Desorption isotherms and thermodynamic properties of Solanum gilo

Abstract

Solanum gilo is a plant belonging to the family Solanaceae with a probable origin in Africa. It was introduced to Brazil by workers. The fruit is cultivated by small producers in Brazil and it is a source of food for the low-income population. Its seeds are harvested with high moisture contents, and the drying process is necessary. Sorption isotherms consist of the relation between water activity (aw) and moisture content of an agricultural product at a constant temperature. This information contributes to the drying process, thus favoring an increased longevity of agricultural products, such as seeds. This research aims to determine the desorption isotherms of Solanum gilo seeds and calculate their thermodynamic properties (enthalpy, entropy and Gibbs free energy). Sorption experiments were performed by the gravimetric static method using saline solutions. Several mathematical models were fitted to the experimental data, and the selection of the best model was performed by statistical criteria. Equilibrium moisture contents were obtained at 10, 20 and 30°C and at water activities between 0.111 and 0.985 (decimal). The modified Oswin model best represents moisture desorption isotherms of Solanum gilo seeds under the studied conditions. The energy required for the process was 0.22-555.68 kJ kg-1. The latent heat of vaporization (L), the enthalpy (Qst), the entropy (ΔS) and the Gibbs free energy (ΔG) increased with the reduction of the equilibrium moisture content of seeds. The theory of isokinetics is valid for the desorption process.