7th Intenatioanal Symposium on Agglomeration, ALBI, FRANCE, MAY 29-31, 2001International audienceThe hydration of semolina particles is an essential step in couscous processing which leads to binding between particles for the formation of agglomerates. Despite this importance, the hydration properties of such food products are rarely studied and in particular, durum wheat semolina has never been investigated. Here we present,a study of the hydration properties of durum wheat semolina by determination of water sorption isotherms, and other characterisation techniques to obtain a better understanding of hydration mechanisms. Equilibrium and dynamic sorption properties, have been measured as a function of relative humidity by means of a controlled atmosphere microbalance. It is found that durum wheat semolina presents a type II isotherm [F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids-Principles Methodology and Applications, Academic Press, 1999] indicative of multi-layer adsorption. The Guggenheim-Anderson-de Boer (GAB) model is used to describe the isotherm and obtain a better understanding of hydration mechanisms and liquid/solid interactions. The effects observed are related to physical properties of the semolina. In particular, the size of the semolina particles is found mainly to influence sorption kinetics: the finer the particles, the faster their sorption kinetics. Increasing temperature in the range 25-45 degreesC accelerates sorption kinetics. Furthermore, hydration causes no irreversible transformation of semolina components. Thus, absorption kinetics seem to be influenced by physical mechanisms, while the biochemical composition determines the amount of water sorbed. (C) 2003 Elsevier Science B.V. All rights reserved

Axe 5 Application intégrée de la connaissance, de l’information et des technologies permettant d’accroître la qualité et la sécurité des aliments Contact: fax: +33 5 63 493 025. E-mail address: jacques.fages@mines-albi.frIn 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