Water binding, desorption isotherm

A complication arises from one of the methods of measuring sorption isotherms for food. Food that has previously been dried and then is rehydrated will have a different sorption isotherm (adsorption isotherm) from that which is in the process of drying (desorption isotherm). This difference is due to a change in water-binding capacity in foods that have been previously dried. 

Fig. 1. Analysis of the water sorption isotherms, (a) Typical shapes of the desorption and adsorption curves of plant tissues. The difference between these two curves shows hysteresis, indicating the irreversibility of water sorption in the tissues during dehydration and rehydration. The sigmoid shape of sorption curves is presumably due to the existence of three types of water-binding sites in tissues (strong (I), weak (II) and multilayer molecular sorption sites (III)), (b) Differential enthalpy (AH), free energy (AG) and entropy (AS) of hydration. Data from Sun (2002).

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