It is possible, therefore, to plot the equilibrium vapour pressure against moisture content or to plot the relative humidity of the air in equilibrium with the food against moisture content of the food. Often, instead of the relative humidity, the water activity of the food surface is used. This is the ratio of the partial pressure of water in the food to the vapour pressure of water at the same temperature.
The equilibrium curves obtained vary with different types of foodstuffs and examples are shown in Fig. 7.4.