In the presence of water and usually to a temperature of 50-60°C, starch sorbs water, which causes swelling of the grain (phenomenon that can be reversed during a desorption). The balance between the water content of the starch and the relative humidity of the environment at a given temperature is determined by sorption-desorption isotherms. These can be made from a predried product, a sorption, or else from a water-saturated product (desorption). The two curves are not superimposed, which shows a hysteresis phenomenon. Several mathematical models have been developed to describe empirically the sorption-desorption isotherms. It is indeed important to be able to predict water transfers between a product and its environment during drying or storage (Le Meste et al., 2002). One of the most used models is that of Guggenheim, Anderson, and de Boer (GAB model), which is based on the BET model (Brunauer, Emmet and Teller). The BET model is itself based on the principle of a multilayer adsorption, the first layer of monomolecular of adsorbate (water) is attached on the solid with a uniform energy (it corresponds to the saturation of the hydroxyls of starch), the following layers are fixed with an energy equal to the heat of condensation of the adsorbate.
The GAB model allows with three parameters an isothermal representation of Aw with water activity = 0.90:
(m/m1) = [(c-1)kAw]/[1+(c-1)kAw] + [kAw/(1-kAw’)]
where m corresponds to the water content and m1 is the water content of the first monolayer. It is an energy constant related to the energy of attachment of the first layer. The constant k represents the sorption heat of subsequent layers on the first.
The sorption capacity for the same water activity decreases with increasing temperature, which implies a water absorption of physical nature. Initially, the grain swells only slightly, the water molecules penetrate primarily into the interstitial spaces of the grain, and each water molecule establishes two hydrogen bonds with the hydroxyl of the starch. The binding energy of these first molecules secured is about 40 kJ/mol (Guilbot et al., 1960). When the water content increases, the binding energy decreases as the water becomes more mobile. The swelling of the grain is more important and finally the energy reaches a value close to the open water (for Aw ≥ 0.2), i.e., 18 kJ/mol. The amount of water that can be sorbed by the grain starch varies depending on the botanical origin of genotype and the degree of organization of the starch grain (Buleon et al., 1987). The B-type starch grain absorbs 29%-54% of water, the wheat starch (type A) and smooth peas (type C) absorb less water than wrinkled pea or potato starch (respectively 26.5%, 27.5%, 30%, and 54%).