Thermodynamic Properties of Four Onion (Allium cepa L.) Varieties Produced in Senegal

Abstract

Thermodynamic data, essential for designing efficient dryers, depend on the physicochemical characteristics of products but remain unavailable for the Senegalese onion varieties Violet of Galmi, Safari, Gandiol F1, and Orient F1. This study determined the isosteric heat, desorption entropy, and Gibbs free energy of these varieties, individually and as a mixture, based on desorption isotherms modeled at 50, 55, 60, and 65 °C using the gravimetric method. Data analysis was performed with Matlab (version 6.0.0). Results show that net isosteric heat and entropy decrease with increasing equilibrium moisture content for all varieties and their mixture, whereas Gibbs free energy remains nearly constant, except for Violet of Galmi, which exhibits an increase between 5 and 35 % moisture content, followed by a slight decrease between 35 and 40 %. At 5 % equilibrium moisture content, the net isosteric heats of desorption were 156.92, 141.37, 115.89, 127.39, and 128.19 kJ·mol⁻¹ for Violet of Galmi, Safari, Gandiol F1, Orient F1, and their mixture, respectively approximately three times the latent heat of vaporization of water (42.52 kJ·mol⁻¹). The linear relationship between net isosteric heat and desorption entropy validates the enthalpy-entropy compensation theory. The isokinetic temperatures (385.04 - 398.37 K) exceed the harmonic mean temperature (329.95 K), indicating that desorption is controlled by net isosteric heat.  Furthermore, negative Gibbs free energy values (ΔG < 0) confirm that desorption is spontaneous. Energetically, although Violet of Galmi exhibits the highest isosteric heat, the mixture behaves similarly to Orient F1, likely due to interactions among constituents that facilitate water desorption. These findings provide a thermodynamic basis for optimizing dryer design and improving post-harvest management of onions, contributing to energy-efficient dehydration processes and the reduction of post-harvest losses in Senegal.