CHAPTER 7:
FREEZING PROCESSES
ERT 426 FOOD ENGINEERING
Low Temperature Food Processing
and Preservation
Low Temperature Food Preservation
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Freezing and cold storage is one of the oldest
methods of preservation
Low temperature can retard chemical reactions as
well as the activity of food enzymes
It can also stop the growth/slow down activity of
microorganism
Low Temp. Preservation: Benefits
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Preservation of food without any adverse effects
on the nutritional values and flavour, colour and
textural characteristics
Control of the rate of chemical/enzymatic
changes in food due to spoilage organism and
endogenous enzymes as in aging of beef
Control of the growth & metabolic activity of
starter cultures of desirable food microorganism as
practiced in cheese ripening & aging of wines
Low Temperature Preservation: Benefits
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Enhanced ease and efficiency of unit operation
such as peeling for canning, as also cutting and
slicing of bread
Reduced loss in flavour & associated changes
during extraction of juice from fruits
Ease of precipitation of waxes from edible oils
Increased solubility of carbon dioxide in water
used for aerated drinks
Low Temperature Preservation:
Methods
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Low temp. processing & preservation methods –
refrigeration & freezing
Refrigeration & freezing are distinguish based on
temp. of operation
Refrigeration means storage at temp. above
freezing of water in the food (16o C to -2.2 o C)
Refrigerators usually operate 4-7 o C
Pure water freezes at 0o C and most food will not
begin until -2.2 o C/still lower temp is reached
Low Temperature Preservation:
Methods
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Refrigerated/cool storage will preserve
perishable/fresh foods for days or weeks
depending on the type of food
Freezing/frozen storage refer to storage at temp
where the food is maintained in solid frozen
condition, generally at -18o C . Frozen storage will
preserve foods for month or even years
Microbial Activity at Low Temperature
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Food spoilage microorganism grow at temp > 10o
C while some food poisoning organism grow slowly
at 3o C
Psychotropic microorganism grow slowly between
4.4 0 C and – 9.4o C provided the food is not
solidly frozen
Complete death of all microorganisms does not
occur merely due to low temp. and when food is
thawed there can be rapid multiplication of
microorganism
Refrigeration
Factors Influencing Changes Occurred in Food
Factors of Importance in Refrigerated Storage
Refrigeration and Cool Storage
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Gentlest method of food preservation, no adverse
effect on taste, texture and nutritive value
Refrigeration (perishable/fresh) should start
immediately after harvest/slaughter and should be
maintained throughout transportation, warehousing
and storage prior to ultimate use
Refrigeration and Cool Storage (cont)
•
Refrigeration influence effect agricultural and
marketing practices and sets to economic climate of
the food industry
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Control of prices during different seasons is also
possible
Refrigeration
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Is the removal of heat out of a body
Process: cold nitrogen gas volatilizing of liquid nitrogen
is allowed to pass over the food product to quick
cooling
Factor influencing changes that occur in
food
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Growing conditions & varieties of plant
Feeding practices of animals
Harvest & slaughter
Sanitation
Damage to tissues
Mixture of foods in the storage
Factor influencing changes that occur in
food (cont)

Too low temperature which <-13o C slow down
natural ripening and result in poor ripened colour
 Loss
in nutrient
 Loss of sweetness of sweetcorn and vitamin for
vegetable
 Loss of crispness and firmness of vegetable and fruits
 Loss of flavour
 Caking of granular food
Factor of Importance in Refrigerated
Storage
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Control of low temperature
Relative humidity and air circulation
Composition of atmosphere in the storage chamber
Food variability (to type of food)
Fruits and vegetable will produce heat due to
respiration
Amount of heat expressed in term of British thermal unit
Most food store best at refrigerated temp when air
humidity is in the range 80-95%
Freezing
Freezing Characteristics of Foods
Factors Affecting the Quality of Frozen Food
Freezing and Frozen Storage
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Proper freezing preserve food without major
changes in size, shape, texture and flavour of a
variety of foods of plant and animal origin
Frozen foods require only a single thawing
operation before eating
Freezing Characteristics of Foods
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Freezing (unless properly controlled) can disrupt
food texture, break emulsions, denature proteins
and cause undesirable physical and chemical
changes
Freezing temp. depend on their different
compositions and solid content
Factor Affecting the Quality of Frozen
Food

5 factors are importance in the maintenance of the
quality of foods in frozen storage
 Solute
concentration effects
 Ice-Crystal Size
 Rate of freezing
 Final Temp
 Intermittent thawing
Solute Concentration Effects
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Maintaining the quality of most food
Food must be frozen to a solid/nearly so to
maintain good quality during frozen storage
A partially unfrozen are will deteriorate with
respect to texture, colour, flavour and other
properties
Ice- Crystal Size
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Freeze rapidly it forms crystal of ice
Rate of freezing is slow, the ice crystal size is large
and clusters are also formed leading to physical
rupture of cells
Rate of Freezing
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Rapid/instantaneous freezing produces ice crystals
of small size and also minimizes concentration
effects of solutes by decreasing the time of contact
between solutes and food tissues and other
constituent
Final Temp
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Maintain of final temp to an accuracy of ± 1.0o C
is important
Intermittent Thawing
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Quick final thawing is better than slow thawing
Freezing Systems
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3 methods of freezing are employed in commercial
practice;
 Air
freezing
 Indirect contact freezing
 Plate
freezer
 Air-blast freezer
 Direct
 Air
contact freezing
blast
 Immersion freezing
Air Freezing
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Cold air is used with diff. velocities
Cold air between -15 to -23 o C
Quick freezing is done by air blast freezing
Freezing air at -18 to -34 o C is blown across the
foods/fluidized bed freezer
Indirect Contact Freezing


Food/food package does not come into contact
with the refrigerant
The food/food package is brought into contact with
a cold surface maintained at temp. in the range of 18o C to -45 o C by a refrigerant
Direct Freezing
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Food is immersed in the refrigerant/sprayed with
the refrigerant
Effect of Freezing on Constituent of
Food
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Not affect nutritive value
Enzyme activity is only retarded by freezing temp.
Hence control of enzyme activity is achieved by
heat treatment (blanching) prior to freezing &
storage in the case of vegetable and fruit
Enzyme activity is simulated in the super cooled
water and the reaction rate is greater than in
crystallized water at the same temp.
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Hence frozen storage even at -9o C permit severe
damage to the quality of the food both in the lost of
nutrients and appearance
Long term storage at -6o C yields unacceptable
food
Fats and oils undergo oxidative deterioration in
frozen foods
Freezing may destabilize emulsions of oil-inwater/water-in-oil
Freezing Time
Plank’s Equation
Pham’s Method
Prediction of Freezing Time of Finite-Shaped Object
Freezing Time
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It is a key calculation in designing a freezing
process
Three distinct period at any point of food during
freezing – prefreezing, phase change &
postfreezing
Frozen temperature of food
 Water
at below 0°C
 Fruits and vegetable at -18°C
 Food with higher fat content i.e ice cream at -25°C
Prefreezing period
•Temp decrease to freezing
point
Super cooling (<0°C)
•Once nucleation and forming
ice crystal, FP increase to 0°C
Phase change
•Temp remain at FP until
complete phase change occur
Postfreezing
period
•Temp decrease
rapidly after
water completely
change to solid
ice
Deviation in temp
profile between
water and potato
due to conc. effect
Prefreezing
period
•Temp decrease
to freezing point
Super cooling (<0°C)
Phase change
•Formation of ice crystal at
temp lower than water due
to presence of solute in food
•Remaining water become more
concentrated with solute and
depress FP
Postfreezing
period
•Temp decrease
gradually until end
poit temp reached
Freezing Time
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Freezing involves removal of both sensible and
latent heat.
Freezing of pure water exhibits sharp transitions
between the different freezing periods, whereas
with foods, the transitions are more gradual.
At the endpoint temperature for freezing foods, the
frozen food may still have some water present as a
liquid; in fact, up to 10 % water may be in liquid
state for foods frozen to -18°C.
Plank’s Equation
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First and most popular equation in predicting
freezing time
This equation describes only the phase change
period of the freezing process
The limitations to Plank’s equation are related
primarily to assignment of quantitative values to the
components of the equation
Even with these limitations, the ease of using Plank’s
equation has made it the most popular method for
predicting freezing time
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Example 7.1
A spherical food product is being frozen in an airblast freezer. The initial product temperature is 10°C
and the cold air -40°C. The product has a 7 cm
diameter with density of 1000 kg/m 3 , the initial
freezing temperature is1.25°C, the thermal
conductivity of the frozen product is 1.2 W/(m K), and
the latent heat of fusion is 250 kJ/kg. Compute the
freezing time.
Solution to Example 7.1
Pham’s Method
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Pham’s method can be used for finite-size objects of
irregular shapes by approximating them to be similar to
an ellipsoid.
Another advantage is that it is easy to use, yet it
provides answers with reasonable accuracy
Assumption in developing this method:
The environmental conditions are constant.
 The initial temperature, Ti, is constant.
 The value for the final temperature, T c, is fixed.
 The convective heat transfer at the surface of an object is
described by Newton’s law of cooling

Ef for infinite slab = 1
Ef for infinite cylinder = 2
Ef for infinite sphere =3
Example 7.2
Recalculate the freezing time in Example 1, using
Pham’s method with the following additional
information. Final center temperature is 18°C, density
of unfrozen product is 1000 kg/m3 , density of
frozen product is 950 kg/m3 ,moisture content of the
product is 75%, specific heat of unfrozen product is
3.6 kJ/(kgK), and specific heat of frozen product is
1.8 kJ/(kgK).
Solution to Example 7.2
Solution to Example 7.2 (cont)
Solution to Example 7.2 (cont)
Prediction of Freezing Time of Finiteshaped Object
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
Pham’s method can also be used to calculate
freezing times of other shapes such as a finite
cylinder, infinite rectangular rod, and rectangular
brick, which are commonly encountered in freezing
foods.
Pham’s equation, may be used for this purpose using
an appropriate value of the shape factor, Ef. In
order to calculate Ef, two dimensional ratios are
required, β 1 and β 2.
Example 7.3
Lean beef in the shape of a large slab with 1 m
length, 0.6 m width and 0.25 m thickness is to be
frozen in an air-blast freezer with a Biot Number of
2.5. Calculate the shape factor from the given
dimensions.
Solution to Example 7.3
Solution to Example 7.3 (cont)