Impact of Swell-Drying Process on Water Activity and Drying Kinetics of Moroccan Pepper (Capsicum annum)

Abstract

The impacts of total hot air drying (THD) and the swell-drying process (SD), which is hot air drying coupled with an instant controlled pressure drop DIC process, on the drying kinetics and adsorption isotherm behavior, were determined in the case of Moroccan peppers. Drying kinetics were studied through a model that takes into account the starting accessibility and the internal diffusion model with Fick's law. Adsorption experimental data were obtained by using the gravimetric method at 25, 40, and 50°C. Data were fitted to the GAB, Halsey, and Oswin models. Studied responses were: effective diffusivity (Deff), starting accessibility (δWs), monolayer moisture content (Xm), specific surface area (σ), internal porosity (ϵ), and microstructure. Mean relative percentage deviation (E) and root mean square (RMS) were used to evaluate the fitting of models. Results showed that: pepper adsorption isotherms followed the type II isotherm curve;the equilibrium moisture content (Xeq) depended on the temperature and the drying method; the GAB and Halsey models fitted well the experimental data; at selected conditions of the DIC treatment (0.35 MPa and 5 s), the monolayer moisture content (Xm), the specific surface area (σ), and the internal porosity values were higher than those of THD samples; the DIC process intensified the drying operation, by increasing the effective diffusivity and the starting accessibility. Although the DIC process improves the drying kinetics, this behavior could be ignored related to a higher aw. This study shows that, thanks to the new expanded structure obtained by the DIC process (higher porosity), the adsorption capacities of products are improved (aw is reduced). Thus, the SD process improves both the drying operation and the stability of dried pepper products.