Evaluate the Moisture Requirement - The Concept of Water Activity

We already know that moisture is one of the important factors for the survival of living species. As such, it has a great influence on them, including microorganisms like bacteria. Water accounts for 80-90% of the total weight of cells. The water requirement of bacteria varies fiom species to species. Although water is required by bacteria, the growth largely depends upon the available water, which can be utilized for the growth of microorganisms. This is expressed as water. activity which is defined as the percent equilibrium relative humidity (%ERH) divided by 100. ERH is an expression of the amount of free water vapour present on a product, both at its surface and within its structure. Numerically, ERH of a food divided by 100 equals "Water Activity". This can be represented by the following equation:

water Activity (a_w) = Equilibrium Relative Humidity/100