Drying of Curcuma longa L. slices by refractance window: Effect of temperature on thermodynamic properties and mass transfer parameters

Abstract

Curcuma longa L. rhizomes (CL) are aromatic spices known for their natural dye properties. These rhizomes contain considerate quantities of nutrients and bioactive compounds. Nevertheless, due to their perishable nature, they necessitate water removal process. This research aims to evaluate the adequacy of the analytical model established by Dincer and Dost in 1995, using experimental data from the Refractive Window (RW) drying method, focusing on solid samples in the flat plate configuration of Curcuma longa L. A comparative analysis will be carried out with the model devised by Crank in 1975, which relies on the traditional solution of Fick's second law. The primary objective is to determine the mass transfer parameters and the thermodynamic characteristics relevant to the drying procedure. The drying process for the samples was observed to be relatively rapid. In their natural state, the samples had a moisture content of 80.71 ± 1.12% (on a dry basis). According to the Dincer and Dost model, the moisture diffusivity ranged from 0.85 × 10^–8 m² s⁻¹ to 2.15 × 10^–8 m² s⁻¹, and the mass transfer coefficient varied from 1.60 × 10^–6 to 1.12 × 10^–6 m s⁻¹. The entropy value (ΔS) obtained was negative (ΔS < 0). The thermodynamic properties indicated a non-spontaneous process with positive values for enthalpy and Gibbs free energy, along with negative values for entropy.