Extraction of Origanum onites L. using an industrial-type microwave-assisted distillation (MWAD) system: Increasing energy saving and essential oil yield compared to conventional steam distillation

Oregano species are commonly utilized for different medical purposes as well as for other essential oil products. One of the oregano species, Origanum onites L. has a dense natural distribution in the Aegean and Mediterranean regions of Turkey and Greece and is cultivated mainly in Denizli province. Turkey meets 80 % of the world's oregano market. Since Origanum onites L., whose essential oils are of high quality, has a high economic value, assessing the quality and production of essential oils extracted using various methods is important. Microwave-assisted distillation (MWAD) processes are generally carried out under laboratory conditions using clevenger or small-scale specialized instruments made for research in the lab. Thus, designing industrial-scale distillation systems with microwave power is necessary. For this reason, an industrial-type MWAD system with 12 magnetrons (power output of 1 kW) was developed in this work, and both conventional steam distillation (SD) and MWAD of oregano plant were realized, as a novelty. This study experimentally investigated the effects of microwave power and applied time on both energy saving (%) and essential oil yield (%). System was designed to distill 9 kg of dried plants chopped at once and the results obtained were used to obtain reference data for MWAD systems that can be used in industrial systems. The obtained essential oil samples were subjected to GC-MS analysis for chemical analysis. A statistical analysis of the experimental data was performed via Design Expert software to elucidate the impact of microwave power and time on essential oil yield and energy savings. When the results were evaluated, the essential oil yield of oregano plant increased by 4–22 % with the MWAD system compared to SD. The highest increase in yield was achieved with the test (500 W + 40 min). Also, the distillation time with MWAD system was 50 % shorter than with SD system. The MWAD system consumed at least 25 % less energy. According to the GC-MS analysis results, carvacrol increased steadily depending on the time applied microwave power as well as oxygenated monoterpenes. The chemical composition of the essential oils obtained from the proposed MWAD system did not show any incompatible value according to the ISO-7925 standard. As a result, this proposed method can be safely used in industrial systems.