2. Water activity is defined as the
vapor pressure of a liquid divided by
that of pure water at the same
temperature
Simply, stated it is a measure of the
energy status of the water in a
system.
Water activity was developed to
account for the intensity with which
water associates with various non-
aqueous constituents and solids.
3. Definitionof aw:
aw= p/ p₀
where p is the vapor pressure of water
in the substance, and p₀ is the vapor
pressure of pure water at the same
temperature
Higher aw substances tend to support
more microorganisms. Bacteria
usually require at least 0.91, and
fungi at least 0.7
4. Measurement of Water Activity
The measurement of water activity in
foods has been the subject of many
studies
The choice of one technique over another
depends on the
range, accuracy, precision, and speed of
measurement required
The accuracy of most methods lies in the
range of 0.01–0.02 aw units.
On the basis of the underlying
principles, the methods for water activity
5. Measurements Based on
Colligative Properties
1) Vapor Pressure Measurement
The measurement using a vapor pressure
manometer (VPM) was first suggested by
Makower and Meyers, Taylor, Sood and
Heldman, Labuza , and Lewicki
aw is expressed as ratio of partial pressure
of water in food to vapor pressure of pure
water having same temperature
Therefore measuring the vapor pressure
of water in food system is the most direct
measure of aw
6. The method for measurement of water vapor
pressure of food consists of placing 10–50 g
of sample in the sample flask
The desiccant flask is filled with a desiccant
material, usually CaSO4
Keeping the sample flask isolated, the
system is evacuated to less than 200 μmHg;
this is followed by evacuation of the sample
for1–2 min.
The vacuum source is then isolated by
closing the valve between the manometer
legs
Upon equilibration for 30–50 min the
pressure exerted by the sample is indicated
7. The sample flask is subsequently
excluded from the system, and the
desiccant flask is opened
Water vapor is removed by sorption onto
the desiccant, and the pressures exerted
by volatiles and gases are indicated by
the difference in manometer legs (ΔL2)
Apiezon B manometric oil (density 0.866
g/cm3) is generally used as the
manometric fluid
Temperatures of the sample (Ts) and the
vapor space in the manometer (Tm) are
recorded
8.
9. water activity of the sample, after
thermal and pressure equilibration are
attained at each measurement, is
calculated from formula
If Ts and Tm are different, water activity
is corrected as
10. 2)Freezing Point Depression
Measurements
For aw > 0.85, freezing point depression
techniques have been reported to provide
accurate results
For real solutions, the relationship
between the freezing point of an aqueous
solution and its aw is
This method is applicable only to liquid
foods and provides the aw at freezing
temperature instead of at room
temperature,
11. This method, however, has the advantage
of providing accurate water activity in the
high range (>0.98)
And can be effectively applied to systems
containing large quantities of volatile
substances
Other colligative properties such as
osmotic pressure and boiling point
elevation have not been used for food
systems.
12. Measurements Based on
Psychrometry
Measurements of dew point and wet bulb
depression of thermocouple
psychrometers along with hair and
electric hygrometers have been used in
the measurements of aw
1) Electric Hygrometer
There are 2 type of electric hygrometer
sensors capacitance or resistance
Material(either salt film or hygroscopic
polymer fiber)
Depending on the water vapor pressure
of surrounding air, water will absorb or
13. The sensor must be calibrated to
resistance or capacitance value to unit of
aw
In these instrument sample is placed in
sealed chamber containing sensor
The sample air and sensor must come to
vapor and thermal equilibrium for accurate
aw measurement
These instrument measure the entire
range from 0.1 aw and accuracy between
0.01 and 0.02 aw
14. Hair or polymer hygrometer
Several strands of hair are attached
between a fixed point
Leaver arm to produce a deflection when
relative humidity changes
Sample is placed in a hygrometer sample
cup and seals lid containing build-in-
polymer hygrometer
This instrument requires 3hr of
equilibration at constant temperature
15. Thermocouple psychrometer
Psychrometer measure the wet bulb
temperature which related to vapor
pressure using the equation
Thermocouple is positioned above a
sample in a small sealed champers
Small droplet of water is placed or
condensed on thermocouple junction
As water evaporate latent heat of
vaporization cools the thermocouple to
wet bulb temp
16. 2) Capacitance hygrometer
Consist of 2 charged plate separated by a
polymer membrane dielectric
As membrane absorb water, its ability
increases and capacitance is measured
17. 3) Dew point hygrometer
The temperature at which dews forms on
a clean surface is directly related to the
vapor pressure of air
Placing the mirror over a closed sample
chamber
The mirror is cooled until dew point
temperature is then used to find the
relative humidity of chamber using psycho
meter chart
Major drawbacks include sensor fatigue
and sensor poisoning by volatiles such as
glycol, ammonia, acetone, and other
organic substances
18. Measurements Based on Isopiestic
Transfer
The isopiestic method relies on the
equilibration of the water activities in two
materials in a closed system
Transfer of moisture may take place either
through direct contact of the materials,
Thus allowing for movement of
bulk, micro capillary, and gaseous water
Or by maintaining the two materials
separately, thus permitting transfer to
occur only through the vapor phase
19. Analysis of the concentration of water in
some reference material such as
microcrystalline cellulose or a protein at
equilibrium permits determination of water
activity from the calibration curve
This technique is not accurate at aw
levels of less than 0.50 or over 0.90
20. Measurements Based on Suction
(Matric) Potential
The water potential of soil the capillary
suctional potential of gel and the matric
potentials of food gels have been
determined using the principle of a
tensiometer
Accurate for high aw range, the technique
is useful for materials that bind large
quantities of water
