Detailed knowledge of thermal, hydric and mechanical phenomena within a product during drying helps decrease energy costs by reducing the exposure time and improve product quality by respecting the severe standards requirements. In this context, we simulate the spatiotemporal behavior of a highly deformable product saturated with water during convective drying. The numerical simulation of heat and mass transfer states consists of solving the full conductive solid phase conservation equation, the convecto‐diffusive conservation equation of the liquid water together with the energy equation. These equations are coupled by the solid phase contraction rate due to water removal, which represents the shrinkage. The thermo‐physical properties of the considered product, grape in our case, were determined from characterization experiments. Measurement series were carried out in a laboratory‐drying loop and the obtained numerical results are found to be in good agreement with the experimental ones. The simulation allows the monitoring of the time evolution of temperature and humidity inside the product for different drying conditions. At 20% relative humidity of drying air, the ideal temperature was found to be 55 °C and not 60 °C, which minimizes the cost of drying and conserves better the structure of the product. The simulation results of a deformable product can be extended to deformable foodstuffs to minimize the exposure time needed for predefined final water content. Practical applications The research of optimization of drying process of grapes led to the finding of the corresponding temperature and relative humidity of convective drying air. A proposed simulation model which takes the shrinkage of this product into account describes its behavior, gives the exposure time needed for predefined final water content. The model can be extended to other deformable products.
Seedless grape (Sultana grape) is a very important commercial fruit grown in large quantities in Tunisia. This product is characterized by a high initial moisture content (the initial wet basis moisture content of the fruit is more than 80 %), and thus a high shrinkage during drying. The mature seedless grape is a spherically shaped fruit. Thermo-physical properties and drying kinetics of seedless grape is essential for the optimization of its drying processes. This paper is composed of two parts, the first one is reserved to the experimental study of seedless grapes, such as the establishment of the desorption isotherms which were determined at 40, 50, 60 and 70 °C by using static gravimetric method and these desorption data were fitted by GAB model. Then we were interested in measurement of the axial hydrous shrinkage of a grape berry and it was expressed as a function of moisture content. Indeed, the drying kinetics under different controlled conditions of air temperature and relative humidity were realized. In the second part, the moisture diffusivity of the seedless grape was determined by minimizing the sum of square of deviations between the predicted and experimental values of moisture content of convective drying kinetics. The adopted approach was based on a numerical solving of the conservation equation of the solid phase and the equation of diffusion/convection of liquid phase for spherical geometry, coupled by the solid phase velocity due to shrinkage. The moisture diffusivity of seedless grape increased with temperature and was correlated by an Arrhenius-type equation. Indeed, the effect of moisture diffusivity was expressed by an exponential function. The moisture diffusivity of seedless grape ranged between 3.5610−10 (m2/s) and 12.610−10 (m2/s). Activation energy was found equal to 57.76 kJ/mol.
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