7/30/2019 Food Freezing (2009)
1/69
FOOD FREEZING
Mechanisms, Food Quality, and
Engineering Aspects
7/30/2019 Food Freezing (2009)
2/69
Understanding Freezing
Retard deterioration of foods - preservation
Chemical, physical, microbial, etc.
Improve organoleptic properties Desirable characteristics
7/30/2019 Food Freezing (2009)
3/69
Controlling Freezing
Maximal quality of product
Initial freezing process
Storage and distribution conditions Efficient and economic processing
7/30/2019 Food Freezing (2009)
4/69
Food Freezing
For a food to freeze, must lower the temperaturebelow its freezing point
Foods are mixtures of various ingredients, some
of which affect phase behavior of water Sugars, salts, proteins, fats, flavors, etc.
Freezing point depression
Dependent on composition
Particularly smaller molecular weightingredients like sugars and salts
7/30/2019 Food Freezing (2009)
5/69
Average freezing point of some food categories
Food Moisturecontent (%)
Tf, oC
Vegetables 78 - 92 -0.8 to -2.8
Fruits 87 - 95 -0.9 to -2.7
Apple juice 87.2 -1.44Apple sauce 82.8 -1.67
Apple Juice concentrate 49.8 -11.3
Meat 55 - 70 -1.7 to -2.2
Milk 87 -0.5
Egg 74 -0.5
7/30/2019 Food Freezing (2009)
6/69
Freezing Process
As freezing proceeds, heat is released andconcentration of unfrozen liquid phase increases
Phase change (333.2 kJ/kg of ice) causes
temperature of local environment to increaseTemperature increase depends on amount
of ice freezing and the rate of heatremoval
Freeze concentration of remaining fluid phasecauses decrease in freezing point of remainingliquid
7/30/2019 Food Freezing (2009)
7/69
Freezing Point Depression
-6
-5
-4
-3
-2
-1
0
0 10 20 30 40
FreezingP
oint,C
Concentration, %
Glucose
Sucrose
Corn Syrup
7/30/2019 Food Freezing (2009)
8/69
Freezing Process
As more heat is removed, the unfrozen phase
continues to become more concentrated
Continued freezing causes decrease in
molecular mobility (increase in viscosity of
unfrozen phase)
Molecules move more slowly
Approaches glassy state where molecular
mobility is very low
7/30/2019 Food Freezing (2009)
9/69
7/30/2019 Food Freezing (2009)
10/69
Freezing Process
The endpoint of freezing is either:
When freezing point temperature reaches freezertemperature
Product temperature goes below the glasstransition temperature and the unfrozen phasebecomes glassy
A state diagram helps understand which willoccur
Follow trajectory of freezing process
7/30/2019 Food Freezing (2009)
11/69
7/30/2019 Food Freezing (2009)
12/69
7/30/2019 Food Freezing (2009)
13/69
State Diagram
State diagrams tell us where to expect the systemto head for phase equilibrium
At a given storage temperature, the system will
move to approach the equilibrium curveMaximum amount of ice formed
Any point other than on the freezing pointdepression curve, including the glassy state, isnonequilibrium - metastable
Stability depends on process/storage conditions
7/30/2019 Food Freezing (2009)
14/69
Freezing Rate
But freezing rate determines how much of the
allowable water freezes in a food
Slow freezing - equilibrium ice formation
Follows freezing point depression
Fast freezing - any amount of ice, depending
on freezing rate
Any trajectory
7/30/2019 Food Freezing (2009)
15/69
Water Frozen
Amount of water frozen into ice thus depends on
freezing rate
Slow freezing - maximum ice
Fast freezing - any ice content
7/30/2019 Food Freezing (2009)
16/69
Unfreezable Waterassuming phase equilibrium
All products have some water that remainsunfrozen even at very low temperatures (
7/30/2019 Food Freezing (2009)
17/69
Freezing Rate Due to conduction heat transfer, the freezing
rate is also a function of the position in the
food Center sees much slower freezing rate than surface
Mechanisms of freezing may be different Ice distribution may also be different at surface from interior
Temperature differential allows moisture migration
7/30/2019 Food Freezing (2009)
18/69
Freezing Rate
Freezing rate defined as: Ratio between the minimum distance from the surface to
the thermal center, and the time elapsed between thesurface reaching 0C and the thermal center 10C colderthan the temperature of initial ice formation. (InternationalInstitute of Refrigeration, as quoted by Zaritzky, 2000)
Typical food freezing rates 0.2 - 0.5 cm/h slow static
0.5 - 3 cm/h quick air blast and plate
5 - 10 cm/h rapid IQF fluidized bed 10-100 cm/h ultra-rapid cryogenic
7/30/2019 Food Freezing (2009)
19/69
Question
Freezing rate has many impacts on a freezing
operation - how many can you list?
Product quality
Throughput rate
Refrigeration costs
Equipment costs Others?
7/30/2019 Food Freezing (2009)
20/69
Freezing Mechanisms
The process of freezing requires these steps:
Subcooling - bring temperature down belowfreezing temperature
Nucleation - formation of the smallest crystalsfrom the liquid state
Growth - increase in size of those nuclei until thesystem approaches phase equilibrium
Ripening - change in dispersion of crystal sizeswith time due to thermodynamic effects
7/30/2019 Food Freezing (2009)
21/69
Subcooling
Nuclei do not form under most circumstances
until temperature is lowered substantially
below the melting point (Tm)
Related to an energy barrier to be overcome to form
a stable nucleus
The temperature at which nuclei form depends on
process conditions Cooling rate, agitation, etc.
7/30/2019 Food Freezing (2009)
22/69
Subcooling
High T (20-30C)
Rapid freezing
High nucleation rate
Many nuclei formed
Low T (1-5C)
Slower freezing Lower nucleation rate
Fewer nuclei formed
T = the difference between freezing point and freezing temperature
7/30/2019 Food Freezing (2009)
23/69
Nucleation Onset of nuclei formation in a frozen food is
when the water molecules attain the correctenergy and position to form into a crystallattice
7/30/2019 Food Freezing (2009)
24/69
Nucleation Mechanisms
Homogeneous nucleation - water molecules cluster together
Heterogeneous nucleation - dust particles promote nucleation
7/30/2019 Food Freezing (2009)
25/69
Growth
After nuclei form, they grow until all
subcooling has been relieved Equilibrium temperature and product temperature are the same
Mechanisms
Heat removal rate Counter-diffusion of solutes
7/30/2019 Food Freezing (2009)
26/69
Effects of Freezing
on Food Quality
Numerous changes take place during all stages
of freezing that can affect food quality
Prefreezing conditions
Freezing rate
Storage conditions
7/30/2019 Food Freezing (2009)
27/69
Prefreezing
If initial temperature is well above the freezingpoint when a product is frozen
Water migration occurs due to thermal gradientsduring cooling and freezing
The warm water inside migrates toward surface
Redistribution of solutes
May be a problem in regions of different watercontent, e.g., crumb and crust
Can cause separation and unsightly appearance
7/30/2019 Food Freezing (2009)
28/69
Freezer Bloom Freezing of frozen cakes with sugar frosting
Freezing from warm state causes watermigration, which carries dissolved sugar
When the water evaporates (or ice
sublimes) leaves unsightly spots
7/30/2019 Food Freezing (2009)
29/69
Freezing Freezing affects properties of the food
Effects on cell structure
Osmotic pressure differences between intracellular andextracellular fluid cause moisture migration
o May lead to cell lysis (rupture)
Moisture migration
Osmotic differences; thermal gradients; etc.
Volume expansion of ice may rupture cells
Freeze concentration of solutes in unfrozen phase
Salts, sugars, etc. may lead to crystallization
Protein denaturation Freeze concentration of solutes like salts
7/30/2019 Food Freezing (2009)
30/69
Slow vs. Rapid Freezing
Rapid freezing leads to formation of manymore and smaller crystals
Fewer internal changes in structures (cells,etc.)
Smoother product
7/30/2019 Food Freezing (2009)
31/69
Freezer Storage
Over time, changes can occur in the frozen
product that cause product quality todeteriorate
Equilibration of ice phase volume
Changes in ice crystal dispersion due to ripening
Starch retrogradation
Protein denaturation
Water migration and loss
7/30/2019 Food Freezing (2009)
32/69
Phase Equilibration
If phase equilibrium was not attained during
freezing, the system will drive towards that
equilibrium over time if T > Tg
Increase in ice content
Changes in ice crystal
size distribution
7/30/2019 Food Freezing (2009)
33/69
Recrystallization
Definition:
"Any change in the number, size, shape, orientation
or perfection of crystals following completion of
initial solidification." (Fennema, Powrie and Marth,
1973)
Enhanced ramatically
by fluctuating
temperatures during
storage
7/30/2019 Food Freezing (2009)
34/69
Fluctuation in Ice Content
7/30/2019 Food Freezing (2009)
35/69
Recrystallization
Effects of recrystallization
Increase in mean size causes disruption of
microstructure and loss of texture
Smooth frozen product becomes coarse
7/30/2019 Food Freezing (2009)
36/69
Moisture Migration and Freezer Burn
During freezing and frozen storage, regions ofdifferent water activity tend to equilibrate
Crust and crumb will change during storage
Bread and icing or filling
Pizza crust and sauce
Freezer burn - color and quality change Loss of moisture to air
7/30/2019 Food Freezing (2009)
37/69
Summary
An understanding of the physico-chemical
factors that affect quality during freezing
allows production of the highest quality
product with the most efficient process
7/30/2019 Food Freezing (2009)
38/69
Engineering Aspects
Freezing process has a dramatic affects on the
thermal properties of food products
As the water within the product changes from
liquid to solid, the density, thermal
conductivity, heat content (enthalpy), and
apparent specific heat change gradually as the
temperature decreases below the initialfreezing point for water in the food.
7/30/2019 Food Freezing (2009)
39/69
Density
The density of solid water (ice) is less than thedensity of liquid water. Thus the density of a frozen
food will be less than the unfrozen product.
950
960
970
980
990
1000
1010
1020
1030
1040
1050
1060
-45 -35 -25 -15 -5 5 15 25
ProductDensity,
kg/m3
Temperature, C
Strawberries
Water content 89.3%
Init. Freezing temp = -0.89C
7/30/2019 Food Freezing (2009)
40/69
Thermal Conductivity Thermal conductivity of ice is approximately four time larger
than that of liquid water.
This relationship has a similar influence on the thermal
conductivity of frozen food.
The majority of thermal conductivity increase occurs within
10oC below the initial freezing temperature of the product.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-40 -30 -20 -10 0 10
ThermalCondu
ctivity,W/m-oC
Temperature, C
Froozen lean beef
7/30/2019 Food Freezing (2009)
41/69
Enthalpy The heat content (enthalpy) of a froozen food is an important property in
computations of refrigeration requirement for freezing of product. The heat content normally zero at -40C and increases with increasing
temperature.
Significant changes in enthalpy occur at 10oC below the initial freezing
temperature, when most of the phase change in product water occurs.
0
50
100
150
200
250
300
350
400
-45 -35 -25 -15 -5 5 15
Entha
lpy(kJ/kg)
Temperature, C
Sweetcheries
Moisture content = 77%
Init. Freezing temp. = -2.61C
7/30/2019 Food Freezing (2009)
42/69
Apparent Specific heat
Specific heat of a frozen food at a temperature greater than
20oC below the initial freezing point is not significantlydifferent from the specific heat of the unfrozen product.
0
5
10
15
20
25
30
35
-45 -35 -25 -15 -5 5 15
ApparentSpecificHeat,kJ/k
g.oC
Temperature, C
Sweet cherries
Moisture content = 77%Init.freezing temp. = -2.61oC
7/30/2019 Food Freezing (2009)
43/69
Freezing Process Prefreezing
Phase change
Postfreezing
7/30/2019 Food Freezing (2009)
44/69
Prefreezing : The temperature of the waterdecreases to the freezing point as sensible heat isremoved. Small amount of supercooling takeplace; but once nucleation occurs and ice crystals
begin to form, the freezing point increases to 0o
C. Phase change : The temperature remains at thefreezing point until a complete phase changeoccurs. Latent heat of fusion is removed from
liquid water to convert it into solid ice. Postfreezing : When all the liquid water haschanged into solid ice, the temperature of the icedecreases rapidly as sensible heat is removed.
Freezing Process of Pure Water
7/30/2019 Food Freezing (2009)
45/69
Prefreezing : Temperature decreases during prefreezing
as sensible heat is removed. Nucleation and ice crystalsbegin to form at lower temperature that that of purewater.
Phase change : After a brief supercooling, latent heat isgradually removed with decreasing temperature. This
is due to concentration effect during freezing of food. As water in the food converts into ice, the remaining
water becomes more concentrated with solutes anddepresses the freezing point.
After this time, sensible heat is further removed untilpreselected endpoint is reached (Typically -18oC forfruits and vegetables and -25oC for foods with higherfat contents such as ice cream, fatty fish, ect.)
Freezing Process of Potato
7/30/2019 Food Freezing (2009)
46/69
Freezing Time
The most important calculation in the designof freezing process is the determination offreezing time.
Freezing time is the most critical factor
associated with the selection of a freezingsystem to ensure optimum product quality.
Freezing time requirements help establishsystem capacity.
Methods of prediction : Planks Equation andPhams Method.
7/30/2019 Food Freezing (2009)
47/69
Planks Equation
Proposede by Plank (1913) and adapted to food by
Ede (1949).
This equation describes only the phase change
period of the freezing process.
7/30/2019 Food Freezing (2009)
48/69
Planks Equation
Assume the homogeneous slab is
at initial freezing temperature (TF).
The slab is exposed to a freezing
merdium at temperature Ta.
After some time, the will be 3
layers; 2 frozen layers at the
surface and a middle unfrozen
layer.
The moving front inside the slab
separates the frozen from the
unfrozen region.
As water is converted into ice at
the moving front, latent heat of
fusion (L) is generated.
7/30/2019 Food Freezing (2009)
49/69
The latent heat of fusion generated at themoving front must be transfered through thefrozen layer to the surface (by conduction).
The heat is then transfered to the freezingmedium via convection. The convective heattransfer coefficient at the surface of the slab ish.
The temperature of the unfrozen regionremains at TF until the freezing front moves allthe way to the center plane of the food.
Formulation of Planks Equation
7/30/2019 Food Freezing (2009)
50/69
Two layers of heat transfer : conduction through the
frozen layer and a convective boundary layer.
Formulation of Planks Equation
Ak
XqTT
X
TTAkq sF
Fscond
.)(
)(.
Ah
qTTTTAhq asasconv
.)()(.
hk
X
A
qTTTT assF
1)()(
k
X
h
TTAq aF
1
)(
7/30/2019 Food Freezing (2009)
51/69
The heat released due to advancement of the
freezing front :
where :A = cross section area, m
L = latent heat of fusion, kJ/kg
f = density of water, kg/m3dx/dt = velocity of moving front, m/s
Formulation of Planks Equation
dt
dxLAq f
7/30/2019 Food Freezing (2009)
52/69
Since all the heat released at the freezing front must be
transfered out to the surrounding medium, then :
The freezing process is completed when the moving front
advances to the center of the slab (a/2). Then
Formulation of Planks Equation
k
X
h
TTA
dt
dxLA aFf
1
)(
2/
001
a
faF
ft
dxkx
hTT
Ldtf
faF
f
f
k
a
h
a
TT
Lt
82
2 Freezing time for
infinite slabs
7/30/2019 Food Freezing (2009)
53/69
For food material with moisture content m,Planks equation becomes :
Formulation of Planks Equation
faF
f
fk
a
h
a
TT
mLt
82
2
Where :
L = latent heat of fusion for water (333.2 kJ/kg)
m = moisture content of food, fraction
f = density of frozen product, kg/m3
TF = freezing temperature, C
Ta = medium temperature, C
a = thickness of the slab, m
h = convective coefficient, W/m2.K
kf = thermal conductivity of frozen product, W/m.K
7/30/2019 Food Freezing (2009)
54/69
General Form of Planks Equation
where :
f = density of the frozen maerial
kf = conductivity of frozen material
P and R = shape factors
Formulation of Planks Equation
faF
f
fk
aR
h
aP
TT
mLt
2'.'...
7/30/2019 Food Freezing (2009)
55/69
Values for P and R
Infinite slab : P = R = 1/8
Infinite cylinder: P = R = 1/16
Sphere : P = 1/6 R = 1/24
For finite slab and cylinder, use diagram to
determine the values for P and R.
7/30/2019 Food Freezing (2009)
56/69
Material geometry constants P and R for a brick-shaped object for the Planks Freezing
time equation with dimensions a, b, c; where a is the shortest side.
E l #1
7/30/2019 Food Freezing (2009)
57/69
Example #1
A spherical food product is being frozen in an air-blast freezer. The initial rpoduct temperature is 10oC
and the cold air -40oC. The product has a 7 cm
diameter with density of 1000 kg/m3, the initial
freezing temperature is -1.25oC, the thermalconductivity of the frozen product is 1.2 W/m.K, and
the latent heat of fusion is250 kJ/kg. Convective heat
transfer coefficient is 50 W/m2.K. Compute the
freezing time.
E l #1
7/30/2019 Food Freezing (2009)
58/69
Given:Ti = 10
oC
Ta = -40oC
TF = -1.25oC
a = 0.07 m
f= 1000 kg/m3
kf= 1.2 W/m.K
HL = 250 kJ/kg
P = 1/6
R = 1/24
Example #1
E l #1
7/30/2019 Food Freezing (2009)
59/69
Using Planks Equation :
Example #1
hoursWJt
KmW
m
KmW
mx
C
kgkJmkgt
F
oF
72.02600/2600
)./2.1(24
)07.0(
)./50(6
07.0
)]40(25.1[
)/250()/1000(
2
3
E l #2
7/30/2019 Food Freezing (2009)
60/69
Blueberries are to be frozen in 2 liters cardboard boxes having thefollowing dimensions: 6 cm x 15 cm x 22.5 cm
Blueberries have the following characteristics:moisture content m = 85%Density = 1015 kg/m3Thermal conductivity
frozen: kf= 1.95 W/m.Kunfrozen: k
u= 3.85 W/m.K
Freezing temperature: Tf= - 0.9CThe boxes are made of cardboard 0.8 mm thick with a thermalconductivity kcard = 0.05 W/m.KThe blueberries are frozen in an air blast freezer where the
convective heat transfer coefficient (hc) between the cold air andthe boxes surface is equal to 25 W/m2.K.
The temperature of the cold air in the freezer can be adjustedbetween -20C and -40C.
Example #2
E l #2
7/30/2019 Food Freezing (2009)
61/69
Box dimensions: 0.06 m x 0.15 m x 0.225 m
a = 0.06 m b = 0.15 m c = 0.225 m
b/a = 2.5 c/a = 3.75
Example #2
7/30/2019 Food Freezing (2009)
62/69
E l #2
7/30/2019 Food Freezing (2009)
63/69
From above chart we get: R = 0.085 P = 0.304
Evaluation of the overall heat transfer coefficient:
= 1/25 + 0.8 x 10-3/0.05 = 0.056
U = 17.86 W/m2.Kkf= 1.95 W/m.K
Latent heat of fusion for blueberries = 85% x latent heat of fusion of waterL . m = 335 x 0.85 = 284.75 kJ/kg = 284750 J/kg
We want the blueberries to freeze in less than 2.5 hours: q f= 2.5 x 3600 = 9000 s
Example #2
E l #2
7/30/2019 Food Freezing (2009)
64/69
Example #2
CT
xxx
k
aR
U
aP
t
mLTT
kaR
UaP
t
mLTT
o
a
ff
f
Fa
ff
faF
7.38
95.1
06.0085.0
86.17
06.0304.0
9000
10152847509.0
''..
''..
2
2
2
Ph M th d
7/30/2019 Food Freezing (2009)
65/69
Phams Method
Proposed by Q. T. Pham in 1986
Can be used for finite-size objects of irregular
shapes by approximating them to be similar to
an ellipsoid.
Easy to use yet provides answer with
reasonable accuracy.
Ph M th d
7/30/2019 Food Freezing (2009)
66/69
Basic Assumptions:
Environmental conditions are constant
Initial temperature, Ti is constant
Final temperature, Tc
is fixed
Convective heat transfer at the surface of
the object obeys Nowtons law of cooling
Phams Method
C li d F i Di
7/30/2019 Food Freezing (2009)
67/69
Cooling and Freezing Diagram
Ph M th d
7/30/2019 Food Freezing (2009)
68/69
Based on experimental data for variety of foods
where Tc is final center temperature and Ta is mediumtemperature.
Freezing time for simple-shaped objects :
where dc is the shortest distance to the center, or radius. h isconvective heat transfer coefficien, Efis shape factor (Ef= 1 forinfinite slab, 2 for infinite cylinder, and 3 for sphere
Phams Method
acfm TTT 105.0263.08.1
2
12
2
1
1 Bi
f
c N
T
H
T
H
hE
dt
7/30/2019 Food Freezing (2009)
69/69
k
lhNBi
.
Top Related