Water sorption isotherms and drying characteristics of rupturewort (Herniaria hirsuta) during a convective solar drying for a better conservation

Abstract

Free water contained in porous structure of food plays a fundamental role in proliferation of microorganisms and bacteria that can damage the product materials. This work presents a thermo-kinetic study carried out in a natural and forced convection solar dryer; the objective was to establish the optimal conditions for drying and storing Herniaria hirsuta by testing the impact of different aerothermal conditions (air temperature, air velocity) on water loss of fresh Herniaria hirsuta. The kinetics of drying is studied for three temperatures (50, 60 and 70 °C) in two-air velocities (0.09 and 0.18 m/s). The optimal water activity which is related to conservation is 0.28. The LESPAM model was found to be the most appropriate for describing the sorption curves. The air drying temperature is a principal factor influencing the drying kinetics; the drying rate decreases in low air drying temperature, the air velocity had a small impact on the drying kinetics of Herniaria hirsuta. The Midilli-Kuck model was found to be the best fitted drying curves in thin layers for Herniaria hirsuta. The effective diffusivity of moisture water values changed from 2.5312 10−9 to 18.0511 10−9 m2·s−1, and increased simultaneously with the increase of temperature. The average activation energy of the diffusion process is obtained to be 2938.46 kJ/kg; it expresses the temperature effect on the diffusion coefficient. Dried Herniaria hirsuta under the maximum drying parameters showed highest retention of antioxidant activity, while the total phenolics and flavonoids were decreased by 10 and 46%, respectively.