Water activity is an important factor in food shelf life and it is an indicator of the mobility of water from humid area to dry area.
There are two types of water in food.
One is the water combined with substance like protein and carbohydrate.
The other that is easy to move and evaporate is called 'Free' water. Water activity(Aw) is measured one hundredth of the relative humidity of the food surface.
Microbe can use only free water to grow and proliferate so to control shelf life, it is important to check how much free water is there in food.
Also this control step is an integral part of HACCP.
For example, jam has a high moisture content but the water activity is not so high because most the moisture is combined with the sugar. On the other hand, white bread does not use a lot of sugar or salt so the value of water activity increases considerably to around 0.96.
[About water activity (AW)]
Ratio of food surface water vapor pressure (P) and pure water vapor pressure (P0).[Water activity Aw = P/P0: numerical value 0 - 1].
When enclosing food in airtight containers, equilibrium relative humidity [E.R.H. % = Aw × 100] in the container.
In addition to water quantity (water content) as a physical property of food, it is used for product design, quality control, etc.
The water contained in food is not necessarily always pure water , salts and saccharide in food composition are dissolved into the water, and protein is hydrated. Water activity is a reflection of the status of free water which is water with high fluidity that is not fixed in food and the relationship between the growthof microbes is clear.
<Lowest Water Activity for growth>
*Numerical values in brackets are: halophile, glucose tolerance, xerophilic. There is a trend of higher water activity with the increase in water content but this is not a uniform correlation.