Water Activity

Low-moisture foods last longer than high-moisture foods. However, water activity is a far more reliable predictor of how long a food will last than moisture content. Some foods with the same moisture content can have dramatically different shelf-lives, due to differences in their water activities.

Free and bound water

Water in a food can be considered as either free or bound. This can be measured based on vapor pressure. Free water vaporises more easily and generates more vapor pressure in a container.

Bound water is tightly associated with components in the food — like proteins and sugars — which makes the water molecules less likely to vaporise. This results in a comparatively low vapor pressure.

Foods have widely varying water activities, due to differences in composition and structure.

Water activity

Water activity can then be defined based on vapor pressure ($V_p$) as:

$$a_w = \frac{V_p \ of \ food}{V_p \ of \ pure \ water}$$

with:

$$0 < a_w < 1$$

A water activity of 1 is equivalent to that of pure water.

Examples

Drying is effective at preserving food only when it significantly reduces water activity. Dried foods are not completely dehydrated but have very low levels of free water available to support undesirable changes, such as microbial growth and chemical reactions.

• Fresh meat has a water activity of 0.99 (close to water), while beef jerky has a value of <0.80.
• Liquid milk has a value of ~0.99, while milk powder has a value of <0.30.

Thresholds of safety and quality

In general, the rates of reactions and the extent of microbial growth increase with increasing water activity.

It is also well known that there are thresholds of water activity beyond which the rate of undesirable changes are significantly decreased.

There is limited bacterial growth when water activity is maintained below 0.9, while molds will not grow below 0.7. Notably, some chemical reactions can still proceed at very low water activity, such as browning (>0.2).

A hygienically-produced powder with a water activity of 0.3, is far below the level where the growth of microorganisms can be expected. Even though such a powder is microbially stable, it may still exhibit:

• Significant chemical changes
• Absorption of moisture from the air, which can increase water activity

Despite this, a well-manufactured powder should have a shelf-life of 12-24 months, especially if carefully packaged and stored.