Water Activity and Moisture Content are two very different parameters which are often confused and misused.
In a meeting today with a manufacturer of coffee capsules and pods, these differences were critical.
– Their whole bean supplier provided each batch with moisture content readings.
– However customers buying the coffee capsules and pods were asking for detailed water activity measurements for each batch. In addition for BRC Food Safety and shelf life validation of the final product, water activity was required.
Why are there these differences and can one measurement be used to determine both values?
Moisture content is probably the simplest value to understand. It is simply the quantity of water contained in a material. Traditionally measured through loss of weight on drying. This method raises some issues, depending on the drying temperature you may not remove all water or may also remove other non water compounds.
More modern methods resolve these issues and use infra red absorption. This way the water content is directly measured, the method is non-destructive and far quicker.
Moisture content is typically given as a percentage in terms of weight.
Water Activity (aW) is a measure of the free water in a sample, and ranges between 0…1. Pure water would have an aW of 1.0. As water activity measures the ‘free’ or ‘active’ water in a sample it is more relevant to growth of organisms, chemical processes, enzyme activity and physical parameters like size and clumping as these are only effected by the water that can be chemically interacted with. Interestingly Water Activity is related to Moisture Content but it is product and temperature specific.
aW… is more relevant to growth of organisms, chemical processes, enzyme activity and physical parameters
What is free water? Water can be bound in materials in two broad ways.
1. Chemically Bound Water. Is bound so tightly that it cannot be utilised by bacteria, enzymes etc. It can be removed through high temperature heating.
2. Free Water. Is bound through weak bonds, structural diffusion, capillary condensation and surface binding. It can be utilised by bacteria and can exchange with the environment, it is also removed through heating.
For instance a whole grape would have the same moisture content as two halves of the same grape. However the aW would be far lower in a whole grape as much of the water is bound inside the grape skin and only made free when the grape is cut in half!
Work in progress… More to follow soon!