Effect of Drying and Microwave-Assisted Extraction Parameters on Lippia adoensis Variety Koseret Essential Oil Yield

Green extraction technologies, such as microwave-assisted extraction, have been used to replace conventional methods of isolating essential oils from plants. In this study, the essential oil was extracted from the Lippia adoensis variety koseret using the advanced method of microwave-assisted hydrodistillation. The main objective was to investigate the effect of irradiation time, microwave power and particle size on the yield and chemical composition of the essential oil extracted from leaves dried in an oven at 50 °C and room temperature.

The drying experiments were initially carried out by oven drying and drying at room temperature until 10 % of moisture was achieved. Several essential oils were then extracted from the oven-dried samples for a preliminary study, followed by optimisation of the essential oil yield. The Box-Behnken design with response surface methodology was used to optimise the process and effectively analyse the influence and interaction of the factors on the essential oil yield. The chemical composition and functional groups of the essential oil from both samples were then analysed.

The micrographs of the oven-dried leaves showed damage to the glandular trichomes. The analysis of variance showed significant linear, quadratic and interaction effects of particle size and time, as well as of microwave power and time on essential oil yield. The optimal conditions were: 30 min irradiation time, 550 W microwave power and a particle size of 0.5-0.7 mm, resulting in essential oil yields of (0.49±0.01) % from the oven-dried leaves and (0.40±0.01) % from the room-dried leaves, which accounted for (85.1±0.8) and (87.2±0.7) % of the oils, respectively, with linalool being the predominant component. Fourier-transform infrared spectra showed the presence of hydroxyl functional groups, confirming the chemical composition results.

The results showed the effect of drying methods and microwave-assisted process parameters on the quantity of the produced essential oils. The essential oil yield obtained by the innovative microwave-assisted hydrodistillation results in an efficient process and product quality that can be a very useful ingredient for the food and pharmaceutical industry.