Abstract  

The time required by water activity measurements can be cut in half simply by ventilating the product sample during measurements. Several predictive methods can be used to further reduce the time to measure Aw. Regardless of the technique used for measuring water activity, these methods involve giving a result well before actual equilibrium takes place.

Conventional    Water Activity  Measurement  

Conventional water activity instruments use a small sealed container (or sample holder) in which a sample of the product is placed. Water activity can be measured as soon as the water vapor pressure in the air trapped in the sample holder has equilibrated with that of the product. Usually, this equilibration process is monitored by measuring the humidity of the air above the product. Because temperature is important, either the temperature of the product or the temperature of the air above the product, is also monitored. At equilibrium conditions, %RH = 100 x Aw and water activity can be directly measured with a relative humidity sensor or by any other means. If a chilled mirror is used to that purpose, both the value of the dew point and the value of temperature are used to compute Aw.

To conform with regulatory requirements (FDA), products should be measured at a constant 25° C. For other purposes, the temperature of measurement usually does not matter very much as long as it is kept within a narrow range such as 20 to 30° C and maintained as constant as possible.

The natural (or static) equilibration of most products typically requires from 45 to 60 minutes and can take as long as a couple of hours. Because humidity changes at an extremely slow pace towards the end of the process, determining when the measurement is truly ended can be tricky. An additional difficulty is the requirement to maintain quasi constant temperature conditions during the entire measurement.

Accelerated 
Water Activity  Measurement 

The substantial amount of time required by the static equilibration process explains why there is strong interest in faster methods to measure Aw.

Ventilation of the product sample during measurements reduces the amount of time required for equilibration. Ventilation facilitates the exchange of moisture between the sample, the air above the sample and the humidity sensor. Typically, ventilating the sample results in a 50% reduction of the time required for full equilibration. For example, chocolate syrup equilibrates in about 35 minutes as opposed to 70 minutes without ventilation. Measurements also tend to be more repeatable.

Further reduction in the measurement time requires giving a result well before the full equilibration of the sample. The following describes 3 different methods of doing this, with varying degrees of sophistication:

Method #1 

End the measurement when the product is very close to equilibrium but not quite at full equilibrium. To do this, the humidity rate of change (%RH / minute) is constantly monitored and compared to a fixed value or limit. The measurement is ended when the rate of change, in absolute terms, falls below the limit. If the limit for the humidity rate of change trend is set at a very small value, the error on the measurement is also small but the time to measure is not shortened by much. By properly choosing the limit for the humidity trend, most products can be measured in typically 10 to 20 minutes with a deviation from the actual Aw of typically 0.01 Aw or less.

Method #2 

End the measurements in a manner similar to method #1 with the additional step of applying a correction to the result. This correction is likely to be based on experience with many different products. Because a correction is applied to the result, the measurement can be interrupted earlier than with method #1 (possibly after 5 to 6 minutes) without loss of accuracy.

Method #3 

Use the data from the humidity probe to compute a projection of the value of Aw, based on a mathematical model of the equilibration process. The measurement is ended when successive projections are in agreement with one another. When this method is properly applied, measurements are typically obtained within 5 minutes and the deviation from the actual value of Aw is typically 0.005 Aw or better.

In summary, water activity can be measured in about half the usual time just by ventilating the product sample during measurements. Several methods are also available to further reduce the measurement time down to a few minutes. In one way or another, these methods anticipate the results of final equilibrium. 

Accelerated water activity measurement works well with most products. The time savings may come at the cost of some decrease in accuracy (or deviation from the actual value of Aw). With this in mind, R&D laboratories should still rely on conventional water activity measurement as a the ultimate reference.