Penetration rates into heterogeneous model systems and soluble food material
Wetting as the first step during food powder reconstitution strongly depends on the interactions of wetting liquid and particle surface, expressed by the contact angle. Due to the heterogeneous composition of food materials including hydrophilic and hydrophobic surfaces, also the wetting process is of heterogeneous nature. Furthermore, the solubility of food ingredients, such as sugars, increases the complexity of understanding and describing the wettability. In this study, we compared experimental water penetration into inert model powders containing hydrophilic and hydrophobic surfaces with theoretical penetration rates which were calculated by modifying a Washburn-based approach for the application to heterogeneous systems. A fair agreement was found when adjusting the porosity of the powder bed. In a second step, the inert powder was replaced by sucrose to introduce the effect of solubility. Since in presence of sucrose the model for inert systems can no longer describe experimentally observed penetration rates, we developed a model for water penetration into food powders considering solute concentration dependent liquid properties. The model is based on the solution of a coupled system of two differential equations representing capillary rise into a pore network and the mass transfer equation. Viscosity was found to have a major influence on the wetting kinetics. Our model predicts penetration rates during the first few seconds which are close to the experimental data. This allows us to conclude that it is suitable for predicting the first seconds of capillary penetration into a particulate system consisting of soluble sucrose.