A guide to water activity in moisture-sensitive products
Water activity is one of those measurement techniques that are used in a large number of industries, but which are little understood except by a relatively small number of experts. Although it’s a relatively complex topic, water activity is nonetheless a critically important determinant of product quality and shelf-life in the food industry, while in the pharmaceutical and cosmetics sectors it is essential to ensure a solid dosage form with optimal physical and chemical characteristics. As such, water activity deserves far wider understanding.
What do we mean by water activity?
The term water activity is often abbreviated to ‘aw’. In simple terms, it is a measurement of the water available within a product or ingredient that might, for example, either lead to microbial growth or, through migration, affect the quality of other ingredients. A classic example is muesli, containing raisins, seeds and bran flakes. Raisins normally contain more moisture than bran flakes, so that over time moisture from the raisins will migrate to the bran flakes; depending on the time involved, the result is likely to be hard, dried-out raisins and soggy bran flakes!
It's important not to confuse water activity with the amount of water in a product – the moisture content. The latter determines the physical properties of the product and can generally be characterised by the strength of molecular bonds between water and other key ingredient molecules; by definition, this type of ‘bound’ water is not normally available to microorganisms to support their growth.
By comparison, water activity is a measure of the quantity of water that can be considered ‘free’ or ‘unbound’, and which can cause the problems described above. Although microbial growth can be driven by a number of factors including temperature, oxygen content and pH it is the availability of free water that is the most important determinant in prompting microbial growth. In pharmaceutical applications in particular it is water activity that offers a superior method of calculating microbiological growth.
Water activity is calculated by measuring the vapour pressure produced by the free moisture in the product and comparing it with the vapour pressure of distilled water under identical conditions; vapour pressure is the force exerted by molecules of water in a gaseous form at the surface of a product and indicates how easily water will evaporate from the product.
The formula for calculating water activity is: aw = p/ps.
Where p is the water vapour pressure above the surface of the product and ps is the water vapour pressure above the surface of a body of pure water.
Both are always measured at the actual temperature of the product, as temperature can affect the way in which water interacts with the product. These results are presented on scale of 0-1, where 0 shows that there is no available free water, while 1 indicates that all of the water in the product is available.
To put this into perspective, most fruit and vegetables, meat and dairy products have a value of 0.99 aw; this is relatively high and explains why the growth of bacteria and mould can occur quickly. Conversely, products such as honey and roasted coffee beans have water activity values of around 0.5 aw.
The importance of aw
As mentioned above, water activity is an important parameter in determining the quality and shelf-life of food and other moisture sensitive products such as pharmaceuticals, cosmetics, seeds and advanced industrial coatings.
Using water activity values for different ingredients, as in the example of muesli, and understanding that water will normally migrate from a substance with a high aw value to one with a low aw value, enables both product formulation and control to be improved. Additionally, water activity is often used as a key measurement parameter for Hazard Analysis and Critical Control Point (HACCP) regimes and as an essential measurement for accurate process control; for example, enabling production staff to determine exactly how long to cook or dry a product, or to control oven temperatures and the speed of conveyor systems.
In the pharmaceutical sector, water activity measurements can be used in product formulation, manufacturing and packaging. In particular, water activity provides an effective means of predicting the stability of proteins, enzymes and biopharmaceutical components, and of managing the compatibility of hygroscopic excipients, such as starch and cellulose, in moisture-sensitive products.
Measuring water activity
Water activity can be measured using a range of advanced laboratory, in-line and portable spot-measurement devices. In each case, these are simple to use and are capable of extremely high levels of accuracy, with consistent results being produced quickly in a form that is easy to understand by scientists and production staff alike.
Rotronic has been designing and manufacturing water activity measurement instruments for many years. As a result, we’ve built up a wealth of knowledge and experience across all areas of application. Our latest product range offers a wide choice, from entry-level devices to sophisticated laboratory analysers, all backed by optional accessories, software and support services.
To learn more about our latest water activity products:
Our Water Activity ProductsContact us for support on your aw measurements
Interested to read more theory on water activity? Check the following blog series:
Aw Measurement - Handling SeedsHow to carry out a Water Activity (aw) Measurement
Importance of Water activity Measurement
Related Products
Temperature-Stabilized Water Activity Analyzer - Rotronic AW-Therm
Rotronic Accessories for Water Activity Measurement
Bluetooth Water Activity Measurement Head - Rotronic AwEasy
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