Everything about Water Activity totally explained
Water activity or a
w is a measurement of the energy status of the water in a system. It is defined as the
vapor pressure of water divided by that of pure water at the same
temperature; therefore, pure
distilled water has a water activity of exactly one.
There are several factors that control water activity in a system. Colligative effects of dissolved species (for example salt or sugar) interact with water through dipole-dipole, ionic, and hydrogen bonds. Capillary effect where the vapor pressure of water above a curved liquid meniscus is less than that of pure water because of changes in the hydrogen bonding between water molecules. Surface interactions in which water interacts directly with chemical groups on undissolved ingredients (for example starches and proteins) through dipole-dipole forces, ionic bonds (H3O+ or OH-), van der Waals forces (hydrophobic bonds), and hydrogen bonds. It is a combination of these three factors in a food product that reduces the energy of the water and thus reduces the relative humidity as compared to pure water. These factors can be grouped under two broad categories osmotic and matric effects.
Due to varying degrees of osmotic and matric interactions, water activity describes the continuum of energy states of the water in a system. The water appears “bound” by forces to varying degrees. This is a continuum of energy states rather than a static “boundness”. Water activity is sometimes defined as “free”, “bound”, or “available water” in a system. Although these terms are easier to conceptualize, they fail to adequately define all aspects of the concept of water activity.
Water activity is temperature dependent. Temperature changes water activity due to changes in water binding, dissociation of water, solubility of solutes in water, or the state of the matrix. Although solubility of solutes can be a controlling factor, control is usually from the state of the matrix. Since the state of the matrix (glassy vs. rubbery state) is dependent on temperature, one shouldn't be surprised that temperature affects the water activity of the food. The effect of temperature on the water activity of a food is product specific. Some products increase water activity with increasing temperature, others decrease aw with increasing temperature, while most high moisture foods have negligible change with temperature. One can therefore not predict even the direction of the change of water activity with temperature, since it depends on how temperature affects the factors that control water activity in the food.
As a potential energy measurement it's a driving force for water movement from regions of high water activity to regions of low water activity. 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. Other examples of this dynamic property of water activity are; moisture migration in multidomain foods (for example cracker-cheese sandwich), the movement of water from soil to the leaves of plants, and cell turgor pressure. Since microbial cells are high concentrations of solute surrounded by semi-permeable membranes, the osmotic effect on the free energy of the water is important for determining microbial water relations and therefore their growth rates.
Higher a
w substances tend to support more
microorganisms.
Bacteria usually require at least 0.91, and
fungi at least 0.7. See
fermentation.
Formulae
Definition of a
w:
»
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:
»
where l
w is the
activity coefficient of water and x
w is the mole fraction of water in the aqueous fraction.
Relative humidity:
»
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.
In addition, water activity helps limit or slow certain undesirable reactions, such as non-enzymatic browning, fat oxidation, vitamin degradation, enzymatic reactions, protein denaturation, starch gelatinization and starch retrogradation. This too maintains product quality and extends shelf life.
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 that 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. WJ 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.
FDA regulations for
Intermediate Moisture Foods are based on these values.
Lowering the water activity of a food product shouldn't be seen as a
kill step. Studies in
powdered milk show that viable cells can exist at much lower water activity values but that that'll 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 don't require cleaning, but are less accurate than
dew point hygrometers (+/- .015 a
w). They require regular calibration and can be affected by residual water in the polymer membrane.
Dew Point Hygrometers
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 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.
Selected aw values
Example Foods
| Substance |
aw |
| Distilled Water |
1 |
| Tap water |
0.99 |
| Raw meats |
0.97 - 0.99 |
| Milk |
0.97 |
| Juice |
0.97 |
| Cooked bacon |
< 0.85 |
| Saturated NaCl solution |
0.75 |
| Point at which cereal loses crunch |
0.65 |
| Typical indoor air |
0.5 - 0.7 |
| Honey |
0.5 - 0.7 |
| Dried fruit |
0.5 - 0.6 |
aw Values of Microorganism Inhibition
Further Information
Get more info on 'Water Activity'.
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