Water activity or aw is the partial vapor pressure of water in a
substance divided by the standard state partial vapor pressure of
water. In the field of food science, the standard state is most often
defined as the partial vapor pressure of pure water at the same
temperature. Using this particular definition, pure distilled water
has a water activity of exactly one. As temperature increases, aw
typically increases, except in some products with crystalline salt or
Higher aw substances tend to support more microorganisms. Bacteria
usually require at least 0.91, and fungi at least 0.7. See also
Water migrates from areas of high aw to areas of low aw. For example,
if honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7), the honey
absorbs water from the air. If salami (aw ≈ 0.87) is exposed to dry
air (aw ≈ 0.5), the salami dries out, which could preserve it or
2.1 Food product design
2.2 Food safety
3.1 Resistive electrolytic hygrometers
Dew point hygrometers
4 Moisture content
5 Use in humidity control
6 Selected aw values
8 External links
Definition of aw:
displaystyle a_ w equiv p/p*
where p is the vapor pressure of water in the solution, and p*₀ is
the vapor pressure of pure water at the same temperature.
displaystyle a_ w equiv l_ w x_ w
where lw is the activity coefficient of water and xw is the mole
fraction of water in the aqueous fraction.
Relationship to relative humidity: The relative humidity of air in
equilibrium with a sample is called the Equilibrium Relative Humidity
displaystyle mathrm ERH =a_ w times 100%
Estimated mold-free shelf life in days at 21° C:
displaystyle mathrm MFSL =10^ 7.91-8.1a_ w
Water activity is an important consideration for food product design
and food safety.
Food product design
Food designers use water activity to formulate shelf-stable food. If a
product is kept below a certain water activity, then mold growth is
inhibited. This results in a longer shelf life.
Water activity values can also help limit moisture migration within a
food product made with different ingredients. If raisins of a higher
water activity are packaged with bran flakes of a lower water
activity, the water from the raisins migrates to the bran flakes over
time, making the raisins hard and the bran flakes soggy. Food
formulators use water activity to predict how much moisture migration
affects their product.
Water activity is used in many cases as a critical control point for
Hazard Analysis and Critical Control Points
Hazard Analysis and Critical Control Points (HACCP) programs. Samples
of the food product are periodically taken from the production area
and tested to ensure water activity values are within a specified
range for food quality and safety. Measurements can be made in as
little as five minutes, and are made regularly in most major food
For many years, researchers tried to equate bacterial growth potential
with water content. They found that the values were not universal, but
specific to each food product. W. J. Scott first established that
bacterial growth correlated with water activity, not water content, in
1953. It is firmly established that growth of bacteria is inhibited at
specific water activity values. U.S. Food and Drug Administration
(FDA) regulations for intermediate moisture foods are based on these
Lowering the water activity of a food product should not be seen as a
kill step. Studies in powdered milk show that viable cells can exist
at much lower water activity values, but that they never
grow. Over time, bacterial levels decline.
Water activity values are obtained by either a resistive electrolytic,
a capacitance or a dew point hygrometer.
Resistive electrolytic hygrometers
Resistive electrolytic hygrometers use a sensing element in the form
of a liquid electrolyte held in between of two small glass rods by
capillary force. The electrolyte changes resistance if it absorbs or
loses water vapor. The resistance is directly proportional to relative
air humidity, and also to water activity of the sample (once
vapor–liquid equilibrium is established). This relation can be
checked by either a verification or calibration using salt-water
mixtures, which provide a well-defined and reproducible air humidity
in the measurement chamber.
The sensor does not have any physically given hysteresis as it is
known from capacitance hygrometers and sensors, and does not require
regular cleaning as its surface is not the effectively sensing
element. Volatiles, in principle, influence the measurement
performance—especially those that dissociate in the electrolyte and
thereby change its resistance. Such influences can easily be avoided
by using chemical protection filters that absorb the volatile compound
before arriving at the sensor.
Capacitance hygrometers consist of two charged plates separated by a
polymer membrane dielectric. As the membrane adsorbs water, its
ability to hold a charge increases and the capacitance is measured.
This value is roughly proportional to the water activity as determined
by a sensor-specific calibration.
Capacitance hygrometers are not affected by most volatile chemicals
and can be much smaller than other alternative sensors. They do not
require cleaning, but are less accurate than dew point hygrometers
(+/- 0.015 aw). They should have regular calibration checks and can be
affected by residual water in the polymer membrane (hysteresis).
Dew point hygrometers
Red line shows saturation
The temperature at which dew forms on a clean surface is directly
related to the vapor pressure of the air.
Dew point hygrometers work
by placing a mirror over a closed sample chamber. The mirror is cooled
until the dew point temperature is measured by means of an optical
sensor. This temperature is then used to find the relative humidity of
the chamber using psychrometrics charts.
This method is theoretically the most accurate (+/- 0.003 aw) and
often the fastest. The sensor requires cleaning if debris accumulates
on the mirror.
With either method, vapor–liquid equilibrium must occur in the
sample chamber. This takes place over time or can be aided by the
addition of a fan in the chamber.
Thermal equilibrium must also take
place unless the sample temperature is measured.
Water activity is related to water content in a non-linear
relationship known as a moisture sorption isotherm curve. These
isotherms are substance- and temperature-specific. Isotherms can be
used to help predict product stability over time in different storage
Use in humidity control
There is net evaporation from a solution with a water activity greater
than the relative humidity of its surroundings. There is net
absorption of water by a solution with a water activity less than the
relative humidity of its surroundings. Therefore, in an enclosed
space, a solution can be used to regulate humidity.
Selected aw values
Shelf-stable cooked bacon
Saturated NaCl solution
Point at which cereal loses crunch
Typical indoor air
0.5 - 0.7
0.5 - 0.7
Clostridium botulinum E
Clostridium botulinum A, B
0.92, (0.90 in 30% glycerol)
No microbial proliferation
^ Rockland, L.B.; Beuchat, L.R. (1987). Water Activity:Theory and
Applications to Food (2nd ed.). New York: Marcel Dekker.
^ Young, Linda; Cauvain, Stanley P. (2000). Bakery food manufacture
and quality: water control and effects. Oxford: Blackwell Science.
^ Man, C.M.D.; Jones, Adrian A. (2000). Shelf Life Evaluation of
Foods. Springer. ISBN 0-834-21782-1.
^ Demchick PH (1984). "Taking control of chamber humidity". The
Science Teacher. 51 (7): 29‑31.
^ a b c d Marianski, 5
Bacon and Food Safety". United States Department of Agriculture
Food Safety and Inspection Service. 2013-10-29. Retrieved
^ a b c d e f g h i Barbosa-Canovas, 1
^ Shaw, 1
^ Ryser, Elliot T.; Elmer, Marth H. (2007). Listeria, Listeriosis and
Food Safety (3rd ed.). CRC Press. pp. 173–174.
^ a b Marianski, 7
Rockland, L.B.; Beuchat, L.R. (1987). Water Activity:Theory and
Applications to Food (2nd ed.). New York: Marcell Dekker.
Marianski, Stanley; Marianski, adam (2008). The Art of Making
Fermented Sausages. Denver, Colorado: Outskirts Press.
Shaw, Angela (2013). Salmonella: Create the most undesirable
environment. Ames, IA: Iowa State University.
Reineccius, Gary (1998). Sourcebook of Flavors. Berlin: Springer.
Fennema, O.R., ed. (1985). Food Chemistry (2nd ed.). New York: Marcell
Dekker, Inc. pp. 46–50.
Bell, L.N.; Labuza, T.P. (2000). Practical Aspects of Moisture
Sorption Isotherm Measurement and Use (2nd ed.). Egan, MN: AACC Egan
Ryser, Elliot T.; Elmer, Marth H. (2007). Listeria, Listeriosis and
Food Safety (3rd ed.). CRC Press. pp. 173–174.
Barbosa-Canovas, G.; Fontana, A.; Schmidt, S.; Labuza, T.P. (2007).
Water Activity in Foods: Fundamentals and Applications. FT Blackwell
Press. pp. Appendix D.
Why measure water activity?, Syntilab
How to measure water activity?, Syntilab