Freezing
Learning objectives
What is freezing
Six component of freezing curve
Theory of freezing
Ice crystal formation
Solute concentration
Volume changes
Calculation of freezing time
Freezing equipment
Changes in foods due to Freezing
Frozen storage
Thawing
Food freezing is a Preservation method that is achieved by a combination of
low temperatures
reduced water activity
pre-treatment by blanching (for some foods)
Major commercially frozen foods include
Fruits either whole or pureed, or as juice concentrates
Vegetables
Fish fillets and seafoods including fish fingers, fish cakes or prepared dishes with an accompanying sauce
Meats as carcasses, boxed joints or cubes and meat products
Baked goods (bread, cakes, fruits and meat pies)
Prepared foods (pizzas, dessert, ice cream, complete meals and cook-freeze dishes
Theory
During freezing
sensible heat is first removed to lower the tempt of food to
the freezing point
In fresh foods,
heat produced by respiration is also removed
called “heat load”
Which is important to determine the correct size of freezing equipment for a particular production rate
Most foods contain a large proportion of water
which has a high specific heat (4200 J kg-1 K-1)
and a high latent heat of crystallisation (335 kJ kg-1)
Substantial amount of energy is therefore needed to
remove latent heat
form ice crystals
freeze foods
Energy for freezing is supplied as electrical energy
which is used to compress gases (refrigerants) in mechanical freezing equipment or to compress and cool cryogens
Freezing curve
If temperature is monitored at the thermal centre of food as heat is removed,
A characteristic curve is obtained (fig 21.1)
The six components of the curve
As:
food is cooled to below its freezing point θf which, with exception of pure water, is always 00C
The phenomenon is known as “supercooling” and may be as much as 100C below freezing point
SB:
tempt rises rapidly to freezing point as ice crystals begin to form and latent heat of crystallisation is released
BC:
Heat is removed from food at same rate as before, but is latent heat being removed as ice forms and temp t
Therefore remains almost constant. Freezing point is gradually depressed by increase in solute concentration in unfrozen liquor and tempt therefore falls slightly (major part of ice forms: fig 21.2)
supercooling
A combination of heat and mass transfer governs the movement of the freezing front. During unidirectional cooling, heat transfer tends to be faster than mass transfer due to the high thermal conductivity of ice and low mass diffusion coefficients. Therefore, solute diffusion will be the limiting factor of growth.
As the object cools past the initial freezing point, phase change starts to occur in the aqueous phase. As water freezes to form essentially pure ice, the solutes in the food become more concentrated in the remaining water causing further depression of the freezing point. Thus there is no sharp freezing point as for pure water, but rather latent heat is removed over a range of
temperatures. There must be supercooling of the food near the object surface to below the initial freezing temperature before ice crystal nucleation,
CD
One of solutes becomes supersaturaed and crystallises out. Latent heat of crystallisation is released and tempt rises to eutectic tempt for that solute
DE
Crystallisation of water and solutes continues. Total time tf taken (freezing plateau) is determined by rate at which heat is removed
EF
Tempt of ice-water mixture falls to tempt of freezer. A proportion of water remains unfrozen at tempts used in commercial freezing;
amount depends on
type and composition of food and tempt of storage.
E.g. at storage tempt of -200C percentage of water frozen is 88% in lamb, 91% in fish and 93% in egg albumin
Ice crystal formation
Freezing point of a food is
“the temperature at which a very small crystal of ice exists in equilibrium with the surrounding water”
For any ice crystal to form,
a nucleus of water molecules must be present
Therefore, “nucleation precedes ice crystal formation
Types of nucleation
Homogeneous
the chance orientation and combination of water molecules
Heterogeneous
the formation of a nucleus around suspended particles or at a cell wall
More likely to occur in foods and takes place during supercooling (fig 21.1)
Length of supercooling period depends on
type of food and
rate at which heat is removed
Rate of ice crystal growth
Controlled by
rate of heat transfer for majority of freezing plateau
Time taken for the tempt of food to pass thru the “critical zone” (fig 21.2),
Therefore
determines both number and
size of ice crystals
Rate of mass transfer
(of water molecules moving to growing crystal and of solutes moving away from crystal)
does not control the rate of crystal growth except towards the end of freezing period when solutes become more concentrated
Solute concentration
Increase in solute conc during freezing causes changes in
pH
Viscosity
Surface tension
Redox potential of unfrozen liquor
As tempt falls,
Individual solutes reach saturation point and crystallise out
Tempt at which a crystal of an individual solute exists in equilibrium with unfrozen liquor and ice is its
“eutectic tempt
E.g.
glucose is -50C
Sucrose: -140C
NaCl: -21.130C
CaCl: -550C
However, it is difficult to identify individual eutectic tempt in complex mixtures of solutes in foods
termed as “final eutectic tempt”
the lowest eutectic tempt of solutes in food
E.g.
Ice cream: -550C
Meat: -500C
Bread: -700C
Max ice crystal formation is not possible until this tempt is reached
As food is frozen below point E (fig 21.1),
the unfrozen material becomes more conc. and forms a “glass”
which encompasses ice crystals
Tempt range at which this occurs depends on
Solute composition
Initial water content of food
However, when tempt of storage is below this tempt range,
formation of a glass
protects the texture of food and gives good storage stability
E.g. meat and vege table 21.2
Many fruits, however have very low glass transition tempts
As a result suffer losses in texture during frozen storage in addition to damage caused by ice crystals
Volume changes
Volume of ice is 9% greater than of pure water
and an expansion of foods after freezing would therefore be expected
However, degree of expansion varies according to factors as:
Moisture content (high moisture produce greater changes in vol)
Cell arrangement (plant material have intercellular air spaces which absorb internal increases in vol without large changes in their overall size when frozen at same temp
e.g. (-200C)
whole strawberries increase in vol by 3.0%
Coarsely ground one increase by 8.2%
Conc of solute (high conc reduce freezing point and do not freeze-or expand- at commercial freezing tempts
Slow freezing
Ice crystals grow in intercellular spaces and deform and rupture adjacent cell walls
Ice crystals have a lower water vapour pressure than regions within the cells, and water therefore moves from the cells to the growing crystals.
Cells become dehydrated and permanently damaged by increased solute conc and a collapsed and deformed cell structure
On thawing, cells do not regain their original shape and turgidity
Food is softened and cellular material leaks out from ruptured cells (drip loss)
Fast freezing
Smaller ice crystals form within both cells and intercellular spaces.
Little physical damage to cells and water vapour pressure gradients are not formed; hence minimal dehydration of cells
Thus texture of food is retained to a greater extent
Freezing time
During freezing,
Heat is conducted from interior of food to surface and is removed by
Freezing medium
Factors affect rate of heat transfer
Thermal conductivity of food
Area of food available for heat transfer
Distance that heat must travel thru food (size of pieces)
Tempt difference between food and freezing medium
Insulating effect of boundary film of air surrounding food
Packaging, if present, is an additional barrier to heat flow
Freezing time
Effective freezing time
Time required to
lower tempt of a food
from an initial value
to a pre-determined final tempt at the thermal centre
Measures time that food spends in a freezer and is used to calculate the throughput of a manufacturing process
Nominal freezing time
Time between surface of food reaching 00C
and thermal centre reaching 100C below tempt of first ice formation

Used as an indicator of product damage as it takes no account of the initial conditions or the different rates of cooling at different points on the surface of food
Complications
Calculating freezing time due to
Differences in initial tempt, size and shape of individual pieces of food
Differences in freezing point and rate of ice crystal formation within different regions of a piece of food
Changes in density, thermal conductivity, specific heat and thermal diffusivity with reduction in temp of food
Assumption based on Plank’s equation
Freezing starts with all water in food unfrozen but at its freezing point, and loss of sensible heat is ignored
Heat transfer takes place sufficiently slowly for steady-state conditions to operate
Freezing front maintains a similar shape to that of food
There is a single freezing point
Density of food does not change
Thermal conductivity and specific heat of food are constant when unfrozen and then change to a different constant value when food is frozen
Individual Quick Freezing (IQF)
The concept of IQF involves exposure of individual pieces of the product to a low temp. medium for a relatively short period of time. In some situations, the rate of freezing is increased by promoting more intimate contact between the product pieces and the cold refrigerant. In other situations, IQF is achieved by using very low temperature cryogenic refrigerants as a freezing medium.
Equipment
Factors affect the selection
Rate of freezing required
Size, shape and packaging requirements of food
Batch or continuous operation
Scale of production
Range of products to be processed
Alternative classifications based on rate of movement of ice front
Slow freezers and sharps freezers (0.2 cm h-1)
Including still-air freezers and cold stores
Quick freezers (0.5-3 cm h-1)
Including air-blast and plate freezers
Rapid freezers (5-10 cm h-1)
Including fluidised-bed freezers
Ultrarapid freezers (10-100 cm h-1),
Cryogenic freezers
Changes in foods
Effects of freezing
Damage caused to cells by ice crystal growth
Freezing causes negligible changes to
Pigments
Flavours
Nutritional components
Food emulsions can be destabilized by freezing
Proteins are sometimes precipitated from solution
Which prevents widespread use of frozen milk
Baked goods, high proportion of amylopectin is needed in starch to prevent retrogradation and staling during freezing and frozen storage
Comparison between plant and animal tissues
Meat have a more flexible fibrous structure which separates during freezing, instead of breaking
and texture is not seriously damaged
In fruits and vegetables,
The more rigid cell structure may be damaged by ice crystals
Extent of damage depends on size of crystals and hence rate of heat transfer
Effects of frozen storage
The lower the tempt of frozen,
the lower the rate of micro-biological and biochemical changes.
However, freezing and frozen storage do not inactivate enzymes and have a variable effect on microorganisms
Resistance of micro
Vegetative cells of yeast, moulds and gram-negative bacteria
e.g Coliform and Salmonella species
are most easily destroyed by low tempt
Gram-positive bacteria and mould spores
e.g. Staphylococcus aureus and Enterococci
are more resistant to low tempt
Bacterial spores
e.g. bacillus species and Clostridium species such as Clostridium botulinum
are virtually unaffected by low tempt
Normal frozen
Tempt is -180C
Slow loss of quality occur due to
Chemical changes
Enzymatic activity
These changes are accelerated by
High conc of solutes surrounding ice crystals
Reduction in water act (0.82 at -200C in aqueous foods)
Changes in pH and redox potential
Enzymes are not inactivated, thus disruptions of cell membranes by ice crystals allow them to react to a greater extent with concentrated solutes.
Major changes to frozen foods during storage
Degradation of pigment
Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degrated to brown pheophytin even in blanched veges
Fruits changes in pH due to precipitation of salts in concentrated solns change the colour of anthocyanins
Loss of vitamins
Water-soluble (C and pantothenic) are lost at sub-freezing tempt (tempt dependent)
100C increase causes 6 to 20fold increase in rate of loss in veges and 30-70fold increase in fruits
Other vitamins loss are due to drip losses (if drip loss not consumed) especially in meat and fish
Residual enzyme activity
In fruits and veges, inadequate blanching result in loss of quality due to
polyphenoloxidase act which causes browning and
lipoxygenases act which produces off-flavors and off-odors from lipids and causes degradation to carotene
In meat
Proteolytic and lipolytic act alter the
Texture and
Flavor over long storage periods
Oxidation of lipids
Takes place slowly at -800C and causes off-odors and off-flavors
Recrystallisation
Physical changes in ice crystals e.g. in shape, size or orientation
Cause of quality loss in foods
3 types
Isomass recrystallisation
Change in surface shape or internal structure, usually resulting in a lower surface area to volume ratio
Accretive
2 adjacent ice crystals and cause an overall reduction in number of crystals
Migratory
Increase in average size and reduction in average number of crystals, caused by growth of larger crystals at the expense of smaller crystals
Migratory recrystallisation
Most common
Largely caused by fluctuations in storage tempt
When heat is allowed to enter a freezer (e.g. opening door), the surface of food nearest to source of heat warms slightly.
This causes ice crystals to melt partially; larger crystals become smaller and smallest (less than 2 µm) disappear.
Melting crystals increase water vapor pressure, and moisture then moves to regions of lower vapor pressure
This causes areas of food nearest to source of heat to become dehydrated
When tempt falls again, water vapor does not form new nuclei but joins onto existing ice crystals, thereby increasing size
Therefore a gradual reduction in numbers of small crystals and increase in size of larger crystals, resulting in loss of quality similar to that observed in slow freezing.
Moisture leaves surface of food to storage atmosphere and produces area of visible damage known as “freezer burn”
It is a problem in foods that have a larger surface-area-to-vol ratio, but minimized by packing in moisture-proof materials
Techniques to minimize tempt fluctuation
Accurate control of storage tempt (±1.50C)
Automatic doors and airtight curtains for loading refrigerated trucks
Rapid movement of food between stores
Correct stock rotation and control
Thawing
When thawed, surface ice melts to form a layer of water
Water has a
lower thermal conductivity and
lower thermal diffusivity than ice and
Surface layer of water therefore reduces rate at which heat is conducted to frozen interior
This insulating effect increases as layer of thawed food grows thicker
Thawing is a longer process than freezing when tempt differences and other conditions are similar
Thawing process (fig 21.9)
AB
Initial rise in tempt due to absence of a significant layer of water around food
BC
Long period when tempt of food is near to that of melting ice
Any cellular damage caused by slow freezing or recrystallisation, results in release of cell constituents to form drip losses (of water-soluble nutrients)
Beef loses
12% thiamine
10% riboflavin
14% niacin
32% pyridoxine
8% folic acid
Fruits lose 30% vit C
In addition, drip losses form
Substrates for enzyme act
Microbial growth
Microbial contamination of food, caused by inadequate cleaning or blanching has pronounced effect during this period
Home thawed at 25-400C
Extend thawing increase risk of contamination by spoilage and pathogenic micro.
Commercial thawed at 50-800C
Thawed to just below freezing point (in a vacuum chamber by condensing steam, at low tempt by warm water or by moist air re-circulated over food) , to retain firm texture for subsequent processing

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