2. What is freezing ?
2
A method of food preservation whereby:
โข The heat is removed (heat of fusion)
โข Temperature of the food is reduced below its
freezing point (T<Tf)
โข a portion of water in food undergoes a
change in state to form ice crystals (aw
lowered)
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3. Preservation by Freezing
3
Preservation achieved by:
โข Low temperature
โข Reduced water activity due to ice formation
& high concentration of solutes in unfrozen
water
โข Blanching of some foods
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4. Goal of Freezing
4
โข To prevent growth of microorganisms by
โ Killing some bacteria (little effect)
โ Reducing water activity
โ Mechanical formation of ice crystals
โ Osmotic changes in cell fluids
โ Tying up some free water( reduce the amount
of free water)
โข To lower temperature enough to slow down
chemical reactions
โ (every 10OC decrease in temperature that
lead to half the reaction rate)12/4/2017 Y.BAVANEETHAN. 4
5. ๏Physical, biochemical and microbiological
degradation of food controlled by heat removing process.
๏Best preservation:
๏งThe zone of maximal ice formation (normally
between -1 and -5ยฐC) then move as quickly as possible
to the equilibrium temperature of at least -18ยฐC
๏Temperature always less than -18ยฐC
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6. Principles of freezing
โข Temperature at the thermal centre of a food when heat is removed ๏
characteristic curve: FREEZING CURVE
โข 4 sections !
REMARK: When the process is performed rapidly, no distinction between the
different sections (graphs X)
X
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7. Freezing curve
โข Section AS
โข The food is cooled to below its freezing point (=sensible heat)
โข At point S the water remains liquid, although the temperature is below
the freezing point
โข Phenomenon is called supercooling and partly determines the crystal
size
โข Section SB
โข The temperature rises rapidly to the freezing point as ice crystals begin
to form and latent heat of crystallization is released
โข Section BC
โข Heat is removed from the food at the same rate as before
โข Latent heat is removed and ice forms, but temperature almost constant
โข The freezing point is depressed by the increase in solute concentrations
in the unfrozen liquor
โข Major part of the ice is formed
โข Section CD
โข The temperature of the ice-water mixture decreases to the temperature
of the freezer12/4/2017 Y.BAVANEETHAN. 7
8. โข Specific heat
โ Is the quantity of heat that is gained or lost by a unit mass
of products to a accomplish a unit change in temperature
without the change in state (kJ/kg C)
โข Sensible heat
โ When an object is heated, its temperature rises as heat is
added. The increase in heat is called sensible heat.
Similarly, when heat is removed from an object and its
temperature falls, the heat removed is also called sensible
heat. (No change their state)
โข latent heat
โ Pure substances in nature able to change their state. Solids
can become liquids (ice to water) and liquids can become
gases (water to vapor). These require the addition or
removal of heat. The heat changes with state changes is
called latent heat.12/4/2017 Y.BAVANEETHAN. 8
10. Supercooling
10
โข Going below freezing point without the formation of
ice crystals ( crystallization)
โข It yields better quality food than if not present
โข This shows that the undesirable effects of
freezing are due to ice formation rather than
reduction of temperature
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11. ๏The freezing point varies in function of the composition of the
food, but is almost never lower than -5ยฐC
๏Examples:
Product ยฐC
Milk, eggs -0.5
Meat -1.7 ๏ฎ -2.2
Fish -0.6 ๏ฎ -2.0
Vegetables -0.8 ๏ฎ -2.8
Fruit -0.9 ๏ฎ -2.7
1 M sachralose
solution
-2.65
1 M NaCl solution -3.45
Y.BAVANEETHAN.
12. Crystallization
๏ Crystallization occurs
โat point B of the freezing curve
โconsists of nucleation and crystal growth
๏ Nucleation: occurs by combining molecules into an
ordered particle of a size sufficient to survive and
serve as a rate for crystal growth
โข Homogeneous nucleation: in pure systems
โข Heterogeneous nucleation: nucleus formation
around suspended particle or at a cell wall, in
food systems, takes place during supercooling.
12/4/2017 Y.BAVANEETHAN. 12
13. Crystallization
๏ Crystallization occurs
โat point B of the freezing curve
โconsists of nucleation and crystal growth
๏ Nucleation Temperature (NT):
โ Nucleation temperature is the temperature at which
the first ice crystals appear in a solution. It is also
referred to as supercooling point (SCP) or
crystallization temperature.
12/4/2017 Y.BAVANEETHAN. 13
14. Homogenous/Heterogeneous Nucleation
๏ There are two types of nucleation: homogeneous and
heterogeneous.
๏ Nucleation caused by electrostatic attraction between
water polar molecules is referred to as homogenous
nucleation.
๏ Since such attractions are weak,
โ A large number of molecules need to be initiate
nucleation.
โ If nucleation happens with adding of extrinsic
nucleator, it is referred to as heterogeneous nucleation
โ An example of an extrinsic nucleator is frost or ice.
โ If ice is dropped into supercooled water, nucleation
occurs instantly.
12/4/2017 Y.BAVANEETHAN. 14
15. (a) Water nucleation forced by pouring frost
(b) seconds after nucleation
(C) & (d) Crystal growth
12/4/2017 Y.BAVANEETHAN. 15
16. ๏ Crystal growth: enlargement of nucleus by the orderly
addition of molecules
โข The length of the supercooling period depends on,
๏ง type of food
๏ง rate of heat removal
12/4/2017 Y.BAVANEETHAN. 16
17. Effect of Supercooling in Food Industry
๏ Supercooling phenomenon is of particular interest in
food industry.
๏ fruits and vegetables storage and transport require cold
environment where the temperature is several degrees
below freezing point.
๏ However, freezing the food lead reduce the quality by
damaging food cells and changing color and sensory.
๏ But, food can store in supercooling or subfreezing
temperature without freezing,
โ There no loss of quality.
โEg => unpeeled garlic was stored in โ6หC for a week,
without freezing
12/4/2017 Y.BAVANEETHAN. 17
18. Ice formation at different freezing
temperatures
Leniger and Beverloo (1995).)
12/4/2017 Y.BAVANEETHAN. 18
19. Slow freezing
๏ Ice crystals grow in intercellular spaces
โ deform and rupture adjacent cell walls
๏ Ice crystals have a lower water vapor pressure than
regions within the cells
โ water moves from the cell to growing crystals
โ cells: dehydrated and permanently damaged
โข by increased solute concentration
๏ On thawing,
โ cells do not regain their original shape & turgidity
โ food is softened and the cellular material leaks out
from ruptured cells (drip loss)12/4/2017 Y.BAVANEETHAN. 19
20. Fast freezing
๏Smaller ice crystals form within both cells and
intercellular spaces
โ little physical damage to cells, and water vapor
pressure gradients are not formed
โ minimal dehydration of the cells
โ texture of the food is retained to a greater extent
12/4/2017 Y.BAVANEETHAN. 20
21. Temperature changes
of food through the critical zone.
Leniger and Beverloo (1995).)
12/4/2017 Y.BAVANEETHAN. 21
22. Effect of freezing on plant
tissues
a) slow freezing
b) fast freezing
The localization of the crystals is determined
by the freezing rate, the cellular structure and
the temperature
a
b
12/4/2017 Y.BAVANEETHAN. 22
27. Thawing
๏ Freeze food in normal atmosphere, surface ice melts to form a
layer of water.
โ Water has a lower thermal conductivity and a lower thermal
diffusivity than ice.
๏ Therefore, reduces the rate of heat is conducted to the frozen
food interior.
โ Called โInsulating effectโ.
๏ Foods are heated immediately to specific temperature which is
sufficient to destroy pathogenic micro-organisms.
๏ Improper thawing,
โ Cold point effect- drip loss
โ contamination by spoilage and pathogenic micro-organisms
โ To overcome this problem,
โข food is thawed by microwave or dielectric heaters
โข heat is generated within the food.12/4/2017 Y.BAVANEETHAN. 27
28. Volume changes
๏ Volume of ice is 9 % higher than that of pure water
โ expansion of foods after freezing
โ the degree of expansion varies depend on
following factors:
1. Moisture content:
โ higher moisture contents ๏ greater changes in
volume
2. Cell arrangement:
โ plant materials have intracellular air spaces
โ absorb internal increases in volume without large
changes in overall size
12/4/2017 Y.BAVANEETHAN. 28
29. Volume changes
โข Example:
โ whole strawberries increase in volume by 3.0 %
โ coarsely ground strawberries increase by 8.2 %
โ both are frozen to -20ยฐC
3. The concentrations of solutes:
โข High concentrations reduce the freezing point
โ No freeze or Expand
โ Commercial use freezing technology
4. Freezer temperature:
โ determines the amount of unfrozen water and
degree of expansion
12/4/2017 Y.BAVANEETHAN. 29
30. Volume changes
5. Crystallized components:
โ ice, fats and solutes - when they cooled
โ reduces the volume of food
๏ The volume changes leads
โ internal tensions mechanical damage
๏ Firm vegetables tissues damage easier than flexible
membranes of muscle tissues.
๏ถVolume change and Solute concentration,
โ main causes of damage of frozen food
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31. Freezing rate
โขUm = 0.1 โ 1 cm/h slow freezing
โข = 1 โ 5 cm/h medium rate
โข = 5 โ 10 cm/h fast freezing
โข = 10 โ 100 cm/h very fast freezing
โข Mostly fast freezing leads to better quality depending on the type
of food
12/4/2017 Y.BAVANEETHAN. 31
32. Freezing equipments
1. Mechanical freezer โ which evaporate & compress a
refrigerant in a continuous cycle
a. Cool air freezers
b. Cool liquid freezers
c. Cool surface freezers
2. Cryogenic freezers โ cryogen- liquid CO2
liquid N2
liquid freon
12/4/2017 Y.BAVANEETHAN. 32
33. Freezing equipments
๏ Factors considered for selection:
a. Rate of freezing required
b. Size, shape & packaging requirement of the
food
c. Batch or continuous operation โ
depends on - Scale of production &
No. of product types
12/4/2017 Y.BAVANEETHAN. 33
34. 3.Fluidized bed freezers
โข Modified form of blast freezer
โข Air at (-25)- (-35)ยฐC is passed at high velocity
(2-5m/s).
โ through 2-13 cm bed of food
โvibrating tray support to uniform freezing.
โข Shape & size of the pieces of food determine,
โ thickness of the fluidized bed &
โ air velocity needed for fluidization
12/4/2017 Y.BAVANEETHAN. 34
35. Fluidized bed freezers
Advantages
โข Foods greater contact with the air & all surfaces are
frozen uniformly & simultaneously
โข This produces
a. higher heat transfer coefficients
b. Shorter freezing times
c. Higher production rates
d. Less dehydration of unpacked foods
Disadvantages
๏ถ Restricted to particulate foods - peas, shrimp,
french fries
12/4/2017 Y.BAVANEETHAN. 35
37. Cryogenic freezers
๏ Characterized by a change of state in the refrigerant (or
cryogen) as heat is absorbed from freezing food
๏ The cryogen is,
โ in intimate contact with the food
โ rapidly removes energy from food
โข to provide latent heat of vaporization or sublimation
โ produce high heat transfer coefficient & rapid
freezing.
12/4/2017 Y.BAVANEETHAN. 37
38. Cryogenic freezers
๏ Most common refrigerants
โ Liquid Nโ
โ solid or liquid COโ,
โ dichlorodiflouromethane (freon 12)
๏ liquid Nโ & COโ- colorless, odorless & inert
๏ 2 types
โBatch type-
๏งimmersed in liquid Nโ for few sec.
โContinuous type-
๏งuse liquid Nโ tunnel- can decrease temp. in 15 sec.
12/4/2017 Y.BAVANEETHAN. 38
39. Cryogenic freezers
Advantages
โFixed rate of heat extraction
โ low capital cost for equipment
โSmaller weight loss from dehydration
โRapid freezing
โExclusion of O2 during freezing
โRapid startup & no defrost time
โLow power consumption
Disadvantages
โHigh cost of refrigerant12/4/2017 Y.BAVANEETHAN. 39
41. Overview
application distance
cooling
medium
temperature
difference cooling
medium- product
advantage
disadvantage
Air blast freezer universal large small dehydration
Fluidization
freezer
specific rather large small dehydration,
individually
freezing, short
freezing time
Plate freezer limited small small high freezing rate,
limited in scale
Scraped wall
freezer
specific small small only liquids and
pastes
cryogenic freezer universal small large high freezing
rates
Immersion
freezer
limited small small contact cooling
mediumY.BAVANEETHAN.
42. Effect of freezing
โขCauses of quality loss:
โ Chemical causes
โ Biochemical causes
โ Microbiological causes
โ Physical causes
โ Specific problems
12/4/2017 Y.BAVANEETHAN. 42
43. Quality loss: chemical causes
๏ Denaturation of proteins ๏
โ modified water bonding capacity and structure
โ fish gets a stringy structure, red meat and poultry
become firmer
๏ Lipids:
โ taste rancid because of oxidation
๏ Color changes in meat:
โ oxymyoglobine (red) is converted to metmyoglobine
(brown)
12/4/2017 Y.BAVANEETHAN. 43
44. Quality loss: chemical causes
๏ Color changes in vegetables:
โconversion of chlorophyl and phenol
๏Loss of vitamins:
โvit C & pandothonic acid lost at subfreezing TO
๏Other vitamin lost due to drip losses in meat & fish
12/4/2017 Y.BAVANEETHAN. 44
45. Microbiological causes
๏ถ During freezing:
โ Growth of microorganisms is temperature dependent.
โ No pathogens can grow around โ5ยบC.
โ No microorganisms growth <-5ยบ.
โ limited amount of micro-organisms are destroyed
โ Lethal damage (cold shock) possible
โข depends on the type of organism but generally sublethal
damage
๏ After thawing: micro-organisms recover (resuscitation)
โ total plate count (TPC) of a deep frozen product is
normally lower compared with TPC after the
resuscitation period.
12/4/2017 Y.BAVANEETHAN. 45
46. Microbiological causes
๏ถDuring storage:
๏ง Generally G+ bacteria more resistant for freezing
than G- bacteria.
โข Bacillus, Clostridium, Lactobacillus,
Staphylococcus, Micrococcus, Streptococcus
๏ง G- bacteria,
โข Echerichia, Pseudomonas, Alcaligenes, Vibrio,
Salmonella
๏ง Pathogenic parasites are killed
12/4/2017 Y.BAVANEETHAN. 46
47. 28
โ loss of moisture from food to storage
environment
โ characterized by: lighter colour (microscopic
cavity previously occupied by ice can change the
wavelength of reflected light)
โ foods with large surface area/volume ratio
e.g. IQF
โ minimized by special packaging methods.
Physical causes: Freezer burn
12/4/2017 Y.BAVANEETHAN. 47