The answer to the question, “How Long Does a Water Activity Test Take?” is, as with most testing questions, “it depends.” Some water activity meters promise a test completed in a very short time; some in as little as five minutes. This time is an arbitrary selection that may or may not produce a satisfactory result.How is Water Activity Measured?
Water activity is the ratio of the vapor pressure of water in a material or substance to the vapor pressure of pure water. Water activity measurements are determined from a calculation of relative humidity. Relative humidity is the percentage of water in the air (vapor pressure) compared with the total amount of water that the air could hold (saturation vapor pressure) at a given temperature.
A water activity test works by placing a sample in a sealed measuring container. When the vapor pressure of the water in the substance and the water in the air reaches equilibrium, the relative humidity of the air surrounding the sample is equal to the water activity of the sample. The process of reaching equilibrium takes time. Just how much time it takes varies from product to product.
During a water activity test, if you plot the changes in relative humidity over time, you get a curve that shows rapid changes at the beginning followed by a continually slower rate of change until you eventually reach an end point. (See the graph on the right.)
For the instruments that set a fixed time to record the water activity reading, that reading could end up taking place during the rapid change period (a potentially distorted prediction of the end point) or, depending on the product’s water release characteristics, the reading could be where the change is very small – a point that would accurately predict the end point.
Some instruments continuously record the relative humidity. With these, it is possible to detect when the rate of change slows down. Most of these instruments can be programmed to automatically select the time for a reading at a point on the flatter part of the Time/Water Activity curve, a place that gives a good representation of the end point.
The state of equilibrium is why test time is so important to get right. It can take quite some time to attain “true” equilibrium (no more changes in moisture over any period of time). In the end, it may not be necessary for every product to attain true equilibrium in order to effectively estimate the end point water activity. Determining the optimum test time based on analysis of the Time/Water Activity curve will provide assurance of a better end point than an arbitrary selection of test end time.
Why is Accuracy Important?
Products with a higher water activity are prone to degradation, and the development of mold and bacteria. Accuracy is very important when it comes to these products. At 0.59aW a product could be considered safe from the potential growth of bacteria and mold. At 0.61aW, the product could be vulnerable to the development of yeasts and molds. In this case, the difference is only 0.02aW.
Reading test results too early in the process can result in only measuring the effect of free water. The distinction between free and bound water isn’t always black and white. Sometimes, water can be loosely bound, and over time become free water. Running the test long enough to get to equilibrium assures that both free and loosely bound water molecules get measured.
A good solution to the issue of adequate test time is to determine when changes in relative humidity start to slow. If, at this point, the water activity is not near “critical levels”, it may be safe to stop the test. However, if the reading is near or above the critical point, more time should be taken so that the reading can have greater accuracy.
Some instruments provide continuous readings of water activity. Many of these instruments provide for automatic shut down when the rate of change in water activity reaches a pre-determined level. For example, you can set a test to stop when, over a period of one minute, the water activity reading changes less than 0.02aw.
Water activity can provide critical information about a product. Accuracy and precision are essential. The instruments with the capability to stop a test when the Time/Water Activity curve flattens out add a dimension of safety and repeatability to water activity testing.
We hope that this explanation of the importance of water activity test times is clear and useful. If it is, please share it with associates who deal with water activity issues.
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