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This study aims to evaluate the quality of lemongrass plants after drying at different speeds (1.0, 1.5, 2.0 m s À1 ), temperatures (40, 50, 60, and 70 C), and drying air control systems (manual and automatic) using electrical conductivity as a parameter for evaluation. Firstly, we evaluated the time necessary for imbibitions of material in deionized water after drying for different air speeds and control systems. Secondly, the effect of temperature in fresh and dry plants was evaluated, comparing them with contents of essential oils and the quantity of ions leached by the electrical conductivity test. According to the results, the value of liquid diffusion increased with the increase of drying air temperature, and the value of activation energy was 62.84 kJ mol À1 . The quality of lemongrass plants evaluated by the electrical conductivity test was inversely proportional to the increase in drying air temperature, and the levels of essential oils were higher in dry plants with lower drying air temperatures. The adaptation of the electrical conductivity methodology to lemongrass plants was effective. A period of 33 h of imbibitions of the material in deionized water was necessary to obtain consistent results from leaching of potassium ions. The electrical conductivity can be used as a quality parameter for drying of medicinal plants.

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Los objetivos del trabajo fueran estudiar la cinética del secado de tomillo con posterior ajuste de diferentes modelos matemáticos a los datos experimentales y determinar los valores de la difusividad efectiva y energía de activación. Para el desarrollo del experimento de secado fueran utilizadas las temperaturas 30, 40, 50, 60 y 70 oC. El diseño experimental fue de bloques completamente aleatorizados, con tres repeticiones. Para el ajuste de los modelos matemáticos a los datos experimentales fue realizado el análisis de regresión no lineal, por el método Simplex-Quasi-Newton. Los resultados mostraron que el modelo matemático de Page obtuve un mejor ajuste a los datos experimentales, los valores de los coeficientes de difusividad efectiva variaron entre 3,69 x 10-12 e 1,19 x 10-10 m² s-1 y el valor de la energía de activación fue de 77,16 kJ mol-1.

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 (R 2 ), 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 m 2 .s -1 for the studied temperature range. The activation energy for the diffusion of water was 31.79 kJ.mol -1 .