Extraction of Hyssopus officinalis L. essential oil using instant controlled pressure drop process

To determine the optimum condition for the extraction of Hyssopus officinalis L. essential oil (EO) by instant controlled pressure drop (in French: Détente Instantanée Contrôlée or DIC), the influential parameters of pressure, heating time per cycle, number of cycles, mean particle diameter of the herb, and water concentration in the steam generator feed were evaluated using response surface methodology (RSM). The impact of using an ethanol-water mixture to generate the required vapor in the DIC process on EO extraction was investigated. The optimum condition was found to be at pressure = 1 bar, heating time = 100 s, cycles = 12, particle diameter = 1 mm, and water concentration = 80 v%. The addition of ethanol turned out to be more efficient at higher heating times and cycle numbers. The optimum yield in DIC was highest compared with hydrodistillation (HD), ultrasound-assisted extraction (UAE), and Soxhlet (SOX) methods; their values, expressed as total area percentages per internal standard area percentage, were 39.04, 10.98, 1.58, and 7.71, respectively. Comparison of the four methods also indicated that the DIC-EO was of a superior quality. The analysis of the DIC-treated residual herb in the optimum experiment revealed that unlike the low amount of valuable oxygenated monoterpenes, the availability of sesquiterpenes increased, which proved that DIC performs selectively while extracting more valuable EO components. The DIC-EO and its extract were examined for total phenolic content (TPC) and antioxidant activity using Folin-Ciocalteu and DPPH assay, respectively. Comparison of these results with those obtained from HD demonstrated higher TPC and antioxidant activity of the DIC-EO. Moreover, scanning electron microscopy (SEM) of the raw material and residual herb in the optimum experiment showed significant structure modifications induced by the DIC process.