The coaxial MW Clevenger-type extractor produced essential oils from five different herbs with a high concentration of oxygenated monoterpenes and different product selectivity regarding the microwave extraction time. The versatility of the different kinds of substrates used and the advantages regarding energy and time savings compared to the conventional HD, makes the coaxial MWHD extraction a desirable technology with promising scale-up in the development of solid–liquid green extraction.
EOs with different qualities in terms of composition can be obtained using different approaches. Camphor and 1,8-cineole were isolated in high concentrations and short extraction times from sage, rosemary and lavender using the coaxial MWHD approach. These EOs have a high value as renewable products especially for medicinal and insecticidal applications. On the other hand, the high amount of fenchone isolated from fennel seeds using the coaxial MWHD increases the value of the essential oil for cosmetics and food applications, while the high content of (E)-caryophyllene in the clove bud EO isolated by the conventional HD increases its anti-inflammatory properties.
The TG of the EO and the TG-FTIR analyses of the evolved gas, for the five herbs investigated, showed that the EO thermal behavior is described by the volatilization of its main components and that thermal degradation does not occur. The most volatile EO was rosemary, while clove EO showed the highest thermal stability. The volatility behavior of the EO analyzed is in agreement with the boiling point and the concentration of its main compound.