase in solute concentrations
in the unfrozen liquor.
• Major part of the ice is formed.
• Section CD
• The temperature of the ice-water mixture decreases to the temperature
of the freeze.
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8. Freezing temperature of selected foods
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9. Advantages of freezing
• No nutrient loss
• Retain freshness of commodity.
• Retain color and flavor constituents.
• No microbial contamination.
• No respiration, hence longer shelf-life.
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10. Types of Freezing
Slow Freezing
• Occurs at -25 or above °C
temperature.
• Ice crystals are large.
• Damage the food cells.
• 3 hours process.
• Ice crystals grow in
intermolecular spaces.
• More drip loss
Fast freezing
• Occurs at -25 or less °c
temperature.
• Small ice crystals.
• Little damage to food cells.
• 5-10 minute process.
• Small ice crystals forms with
in cells and intermolecular
spaces.
• Very less drip loss
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Fast freezing Slow freezing

12. Advantages of quick freezing over slow freezing
• Smaller and less sharp ice crystals are formed, hence there is less
mechanical destruction of intact cells of the food.
• There is more prevention of microbial growth.
• There is more rapid slowing of enzyme action.
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13. Disadvantages of quick freezing
• Higher investment in deep freezer of freezing equipment or frozen stores.
• Need for refrigerated display cabinets for retail marketing.
• Freezing technology at present suitable for upper and middle class
• Products are more prone to dehydration because of large exposed
surface.
• Improper thawing can cause drip loss
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14. Types of 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.
• Common foods- spinach, broccoli, meat and beef.
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1. Air blast freezer

15. Air freezing may result in:
• Excessive drying
• Costly
But it is
• More efficient & more rapid heat transfer
• Less product dehydration & less frequent requirement of frosting.
• Short freezing time so less moisture loss.
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16. 2. BELT FREEZER
• Food trays are placed on a simple wire-mesh belt.
• The belt conveys the product into an air-blast room operated at
-40 °C. While a single belt arrangement is simple, a multitier belt
may be used to save floor space.
• Cold air moves up 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.
• Avoid open space on belt to prevent air channeling.
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17. 3. Cryogenic food freezing
 Uses liquid nitrogen and solid carbon dioxide as cryogen to effectively
freeze the food products.
 The cryogen is in intimate contact with the food and rapidly removes heat
from all surfaces to produce high heat transfer coefficients and rapid
freezing
 This helps to maximise production efficiencies while minimising costs.
 The use of cryogenic gas at very low operating temperatures of -80ºC
provides a high heat transfer rate that ensures quick freezing times.
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19. The main advantages of cryogenic freezing are as follows:
• Short freezing time due to high heat transfer
• Reduction in flavor loss
• Reduction in drip loss
• Reduction in oxidative changes
• Improved texture of the product
• Suitable for freeze sensitive products
The main disadvantage of cryogenic freezing is relatively high cost of
cryogens.
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20. 4. PLATE FREEZER
• It consist of a vertical or horizontal
stack of hollow plates, through which
refrigerant is pumped at -40ºC.
• This method is used for bricked shaped
packaged materials.
• Slight pressure is applied to improves
the contact between surfaces of the
food and the plates and thereby
increases the rate of heat transfer.
• No dehydration takes place as there is
no air is used. So, no weight loss takes
will place.
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21. Changes Associated With Ice Formation
1. Volume changes
• when water is transformed into ice at 0°C, and hence upon freezing there
is expansion of foods.
The degree of expansion depends upon:
 Moisture: High moisture contents in foods produce greater changes in
volume.
 Concentration of solutes: High concentrations reduce the freezing point
and foods do not freeze or expand at commercial freezing temperatures.
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22. 2. Concentration of non-aqueous system
 During freezing water freezes first. So nearly all the dissolved substances
are therefore concentrated in the diminished quantity of water.
 Dissolved gases will be removed/expelled from the space.
 As freezing progresses, concentration of a particular solute increases.
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23. Effect of freezing
• Causes of quality loss:
– Chemical causes
– Biochemical causes
– Microbiological causes
– Physical causes
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24. Quality loss:
chemical causes
• Denaturation of proteins
–Due to ice-liquid interface formation, protein gets exposes to this interface
and start getting denature.
– fish gets a stringy structure, red meat and poultry become firmer
• Lipids:
– taste rancid because of oxidation
• Color changes in meat:
– oxymyoglobine (red) is converted to metmyoglobine (brown)
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25. Quality loss: biochemical causes
• Color changes in vegetables:
–chances of moderate decrease of chlorophyll.
• Loss of vitamins:
–vit C & pantothenic acid lost at subfreezing Temp.
• Other vitamin lost due to drip losses in meat & fish,
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26. Microbiological causes
• During freezing:
–Microorganisms can grow as growth of microorganisms is temperature
dependent.
– No microorganisms growth <-5°C.
– limited amount of micro-organisms are destroyed
• During storage:
– Generally G+ bacteria more resistant for freezing than G- bacteria.
– Bacillus, Clostridium, Lactobacillus, Staphylococcus, Micrococcus, Streptococcus.
– G- bacteria, • Echerichia, Pseudomonas, Alcaligenes, Vibrio, Salmonella
– Pathogenic parasites are killed
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27. Physical causes:
• loss of moisture from food to storage environment
• characterized by: lighter colour
• minimized by special packaging methods.
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