In the manufacturing process of pasta or
couscous, durum wheat
semolina agglomeration comprises successive steps, in particular a wetting and hydration stage. In this step, interactions between water molecules and
semolina grains contribute to the agglomeration properties. Hydration properties of
semolina have been recognised to play an important role in agglomeration.
The hydration properties of semolina have been characterised using various methods which can be divided into 2 groups according to the water addition: liquid or vapour. Several parameters can be used to describe hydration properties and hydration mechanisms, such as: water sorption capacity, hydration kinetics, diffusion of water within the solid (swelling, dissolution, solid-water affinity and thermodynamic properties).
In this study, sorption isotherms and diffusion coefficients have been determined by gravimetric method in specific conditions. The immersion enthalpy of the semolina and its main sub-components, starch and gluten have been measured by mixing micro-calorimetry. Hydration thermodynamic properties, Gibbs free energy, enthalpy and entropy, were calculated from these experimental data. It was found that hydration by vapour or liquid water is an exothermic reaction.
The determined diffusion coefficient was used to estimate the stabilisation time of water within the solid. Being high at low relative humidity, the diffusion coefficient decreases during progressive saturation of solid sites. Sorption energy and solid-water affinity yield information about the type of interaction between water molecules and solid sites. The drop in the sorption energy during the increase in water content can be attributed to a decrease in solid-water interactions. The hydration energy of durum wheat semolina in vapour or liquid water was found to be low. This corresponds to the energy input needed to generate contact between water and grain surface and for mixture homogenisation.
