Thermophysical characterization of the powder resulting from the solar drying of the Moringa oleifera leaves

Abstract

This paper presents the results of thermophysical characterization of the powder resulting from the solar drying of Moringa oleifera leaves. The desorption isotherms of the powder, are determined by the gravimetric static method. The models of B.E.T, Smith, Henderson, Iglesias and GAB are used for the smoothing of the experimental points. The Clausius-Clayperon and GibbsHelmholtz equation are used for the determination of thermodynamic properties. The isotherms obtained are with type II. The theoretical curves of GAB present a better similarity with the experimental curve at 55°C. The monolayer moisture content (Xm) is 6.32 and 7. 24 % at 30° C; 5.13 and 6.3% at 35 °C; 5.1 and 5.9 % at 45 °C; 4.22 and 4.66 % at 55 °C respectively for BET and GAB models. Isosteric and entropic heats are more important for low values of moisture content. The activation energy of desorption of the powder in the range of temperatures tested is -10.1009539 kJ/mol. Isokinetic temperature (Tβ) and the mean harmonic temperature (Thm) are respectively 301.2435K and 300.12342K and the free energy (ΔGβ) is 302.18 J/mol. The desorption process of the Powder is non-spontaneous and is done by controlled mechanisms enthalpic.

Estimation of Thermodynamic Parameters for Better Conservation of Fresh Tomato (<i>Lycopersicum esculentum</i>)

“…They are therefore of sigmoidal shape, characteristic of a multimolecular absorption, which reflects the progressive thickening of the absorbed layer. This type of isotherm is characteristic of the hygroscopic behavior of most food products [11] [18] [38]. The results of the isotherms obtained experimentally are in perfect agreement with the behavior of biological products, particularly products with a high-water content [27] [39] [40] [41].…”

“…The enthalpy-entropy compensation theory is widely applied to evaluate the physical and chemical phenomena that accompany sorption reactions [11]. This theory confirms the natural changes in the interaction between the solvent and the substrate; and provides information on the perfect linearity of the relationship between these two state functions [37].…”

“…The theoretical basis for the determination of the GAB sorption isotherm is the assumption of localized physical adsorption in the multilayers, with non-lateral interactions. This model is widely used nowadays and showed a good fit up to a water activity of the order of 0.9 [11] ( )( )…”

“…The differential entropy is calculated from the Gibbs-Helmholtz equation, Equation (24) where the Gibbs free energy is obtained by the equation Equation ( 25), [11] [24]. ( )…”

“…The transformation of tomatoes for conservation therefore becomes a necessity in order to reduce post-harvest losses and ensure food security for populations. Thus, the access of smallholder farmers to effective and easy-to-use techniques for post-harvest conservation becomes a development priority [11]. Among these techniques, drying remains the most accessible.…”