Management of Surface Drying Temperature to Increase Antioxidant Capacity of Thyme Leaf Extracts (Thymus vulgaris L.)

Abstract

Thyme leaves are an important source of essential oils with antioxidant activity; these compounds are located in trichomes on the leaf surface. The drying conditions affect not only the drying time but also the antioxidant activity. In the literature, a drying temperature of 70°C appears to be the best for drying thyme leaves according to their antioxidant capacity. Considering drying periods at different temperatures also could be beneficial. With these considerations, the goal of this work was to establish a drying strategy with which to manage a drying temperature on the leaf surface that will enable the drying time to be shortened and improve the antioxidant capacity (AC) of the extract of dried thyme leaves. The drying strategy consisted of two consecutive drying periods in order to manage the drying temperature on the leaf surface. The first drying period was carried out at 80°C (T _a1) until the sample surface reached a temperature of 70°C, and the temperature was then immediately set to 70, 60, 50, and 40°C (second drying period, T _a2) at different air velocities (v; 1 and 2 m s⁻¹). Compared to constant drying conditions, two consecutive drying periods were found to improve the drying kinetics: the AC increased from 10.5 to 27.4% while reducing the drying time by 14.5 to 39.2%. The use of this drying strategy was found to be an interesting means of intensifying the convective drying of thyme leaves and its application should be considered when drying similar materials with bioactive compounds on the surface.

Keywords:

• Changing boundary drying model
• Drying time reduction
• Two-step drying

Notes

^a Q was calculated considering the average value of the corresponding drying times and the average values of environmental conditions (temperature = 25°C; relative humidity = 0.55).

^b To calculate ΔQ the estimated Q of the corresponding drying at constant temperature was considered as the reference value.