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 (aw) is one of the most critical factors in determining quality and safety of the goods you consume every day. Water activity affects the shelf life, safety, texture, flavor, and smell of foods. It is also important to the stability of pharmaceuticals and cosmetics. While temperature, pH and several other factors can influence if and how fast organisms will grow in a product, water activity may be the most important factor in controlling spoilage. Most bacteria, for example, do not grow at water activities below 0.91, and most molds cease to grow at water activities below 0.80. By measuring water activity, it is possible to predict which microorganisms will and will not be potential sources of spoilage. Water activity--not water content--determines the lower limit of available water for microbial growth. In addition to influencing microbial spoilage, water activity can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact their color, taste, and aroma. It can also significantly impact the potency and consistency of pharmaceuticals.

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 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.

Definition of aw:

aw = p/po = ERH (%) / 100
where p is the vapor pressure of water in the substance, and p0 is the vapor pressure of pure water at the same temperature.

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.

Examples of Foods

Substance aw
Distilled Water 1
Tap water 0.99
Raw meat 0.97 - 0.99
Milk 0.97
Juice 0.97
Cooked bacon < 0.85
Saturated NaCI solution 0.75
Point at which cereal looses crunch 0.65
Typical indoor air 0.5 - 0.7
Honey 0.5 - 0.7
Dried fruit 0.5 - 0.6
Water activity instruments measure the amount of free (sometimes referred to as unbound or active) water present in the sample.

Sigma offers a portable aw set that includes model 650, a multi-parameter measuring instrument / data logger (also accepts probes for temperature, humidity, velocity, pressure, CO, CO2), a pressure-tight precision humidity probe with certificate, a measurement chamber and 5 sample bowls (plastic).

Model 650, multi-function measuring instrument, incl. data logging, VAC module (volume flow measurement with error calculation), battery, Li cell and calibration protocol.
model 650 is the multi-function measuring instrument with which many specialists and professionals in industry and the trades simplify their measuring tasks. Model 650 includes functions for Temperature reference measurements, i.e. high precision: accuracy .1°C (1/100 resolution), and Humidity/Pressure reference measurements. Using model 650 you can perform all types of velocity measurements (thermal probes, vane and Pitot tube) regardless of whether in a duct, duct outlet or suction aperture. Checks indoor air quality or the air exchange number using CO2 probe - Checks ventilation in underground garages using CO probe- Checks transducers using the mV/mA measurement input- Logs data with date, time and location over long periods using the logger function in the measuring instrument. All probes and printer, except the aw probe set are optional.

pressure-tight precision humidity probe with certificate, measurement chamber and 5 sample bowls (plastic)
(Determines water activity)
Meas. range -20 to +70 °C
Meas. Range 0 to +100 %RH
Meas. Range 0 to +1 aW

1. Checks product quality in pharmaceutical sector
Advantage: Results are traceable to national standards

2. The trend display in model 650 automatically indicates when the equilibrium moisture has been reached and then the measurement is complete. Constant monitoring is therefore not required.

3 +4. Different measurement samples can be read in via barcode. Advantage: Additional information such as min./max. values, mixing ratios…are saved in the barcode.

5. Calibration on location with control and adjustment set, with DKD calibration certificate if required
Advantage: This provides additional assurance.