Measuring Water Activity for Product Quality
Table of Contents
Microbial Growth
Chemical/Biochemical Reactivity
Physical Properties
Free Water vs. Bound Water
Chilled Mirror Dewpoint Theory
Speed and Accuracy
Water Activity - Accepted and Approved
THE IMPORTANCE OF THE CONCEPT OF WATER ACTIVITY cannot be over- emphasized. Water activity is a measure of the energy status of the water in a system. More importantly, the usefulness of water activity in relation to microbial growth, chemical reactivity, and stability over water content has been shown. Click here to see Figure 1.
Microbial Growth
Physical Properties
Water activity is an important parameter in controlling water migration of multicomponent products. Some foods contain components at different water activity levels, such as cream filled snack cakes or cereals with dried fruits. Undesirable textural changes are often the result of moisture migration in multicomponent foods. Moisture will migrate from the region of high aw to the region of lower aw, but the rate of migration depends on many factors. For example, moisture migrating from the higher aw dried fruit into the lower aw cereal causes the fruit to become hard and dry while the cereal becomes soggy.
Free Water vs. Bound Water
Chilled Mirror Dewpoint Theory
Water Activity is a critical factor that determines the shelf life of products. Water activity, not water content, determines the lower limit of available water for microbial growth. While temperature, pH, and several other factors can influence whether an organism will grow in a product and the rate at which it will grow, water activity is often the most important factor. The lowest aw at which the vast majority of spoilage bacteria will grow is about 0.90. The aw for molds and yeast growth is about 0.61 with the lower limit for growth of mycotoxigenic molds at 0.78 aw.
In addition to predicting the rates of various chemical and enzymatic reactions, water activity affects the textural properties of foods. Foods with high aw have a texture that is described as moist, juicy, tender and chewy. When the water activity of these products is lowered, undesirable textural attributes such as hardness, dryness, staleness, and toughness are observed. Low aw foods normally have texture attributes described as crisp and crunchy, while at higher aw the texture changes to soggy. Also, water activity affects the flow, caking and clumping properties of powders and granules.
Water activity instruments measure the energy status (sometimes referred to as free, unbound or active water) of the water present in a sample. A portion of the total water content present in sample is strongly bound to specific sites on the components in the sample. These sites may include the hydroxl groups of polysaccharides, the carbonyl and amino groups of proteins, and other polar sites. Water is held by hydrogen, ion-dipole, and other strong chemical bonds. Additionally, some water is less tightly bound, but is still not available (as in a solvent for water-soluble components). Many preservation processes attempt to eliminate spoilage by lowering the availability of water to microorganisms. Reducing the aw also minimizes other undesirable chemical changes occurring during storage. The processes used to reduce the aw include techniques like concentration, dehydration, humecants, freezing, and freeze drying. These techniques control spoilage by making water unavailable to microorganisms. Because water is present in varying energy states, analytical methods that attempt to measure total moisture in samples don't always agree. Water activity tells the real story.
In a chilled mirror dewpoint system, water activity is measured by equilibrating the liquid phase water in the sample with the vapor phase water in the headspace of a closed chamber and measuring the relative humidity of the headspace. In the AquaLab, a sample is placed in a sample cup, which is sealed against a sensor block. Inside the sensor block is a fan, a dewpoint sensor, a temperature sensor, and an infrared thermometer. The dewpoint sensor measures the dewpoint temperature of the air, and the infrared thermometer measures the sample temperature. From these measurements the relative humidity of the headspace is computed as the ratio of dewpoint temperature saturation vapor pressure to saturation vapor pressure at the sample temperature. When the water activity of the sample and the relative humidity of the air are in equilibrium, the measurement of the headspace humidity gives the water activity of the sample. The purpose of the fan is to speed equilibrium and to control the boundary layer conductance of the dew point sensor.
