Effect of air temperature on drying kinetics, colour changes and total phenolic content of sage leaves (Salvia officinalis)

Abstract

The objective of the present study was to determine the influence of air temperature on the drying kinetics of sage leaves at temperatures of 45, 50, 55, 60, and 65 degrees C in a cabinet dryer. The drying time was significantly affected by temperature. Eight thin-layer drying models were used to describe the changes in moisture ratio as a function of time. The applicability of the models was determined regarding determination coefficient (R-2), reduced chi-square (chi(2)) and root mean square error (RMSE) values. The Midilli & Kucuk model showed the highest R-2, and lowest chi(2) and RMSE and was selected as the best model to describe drying characteristics of the sage leaves. Fick's second law was used to determine the effective moisture diffusivity (D-eff) at each temperature. Deff values were significantly affected by temperature and ranged from 1.62x10(-9) to 5.73x10(-9) m(2)/s. Temperature dependence parameters of Deff was described by the Arrhenius equation. Ea value was 52.52 kJ/mol for the given temperatures. Drying temperature significantly affected total phenolic content (TPC) and antioxidant activity (AA). Highest TPC and AA values were found from the samples dried at temperature 45 degrees C. This study suggested that sage leaves should be dried at a lower temperature due to lower phenolic degradation and colour change.