What Is Water Activity?
Water activity is a measure for the energy status of the water in a system that specifies the relationship between the water vapor pressure of a product and the saturation pressure of pure water at the same temperature. It is stated in “aw” in the range 0…1 aw and is an important indicator for product quality in the industrial production of, for example, cannabis. The determination of water activity is also very important in the food, tobacco, pharmaceutical and cosmetics industries. Water activity should not be confused with the water content – the “chemically bound” water – of a product.
Water activity influences the following properties of a product:
- Microbiological stability
- Chemical stability
- Enzymatic stability
- Color, taste, and nutritional value
- Protein and vitamin content
- Stability of the composition
- Shelf life
- Storage and packaging
- Solubility and texture
What Is Water Content?
Water content, when referring to a solid material, is an expression of the percentage of the materials weight which is water (both in liquid and gaseous phase); usually referred to as ‘percent moisture content.’ This term is widely used in many areas of industry.
Water Activity or Water Content?
There is often confusion between water activity and water content measurements. In many industries, water content is used to control the amount of water present in a product for quantitative reasons. For example, if a product is sold by weight, controlling its water content may be useful in maintaining product quality, but can also impact profitability (more water = more profit). Water activity is more significant for qualitative considerations such as product stability, shelf life (e.g. microbiological & enzymatic stability, aroma retention), handling characteristics (e.g. agglomeration of powders), physical properties (e.g. dimensions of paper) and chemical stability (e.g. pharmaceuticals).
Water activity and water content can be related by a graph called a Sorption Isotherm (see diagram) so if the user has the ability to measure both parameters, the relationship can be defined and each parameter derived from the other (interpolation). In practice, the sorption isotherm may be impractical to use, because not only does the relationship between Aw and moisture content change with temperature of measurement, but also any variations in the material composition has a modifying effect.
This is especially the case with products of natural origin such as food and paper. The user should therefore decide which parameter of measurement is best suited to their products and processes. For quality control purposes, moisture content limits are easily converted to water activity limits by very simple comparative tests. Water activity measurement offers a non-destructive, easy-to-use measurement in a wide range of convenient configurations for both laboratory and on-site use.