EVALUATION OF MATHEMATICAL MODELS FOR PREDICTION OF THIN- LAYER DRYING OF BRAZILIAN LEMON-SCENTED VERBENA LEAVES

The drying target is to reduce the moisture content and thereby increase the life time of products by limiting enzymatic and oxidative degradation. In addition, by reducing the amount of water, the percentages of active principles are increased in relation to the total mass. The aim of the current study was to determine the correct diffusion rate and activation energy by adapting the mathematical model according to the experimental data. Brazilian lemon-scented verbena leaves were harvested with a moisture content of around 85 % wet basis (wb), and were then subjected to drying in a medicinal plant dryer at an air temperatures of 40, 50, 60, 70 and 80 ºC at 0.29±0.03 m.s-1. Approximately 400 g of leaves at a layer thickness of 0.15m were used with three tests performed at each temperature. Several models of drying kinetics were adapted for use with the experimental data, including Henderson & Pabis, Henderson & modified Pabis, Lewis, Midilli et al., Page, Thompson and Wang & Singh. The adjusted determination coefficients (R2), relative mean errors (P), estimated errors (SE) and residue distributions were all compared. The best models, which best represented the Brazilian lemon-scented verbena drying, were Midilli et al. and Page. The calculated effective diffusivity coefficients ranged from 2.91x10-12 to 11.71 x 10-12 m2.s-1 for the studied temperature range. The activation energy for the diffusion of water was 31.79 kJ.mol-1