Transcript Colligative Properties of Solutions

FST 151
FOOD FREEZING
FOOD SCIENCE AND TECHNOLOGY 151
Food Freezing (Basic concepts)
Lecture Notes
Prof. Vinod K. Jindal
(Formerly Professor, Asian Institute of Technology)
Visiting Professor
Chemical Engineering Department
Mahidol University
Salaya, Nakornpathom
Thailand
1
Definitions
• Chilling: Temperatures between 15oC
and slightly above freezing point.
• Freezing: From slightly below freezing
point to -18oC.
2
Purpose
•
Freezing stops / retards:
- Growth of microorganisms
- Rate of chemical reactions
- Enzyme activity
- Moisture loss, if properly packaged
3
Advantages of freezing:
• Preservation of color, flavor and
nutritive value.
• Microorganisms do not grow and
multiply during frozen storage.
• Freezing kills some vegetative cells.
Spores survive, and may grow when
the food is thawed.
4
Disadvantages of freezing:
• Deterioration of texture depending on the
nature of food and the freezing process
• Minor losses in nutritive value and quality
• Expensive preservation operation, and
requires energy even after the operation is
complete
• Depending on the storage conditions, frozen
foods may also lose water
5
The engineering aspects of the freezing
process deal with the following:
• Computing the refrigeration requirements
needed to accomplish the desired reductions
in product temperature
• Determining the freezing time needed to
reduce product temperature to desired levels
• Understanding the changes occurring within
the food product during frozen food storage.
6
Some basic concepts related to freezing…
• A matter can exist in three states
or phases by changing the
temperature and/or pressure:
- Gas
- Liquid
- Solid
7
PHASE TRANSISTIONS
 SOLID TO LIQUID: MELTING
 LIQUID TO SOLID: FREEZING
 GAS TO LIQUID:
CONDENSATION
 LIQUID TO GAS:
EVAPORATION
 SOLID TO GAS: SUBLIMATION
 GAS TO SOLID: DEPOSITION
8
PHASESublimation
CHANGES
Melting
Solid
Boiling
Liquid
Freezing
Gas
Condensation
Deposition
9
PHASE CHANGES
•Freezing point (FP) – the temperature where
liquids change into solids
•Melting point (MP) – the temperature where
solids change into liquids
•Boiling point (BP) - the temperature where
liquids change into gases
10
HEAT TRANSFER
•Exothermic – heat is removed
from the system
•Endothermic – heat is added to the
system
11
SPECIFIC HEAT CAPACITY
•A substance’s resistance to temperature change
when heat is added or removed. Symbol is c.
•Measured in J/g•⁰C
•
Joules are a measurement of energy
•
4.184 J = 1 calorie 1000 calorie = 1 k calorie
•Specific heat is a physical property.
•High specific heat requires more heat to change the
temperature.
•Water has a very high specific heat.
•
cwater
= 4.184 J/g•⁰C
14
•
c
= 2.05 J/g•⁰C
PHASE CHANGE – HEAT CHANGE
•Heat of vaporization (Hvap)– the amount of heat required to
change 1 g of a substance from liquid to gas or gas to liquid.
•
q = mHvap
•Heat of fusion (Hf) – the amount of heat required to change
1 g of a substance from liquid to solid or solid to liquid
•
q = mHf
•Hvap for water = 2260 J/g
•Hf for water = 334 J/g
•Does it take more energy to boil 100 g of water or freeze
15
100 g of water?
PHASE CHANGE PROBLEM
•
How much energy is required to boil 250 g
of water that is at 100⁰C?
• q = mHvap
•q
= heat energy
• m = mass in grams
• Hvap = heat of vaporization of water
q = mHvap
q = 250g x 2260 J/g
q = 565,000 J
16
THREE
STEP
PROBLEM
•How much
energy is
released
when 500 g of
liquid water at 25⁰C is cooled to -15⁰C?
•First calculate 25⁰C to 0⁰C
q = mc∆T
q = 500g x 4.184 J/g•⁰C x 25⁰C
q = 52,300 J
•Next, calculate freezing
•Next, calculate 0⁰C to -15 ⁰C
q = mHf
q = 500g x 334J/g
q = 167,000J
q = mc∆T
q = 500g x 2.03 J/g•⁰C x 15⁰C
q = 15,225 J
•Finally, add them together
52,300J + 167,000J + 15,225 = 234,525 J
17
PHASE CHANGE DIAGRAM
•Things to notice:
•Pressure and temperature
both affect the phase of
matter.
•All three phases of matter
exist at the triple point
Melting/Freezing
Boiling/Condensating
18
In the phase diagram for pure water, three lines indicate the
phase transition between solid, liquid and gas. All three lines
meet at the triple point where all three phases are in
equilibrium. If the pressure is lowered, we note that the
boiling point will be lowered and the melting point raised
(very slightly).
19
Now look at the following diagram indicating the phase
transitions for pure water and for water with some solute
dissolved in it (not to scale).
20
Freezing Point Depression
21
Temperature
Phase Change: Freezing/Melting
FP
Time
22
Temperature
Actual Freezing
FP
Time
23
Freezing & Freezing Point
• A pure liquid will freeze when enough internal
energy is removed from the system to the
surroundings, this is usually initiated by a
decrease in the surrounding’s temperature (an
exothermic process).
• The exact temperature at which the solid phase
is in equilibrium with the liquid phase is
referred to as the “Freezing or Melting Point”
and if the pressure is 1 atm (760 mmHg) then
that temperature is called the “normal
Freezing/Melting Point”.
24
Freezing Point Depression in Solutions
The freezing point of pure water is 0°C, but that
melting point can be depressed by the adding of
a solvent such as a salt. A solution typically has
a measurably lower melting point than the pure
solvent.
A 10% salt solution may lower the melting point
to -6°C (20°F) and a 20% salt solution to to
-16°C (2°F).
25
GENERAL PROPERTIES OF SOLUTIONS
1. A solution is a homogeneous mixture of
two or more components.
2. The dissolved solute is molecular or ionic
in size.
3. The solute remains uniformly distributed
throughout the solution and will not settle
out through time.
4. The solute can be separated from the
solvent by physical methods.
26
Colligative Properties of Solutions
• Colligative properties of solutions are
properties that depend upon the
concentration of solute molecules or ions,
but not upon the identity of the solute.
• Colligative properties include freezing point
depression, boiling point elevation, vapor
pressure lowering, and osmotic pressure.
27
Colligative Properties
• Solution properties differ from those of pure
solvent
• Proportional to molality (concentration) of
solute
–
–
–
–
Vapor pressure reduction
Freezing point depression
Boiling point elevation
Osmotic pressure
28
Colligative Property
• Magnitude of freezing point depression:
ΔT f  Tf,solution  Tf,solvent  0
Depends on concentration of solute (not identity)
ΔT f  K f solute
29
Freezing Point Depression

Relationship for freezing point depression
is:
Tf   K f m
where : Tf  freezing point depression of solvent
m  molal concentrat ion of soltuion
K f  freezing point depression constant for solvent
30
Freezing point depression
ΔT f  K f solute  Tf,solution  Tf,solvent
•Kf: “molal freezing point constant”
specific to solvent
• Units:
°C
molal
•What is molal?
31
Molality
•Concentration in molality, m:
m oles solute (m ol) nsolute
m olality,m

m ass solvent (kg) msolvent
•Independent of solution volume (V varies
with T)
32
Freezing point depression
• Using:
• and:
 nsolute 
ΔT f   K f solute   K f 

 msolvent, kg 
nsolute 
msolute, g
MM solute, g/mol
• Substitution gives:


msolute,g
ΔT f   K f 

 MM solute,g/mol  msolvent, kg 
33
Kf and Kb for some solvents
Solvent
Formula
Mpt(oC)
Bpt(oC)
Kf(°C/m)
Kb(°C/m)
Water
H2O
0.0
100
1.858
0.521
Acetic acid
HC2H3O2
16.6
118.5
3.59
3.08
Benzene
C6H6
5.455
80.2
5.065
2.61
Camphor
C10H16O
179.5
...
40
...
Carbon disulfide
CS2
...
46.3
...
2.4
Cyclohexane
C6H12
6.55
80.74
20
2.79
Ethanol
C2H5OH
...
78.3
...
1.07
•
•
Freezing point is lower
Boiling point is higher
34
Freezing Point Depression
Tf = - kf m
Q. Estimate the freezing point of a 2.00 L sample of seawater (kf = 1.86 oC kg / mol), which
has the following composition:
0.458 mol of Na+
0.052 mol of Mg2+
0.010 mol Ca2+
0.010 mol K+
0.533 mol Cl0.002 mol HCO30.001 mol Br0.001 mol neutral species.
Since colligative properties are dependent on the NUMBER of particles and not
the character of the particles, you must first add up all the moles of solute in
the solution.
Total moles = 1.067 moles of solute
Now calculate the molality of the solution:
m = moles of solute / kg of solvent = 1.067 mol / 2.00 kg
= 0.5335 mol/kg
Last calculate the temperature change:
Tf = - kf m = -(1.86 oC kg/mol) (0.5335 mol/kg) = 0.992 oC
The freezing point of seawater is Tsolvent - T = 0 oC - 0.992 oC
= - 0.992 oC
35
MOLALITY
• Molality = moles of solute per kg of solvent
• m = nsolute / kg solvent
• If the concentration of a solution is given in terms of
molality, it is referred to as a molal solution.
Q. Calculate the molality of a solution consisting of 25 g of KCl in
250.0 mL of pure water at 20oC?
First calculate the mass in kilograms of solvent using the density of
solvent:
250.0 mL of H2O (1 g/ 1 mL) = 250.0 g of H2O (1 kg / 1000 g) = 0.2500 kg of H2O
Next calculate the moles of solute using the molar mass:
25 g KCl (1 mol / 54.5 g) = 0.46 moles of solute
Lastly calculate the molality:
36
m = n / kg = 0.46 mol / 0.2500 kg = 1.8 m (molal) solution
Molality and Mole Fraction
•
Two important concentration units are:
1. % by mass of solute
mass of solute
% w/w =
100%
mass of solution
2.
Molarity
moles of solute
M=
Liters of solution
37
Molality and Mole Fraction (contd)
• Molality is a concentration unit based on the
number of moles of solute per kilogram of
solvent.
moles of solute
m
kg of solvent
in dilute aqueous solutions molarity and
molality are nearly equal
38
Food Freezing Theory
• During freezing, sensible heat is first
removed to lower the temperature of a food
to the freezing point. In fresh foods, heat
produced by respiration is also removed.
This is termed the heat load, and is
important in determining the correct size of
freezing equipment for a particular
production rate.
39
• A substantial amount of energy is therefore
needed to remove latent heat, form ice
crystals and hence to freeze foods.
• The latent heat of other components of the
food (for example fats) must also be
removed before they can solidify but in
most foods these other components are
present in smaller amounts and removal of a
relatively small amount of heat is needed
for crystallization to take place.
40
FOOD FREEZING
• Temperature lowered
• Most water transformed into ice
crystals
• Liquid phase concentrated
• As volume of ice 10% larger than
volume of water, internal pressure in
the food rised to 10 bar or more
41
Freezing Curve
A typical freezing curve of a food consists of the following
regions:
•
•
•
•
The initial sensible heat removal section: To bring it to
the freezing point. the temperature changes but
without change in phase.
Supercooling: In slow freezing, food temperature may
drop temporarily below the freezing point, without
phase change.
Latent heat: When ice crystals form, they release heat
of fusion, and temperature increases to the freezing
point.
Final sensible heat removal: Frozen foods are kept
at or below -18oC. Since the freezing points of most
foods is above that, we need to cool the frozen food.42
Figure 1. Typical freezing curve of foods
43
3-21
Typical Freezing Curve (food)
44
SUPERCOOLING
Temperature
A
Removal of sensible heat
Removal of latent heat
Removal of sensible heat
Cooling time
45
46
• During freezing the product temperature
decreases gradually as the latent heat of
fusion is removed from water within the
product.
• In foods the equilibrium temperature for
initial formation of ice crystals is lower
than the equilibrium temperature for ice
crystal formation in pure water.
• The magnitude of the depression in
equilibrium freezing temperature is a
function of product composition.
47
• After the formation of initial ice crystals in the
food product, the removal of phase change
energy occurs gradually over a range of
decreasing product temperatures.
• The temperature–time relationship during phase
change is a function of the percent water frozen
at any time during the freezing process.
• The shape of the temperature–time curve during
the freezing process will vary with product
composition and with the location within the
product structure.
48
• The gradual decrease in temperature
with time will continue until reaching the
eutectic temperatures for major product
components. In practice, food products
are not frozen to sufficiently low
temperatures to reach these eutectic
temperatures.
49
Time–temperature data during freezing.
50
• AS The food is cooled to below its freezing point f which, with
the exception of pure water, is always below 0ºC . At point S
the water remains liquid, although the temperature is below the
freezing point. This phenomenon is known as supercooling
and may be as much as 10ºC below the freezing point.
• SB The temperature rises rapidly to the freezing point as ice
crystals begin to form and latent heat of crystallisation is
released.
51
• BC Heat is removed from the food at the same rate as before,
but it is latent heat being removed as ice forms and the
temperature therefore remains almost constant. The freezing
point is gradually depressed by the increase in solute
concentration in the unfrozen liquor, and the temperature
therefore falls slightly. It is during this stage that the major part
of the ice is formed .
• CD One of the solutes becomes supersaturated and crystallizes
out. The latent heat of crystallization is released
52
• DE Crystallization of water and solutes continues. The total
time tf taken (the freezing plateau) is determined by the rate at
which heat is removed.
• EF The temperature of the ice–water mixture falls to the
temperature of the freezer. A proportion of the water remains
unfrozen at the temperatures used in commercial freezing; the
amount depends on the type and composition of the food and
the temperature of storage. For example at a storage
temperature of -20ºC the percentage of water frozen is 88% in
lamb, 91% in fish and 93% in egg albumin.
53
54
INITIAL FREEZING POINT OF
DIFFERENT FOODS (OC)
•
•
•
•
•
Beef
Common fruits
Common vegetables
Eggs
Milk
-1.1
- 0. to – 2.7
- 0.8 to – 2.8
- 0.5
- 0.5
55
Effects of Freezing
• The goal of freezing is to reduce the
temperature of the food below -18oC
• This results in the crystallization of part
of the water and some of the solutes in
the food
• Water in the frozen state does not act as
a solvent, can not enter into chemical
reactions, and is not available to
microorganisms.
56
Effect of water
• Water has a major impact on the freezing
behavior of foods. Most foods contain large
amounts of water (fruits/vegetables: up to
90 wt% water, meats: 65 - 75% water).
• Freezing point, the heat capacity above
and below freezing, and the latent heat
of freezing are strongly affected by the
moisture content.
57
Freezing point
• Pure water freezes at 00C under 1 atm.
• The freezing point of foods will be close to
00C depending on the amount of water
present.
• In general, the higher the moisture content,
the closer the freezing point to 00C.
58
Water binding
• The degree of binding of water by the
food is also important. Water that is
tightly bound will not freeze as readily
as "free water".
• In foods, there are inorganic and organic
substances such as sugars, acids, salts,
colloids, etc. dissolved in water.
59
Supercooling
•
It is possible to reduce the temperature of
water below 00C, and still have liquid water
at 1 atm. This is called supercooling.
•
If we cool a solution with no ice crystals, the
temperature can be lowered below the
freezing point.
•
Supercooling is an unstable and temporary
state. It occurs because there is no "nucleus"
for the crystals to form around. Occurrence of
supercooling depends on the rate of freezing.
60
Crystallization of Water
• Formation of a regularly organized solid
phase from a solution.
• There are two steps in the process:
• Nucleation
• Crystal growth
61
Nucleation
•
When the first ice crystal forms, it starts nucleation
and the solidification process. The nucleus occurs
homogeneously or heterogeneously.
•
In very pure water, nucleation is homogeneous,
the "nucleus" is water molecules orienting as
crystals. Homogeneous nucleation does not occur in
foods.
•
In heterogeneous nucleation, crystals form around
foreign particles, surface films, container walls. Ice
crystal formation is easier in this case.
62
Crystal growth
• The rate of crystal growth depends on:
- How fast water molecules get to
the nucleus.
- How fast heat is removed from the
system to favor orientation and
ordering of the water molecule.
63
Crystal growth
• If a material is cooled rapidly, the rate of
nucleation is greater than the rate of
crystal growth, and many small crystals
will form.
• These crystals have no time to attract
more water molecules and grow because
new crystal formation is faster, and it
consumes the available water.
64
Crystal growth
• If the rate of cooling is slow, the rate of
crystal growth is faster than the formation
of new crystals, and there will be a smaller
number of large crystals.
• At temperature near the melting point,
water molecules add to the mass of the
nucleus rather than forming new nuclei.
65
Freezing point
•
In foods, there are solutes, suspended
matter, and cellular material in solution.
•
Ice crystals "squeeze" other molecules
out. Solutes get concentrated. This
reduces the freezing point.
•
In most foods, there is a narrow range
of temperature at which freezing occurs.
66
Ice crystals
•
The number and shape of ice crystals has a
major effect on the quality of frozen foods.
•
Large crystals damage the tissue. Meat,
poultry, fish, shellfish, fruit and vegetable
cells contain jelly-like protoplasm. Large ice
crystals puncture cells. After thawing, they
cannot reach their former state.
•
Small crystals do not injure the tissue as
much. When thawed, they can be re-absorbed
into the protoplasm. Thaw-drip is minimized.67
• The location of ice crystals in food tissues
depends on freezing rate, temperature, nature of
the cells.
•
Slow freezing (less than 10C/min) causes the
crystals to form exclusively in the extracellular
spaces. This shrinks the cells, disrupts tissues
and results in lower quality.
•
Freezing starts at extracellular space. Inside the
cell is a supercooled solution. Its water vapor
pressure is higher than that of extracellular ice.
This difference in vapor pressure causes water to
migrate from inside the cell to extracellular space.
68
Re-crystallization
• Largest possible size, and no defects in the
crystal lattice is the thermodynamically stable
form of a crystal.
• Small crystals try to coalesce into bigger
ones.
• The number, size, shape and orientation of
crystals change during storage. The rate of
re-crystallization depends on temperature.
69
Re-crystallization
•
Lower temperature implies slower recrystallization.
•
Keep foods at as low a temperature as
possible. Fluctuations in temperature
help re-crystallization.
•
Pure ice re-crystallizes at a significant
rate at -70oC. In regular tissue food,
the rate is very slow at -28oC.
70
Eutectic
•
Each solute has a solubility limit in water.
As more water is removed by freezing, a
point is reached where the solute is
saturated in the remaining solution at that
temperature.
•
Further freezing of water results in
crystallization of solute together with
water. Such simultaneous crystallization
is called a eutectic and the temperature is
known as the eutectic point of the solute.
71
Examples
•
For example, a NaCl solution in water has a
eutectic point of -21oC. When this temperature
is reached, water and salt crystallize together.
•
Since the concentration of the remaining
solution does not change after this point,
temperature is constant until all water and
salt crystallize.
•
Sucrose solutions have a eutectic point of
-14oC.
72
Eutectic Point
•
•
•
•
Temperature where there is no further
concentration of solutes due to freezing, thus
the solution freezes.
Temperature at which a crystals of individual
solute exists in equilibrium with the unfrozen
liquor and ice.
Difficult to determine individual eutectic points
in the complex mixtures of solutes in foods so
the term Final Eutectic Point is used
This represents lowest Eutectic temperature
of the solutes in the food.
73
Eutectic temperatures
• Ice Cream
• Meat
• Bread
-55oC
-50 to -60oC
-70oC
MAXIMUM ICE CRYSTALS FORMATION IS
NOT POSSIBLE UNTIL THIS TEMPERATURE
IS REACHED
74
Effects of Freezing
•
Volume change : Most substances shrink
when going from the liquid to the solid state.
Water is anomalous : its volume increases
as it freezes.
•
Conversion of water to ice causes about 9%
volume increase. Volume change in food
during freezing depends on its composition.
Concentrated sugar solutions do not expand
when frozen, they may even shrink.
75
Volume change
The change in volume depends on:
• Percent water. More water = larger expansion.
• Air pockets. They absorb the growth of crystals.
• Unfreezable water. If there are many solutes
present, water is bound, and will not freeze.
• Temperature. Before the freezing point of water
is reached, its volume decreases. Maximum
density is around 4oC. Below that temperature,
it will expand.
76
Effects of volume change
• During freezing some parts of food contract,
and some expand. This causes mechanical
stresses. If these stresses are allowed
enough time to dissipate, there will be mo
major mechanical damage to the material.
• If the rate of freezing is very fast (cryogenic
freezing) the material cracks.
77
Concentration Effects
• Freezing removes water from the food,
and the solute concentration increases.
• Changes in electrolytes may cause
irreversible changes in colloidal structures.
Milk proteins may coagulate. Changes in
pH, ionic strength, viscosity, surface
tension, redox potential, freezing point, etc.
may result.
78