Moisture Content and Water Activity - What Are They?
Many accessing our web site ask questions such as:
“How does water content affect water activity?”
"How is water activity different from water content?”
”Can I convert from moisture to water activity?”
I previously commented on water activity and now hope to clarify the differences between water activity and moisture content. Given that both of these measurements deal with water connected to a material, we must first understand of water content in a product.
Put simply, moisture or water content is the amount of water a substance contains. Water can be present in many ways:
Absorption as a chemical reaction
Binding hydrate absorption and formation
Product structure molecular diffusion
Surface energy binding
Capillary condensation (which forms a solution)
Simple surface water
I trust that this demonstrates that product moisture is a complex concept.
With that understanding, here are a couple of simple definitions.
Moisture content is how much water there is in a given material.
Water activity is how difficult it is to remove the water.
There are two basic direct techniques to measure moisture. Loss-on drying drives off the moisture by applying heat energy. Karl Fischer deconstructs the chemistry to free the moisture. Additionally, there are many indirect methods that must refer to the direct measurements.
Water activity is measured by letting a product sample reach equilibrium relative humidity in a closed temperature-controlled chamber. This allows water that is naturally released at that temperature to form a vapor and stabilize. When the resulting vapor pressure stops changing, no further moisture releases from the sample.
Water activity is particularly important regarding packaging. A high water-activity item will probably emit moisture when placed in a sealed container. This moisture can then react with bacteria, mold and other pathogens to destroy product characteristics as well as cause disease. Thus, the need to know water activity levels is apparent.
Moisture content, by contrast, is important because it influences physical/mechanical properties, yield, texture -- and often the selling price of a material. It is also a significant factor in controlling the repeatability of a production process.
How do we relate water activity to moisture content? This relationship varies between materials and changes with temperature.
The moisture-to-water-activity relationship can developed by testing moisture content and measuring water activity at many different moisture levels and temperatures. Given that both moisture content and water activity need to be developed, the process is often tedious and time-consuming. There are, however, instruments that can develop these relationships automatically over several days. Each water activity measurement takes 5 to 20 minutes -- sometimes longer. Further, these correlations are different when a material is being dried vs. wetted.
Moisture-sorption isotherms are the relationships of moisture content and water activity at a given constant temperature. Although an increase in water activity is almost always accompanied by a rise in water content, these isotherms are non-linear. Therefore, easy rule-of-thumb conversions are not valid.
Correlations between moisture content and water activity can be developed through experimental measurement collection for individual products. The resulting moisture sorption isotherms can then be used to predict water activity and moisture content for a given product. There are no known alternatives to the tedious data development process.
I hope to have shed some light on the subject of moisture content -- what they are and how to convert between them.
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Until next time I remain a puzzled,
P.S. Please share this with any associates who might find it useful.