Water Activity and Moisture Sorption Isotherm

Water in biological systems including foods exists with unhindered and hindered mobility, and referred to as free water and bound water, respectively. Different foods with same moisture content may not possess equal water activity. Depending on temperature and nature of food, moisture content maintains an equilibrium relationship with its water activity. This is commonly known as moisture sorption isotherm (MSI); more clearly, MSI portraits the profile of moisture content versus aw, at a constant temperature Reduction in water activity by drying or adding solutes or combination of both is a practical approach to moderate or slow down deterioration kinetics and preserve desirable qualities. The water activity of food can be measured based on colligative properties, psychrometry and isopiestic vapour transfer. Effect of temperature on sorption isotherm is important as foods are often exposed to a range of temperatures during storage, handling, transportation and processing.