Water activity

Water activity or a_w was developed to account for the intensity with which water associates with various non-aqueous constituents and solids. Simply stated, it is a measure of the energy status of the water in a system. It is defined as the vapor pressure of a liquid divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one.

As the temperature increases, a_w typically increases, except in some products with crystalline salt or sugar.

Higher a_w substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See fermentation.

Water migrates from areas of high a_w to areas of low a_w. For example, if honey (a_w ≈ 0.6) is exposed to humid air (a_w ≈ 0.7) the honey will absorb water from the air.

Formulae

Definition of a_w:

$a_{w}\equiv p/p_{0}$

where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.

Alternate definition:

$a_{w}\equiv l_{w}x_{w}$

where l_w is the activity coefficient of water and x_w is the mole fraction of water in the aqueous fraction.

Relative humidity:

The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH).

$\mathrm {ERH} =a_{w}\times 100\%$

Estimated mold-free shelf life in days at 21° C:

$\mathrm {MFSL} =10^{7.91-8.1a_{w}}$

Uses for water activity

Water activity is an important consideration for food product design and food safety.

Food product design

Food designers use water activity to formulate products that are shelf stable. 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 will migrate to the bran flakes over time, resulting in hard raisins and soggy bran flakes. Food formulators use water activity to predict how much moisture migration will affect their product.

Food safety

Water activity is used in many cases as a critical control point for 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 production facilities.

For many years, researchers tried to equate bacterial growth potential with moisture content. They found that the values were not universal, but specific to each food product. W J Scott in 1953 first established that it was water activity, not water content that correlated with bacterial growth. 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 values.

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 will never grow. Over time, bacterial levels will decline.

Water activity measurement

Water activity values are obtained by either a capacitance or a dew point hygrometer.

Capacitance hygrometers

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 (+/- .015 a_w). 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 psychrometric charts.

This method is theoretically the most accurate (+/- .003 a_w) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.

Equilibration

With either method, vapor equilibrium must occur in the sample chamber. This will take 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 and moisture content

Water activity is related to moisture 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 conditions.

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 a_w values

Example foods

Substance	aw
Distilled Water	1
Tap water	0.99
Raw meats	0.99
Milk	0.97
Juice	0.97
Salami	.87
Cooked bacon	< 0.85
Saturated NaCl solution	0.75
Point at which cereal loses crunch	0.65
Dried fruits	0.60
Typical indoor air	0.5 - 0.7
Honey	0.5 - 0.7
Dried fruit	0.5 - 0.6

a_w values of microorganism inhibition

Microorganism Inhibited	aw
Clostridium botulinum A, B	.97
Clostridium botulinum E	.97
Pseudomonas fluorescens	.97
Clostridium perfringens	.95
Escherichia coli	.95
Salmonella	.95
Vibrio cholerae	.95
Bacillus cereus	.93
Listeria monocytogenes	.92
Bacillus subtilis	.91
Staphylococcus aureus	.86
Most molds	.80
No microbial proliferation	.50

Reference

• Marianski, Stanley (2008). The Art of Making Fermented Sausages. Denver, Colorado: Outskirts Press. ISBN 9781432732578. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Reineccius, Gary (1998). Sourcebook of Flavors. Berlin: Springer. ISBN 0-8342-1307-9.
• Fennema, O.R., Ed. (1985). Food Chemistry - Second Edition, Revised and Expanded. New York: Marcell Dekker, Inc. pp. 46-50.
• Bell, L.N., and Labuza, T.P. 2000. Practical Aspects of Moisture Sorption Isotherm Measurement and Use. 2nd Edition AACC Egan Press, Egan, MN

External links

• Chemistry of water activity
• Water Activity Measurement
• Water Activity Instruments
• Water Activity Meters