Effects of Drying Methods on the Composition of Thyme (Thymus vulgarisL.) Essential Oil

The influence of drying method on aroma compounds of thyme (Thymus vulgaris L.) was evaluated. The drying methods tested were convective drying, vacuum–microwave drying, and freeze drying, as well as a combination of convective predrying and VM finish drying. Thyme's convective drying kinetics are described by a two-term exponential model, and VM drying kinetics consisted of two periods: a constant rate period until a critical moisture ratio (values of 0.311, 0.242, and 0.162 kg kg−1 for 240, 360, and 480 W, respectively) was obtained and a falling rate period beyond that point. Volatile compounds of thyme samples were extracted by hydrodistillation and analyzed by gas chromatography. Thirty-three compounds were tentatively identified; thymol, γ-terpinene, p-cymene, caryophyllene, and α-terpinene were the major components. The total quantity of volatiles of fresh thyme (1,167 mg 100 g−1 db) was reduced by most of the drying treatments, with the exception of VM at 240 and 360 W. The combined method with 40°C and 240 W was the best option for drying thyme; the time required was relatively short (≈ 301 min) and aroma quality was good according to instrumental data (total concentration of volatiles 1,127 mg 100 g−1 db) and sensory evaluation results (high scores for fresh thyme and vegetable odors).