When to use water activity versus water content: 3 examples of moisture measurement.
THE RIGHT ANSWER TO THE QUESTION "which is the best moisture measurement?" comes with an understanding of the processes.
A simple example where water content is clearly the measurement of choice is when one wishes to buy or sell a product on a dry weight or standard moisture basis. The water content of the sample must be measured to compute the amount of water and dry matter present.
Now consider a situation where the measurement of moisture is used to determine whether a product is shelf stable, will store safely without spoilage, or will support microbial production of toxins.
The three vertical lines in Fig. 1, at water activities of 0.60, 0.85, and 0.94 represent limits below which, respectively, no spoilage occurs, products are defined as shelf stable, and toxin production by botulism is stopped.
Figure 1. Isotherms for corn and mustard.
From the figure it is clear that corn can be stored without spoilage if its water content is below 12.5%, and mustard is similarly safe if its water content is below 7.5%. In fact, these are values that have been used for years to determine whether these products are safe to store.
But the water content below which spoilage fungi are inhibited is different for each different product. This is because it is the water activity, not the water content that determines whether these organisms can carry on their physiological functions. When the water activity is below 0.6, water is so tightly bound by the product that it becomes unavailable to even the most xerophytic fungi.
Water content is irrelevant in this application because the microbes respond to the availability, not the amount of water. Thus, even though it is common practice to measure water content for this application, water activity is a much better measure. Water activity is not product specific, and is fundamentally related to the process we are interested in.
Another example where water content is almost useless is in predicting moisture migration in multicomponent products. Moisture moves from regions of high energy to regions of lower energy. Water content has nothing to do with the direction of flow. Flow is completely determined by gradients, or differences in water activity.
If we were to mix mustard at 15% moisture with corn at 20% moisture, which way would the water move? Referring to Figure 1 we see that the water activity of corn at 20% moisture is 0.91. The water activity of mustard at 15% moisture is 0.93. The moisture would therefore move from the mustard to the corn, even though the corn has a substantially higher water content.
This problem is best solved using water activity measurement because the fundamental processes are controlled by energy, not by amount of water.
In general, water activity is the right measurement when microbial processes, including food spoilage, are of concern. Also, moisture migration, physical properties and caking are best addressed using water activity measurements.