Food properties handbook

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Water activity is defined as the ratio of the vapour pressure of water in a system and the vapour pressure of pure water at the same temperature or the equilibrium relative humidity of the air surrounding the system at the same temperature Thus water activity can be expressed as follows (1 1) where a is water activity (fraction) at T C, (P ), P are vapour pressures of water in the system and pure water at, T C (Pa), ERH is equilibrium relative hiimidity of air at T C