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

The influence of drying method on aroma compounds of thyme (Thymus vulgaris L.) was evaluated. The drying methods tested were convective drying, vacuummicrowave 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 0.162kg kg1 for 240, 360, and 480W, 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, -terpinene, p-cymene, caryophyllene, and -terpinene were the major components. The total quantity of volatiles of fresh thyme (1,167mg 100g1 db) was reduced by most of the drying treatments, with the exception of VM at 240 and 360W. The combined method with 40 degrees C and 240W was the best option for drying thyme; the time required was relatively short (approximate to 301min) and aroma quality was good according to instrumental data (total concentration of volatiles 1,127mg 100g1 db) and sensory evaluation results (high scores for fresh thyme and vegetable odors).