Impact of Instant Controlled Pressure Drop Treatment on Moisture Adsorption Isotherm of Cork Granules
Résumé
Some special thermal and mechanical treatments may completely modify the composition, and the structure of organic material changes its interaction with water. In the case of the instant controlled pressure drop (DIC) technique, the main modification, which is structure expansion and high porosity, allows the product to generally reduce moisture activity. In the special case of cork granules, the impact of DIC in terms of porosity is completely absent and expansion is linked to a tear effect obtained from only the large size granules. In order to characterize this type of treatment, we considered natural and DIC-treated cork granules and determined moisture adsorption isotherms at three different temperatures (25, 40, and 60°C) and different water activity levels ranging from 0.05 to 0.9 using the static gravimetric method. We used three samples of the same cork variety with 0.5-1 mm, 2-4 mm, and 4-6 mm as granule diameter. The adsorption isotherms of both natural untreated and DIC-treated cork whatever granules display a sigmoid form type II isotherm, with equilibrium moisture contents at constant water activity decreasing when temperature increases. The impact of DIC treatment depends of cork shape; the larger the shape, the lower the water activity for the same water content. Such a result is linked to the modification of structure. Thus, for the smallest shape, DIC implying only low thermal effect without any structure modification allows cork to lightly increase its water activity compared to the untreated samples. With higher shape cork granules, as DIC treatment induces a tear effect and some expansion, activity of water is lower for the same water content. For describing the experimental data of adsorption isotherms, we used GAB, BET, and Henderson models. We found the GAB to be the most suitable model with predicted values higher than those obtained using the BET model. We note that monolayer moisture content values increase in the case of big shape cork granules after having been treated by DIC.