The moisture content of a product can be defined as the percentage weight of water in relation to the dry weight of the product.
Products in which moisture can be present can be classified in two categories:
Hygroscopic: Examples of hygroscopic materials are salts, vegetal fibers, most metal oxides, many polymers, etc
Non hygroscopic: Examples of non hygroscopic products are metal powders, glass granules, etc.
Regarding the moisture content of a product, we define Static Equilibrium as a set of conditions under which the product does not exchange any moisture with its environment.
Under conditions of Static Equilibrium, the moisture content of a hygroscopic product depends on the nature of the product and also on the two following factors: (a) the partial pressure of water vapor in the immediate environment of the product (b) the temperature of the product.
Water activity (Aw) or Equilibrium Related Humidity (% ERH)
Water activity reflects the active part of moisture content or the part which, under normal circumstances, can be exchanged between the product and its environment.
%ERH = 100 x Aw
Aw = P / P0
P = is the vapor pressure of water in the substance, and
P0 = is the vapor pressure of pure water at the same temperature
At room conditions, research data typically shows that water activity varies only by roughly 0.0005 to 0.005 aw (0.05 to 0.5 %RH) when temperature changes by 1°C.
· It provides useful information regarding the cohesion of tablets and pills, or the adherence of coatings.
· To prevent hygroscopic powders (powdered sugar, salt) from caking or turning into a solid block.
· Provide better information than the total moisture content regarding the micro-biological, chemical and enzymatic stability of perishable products such as foods and seeds