Extraction of Thyme (Thymus pallecens de Noe) Essential Oil by Steam-Distillation, Steam-Diffusion and Hydro-Distillation Processes: Optimization of Operating Conditions and Antioxidant Activity

Response surface methodology (RSM) was used to evaluate the processing parameters' effects of three extraction processes namely: steam-distillation (SD), steam-diffusion (SDiff) and hydrodistillation (HD) on the extraction of thyme essential oil. The tested parameters are the mass of vegetable, the extraction time and the water vapour flow; the response is the essential oil yield. According to the method of statistics, a second order polynomial function was assumed to approximate the extraction yield of the essential oil. The models' matching test, estimated by the determinant coefficient R-2 (93.42 % for SD, 80.46 % for SDiff and 94.73 % for HD), revealed a quite matching between experimental and predictive values. When compared, the effectiveness of the three processes according to the extraction duration showed that SD and SDiff are the most powerful with a value that exceeds 80 % time wise. Analysis highlighted that the oils were characterised by a high amount of oxygenated monoterpenes and carvacrol remains the major component in three oils with 79.4 % for SD, 83.7 % for SDiff and 86.3 % for HD. Furthermore, the antioxidant activity, determined by DPPH test highlighted that the activity of these oils is significant, it increases with the concentration and is stabilized around 80 % for the oil extracted by hydrodistillation and 91 % for those recovered by the two other processes, for concentrations equal or higher to 600 mg/ml. These oils were more active than the carvacrol which is its main component, its activity are comparable with that of BHA and BHT.