*2.3.1.2.6 Solvent-free microwave extraction*

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

The principle of the microwave-assisted hydrodistillation (MAHD) is based upon its direct impact with polar materials/solvents and is governed by two phenomena: ionic conduction and dipole rotation, which in most cases occurs simultaneously [46]. MAHD has been shown to reduce both extraction time and volume of solvent required, minimizing environmental impact by emitting less CO2 in atmosphere [47–49]. Some recently reported studies have successfully utilized a microwave oven for the extraction of volatile active components from plants [50]. It has been regarded as an important alternative in conventional extraction techniques because of its advantages which mainly are a reduction of extraction time, solvents, selectivity, volumetric heating, and controllable heating process (**Figure 8**) [51].

The basic principle of ultrasound-assisted extraction (UAE) to extract EOs from plant raw material consist of generating sound waves (ultrasound frequency about 20 KHz), which create cavitation bubbles in the solution and produce enough energy to break the structure containing the oil in order to release it. Moreover, UAE can act as an emulsifier dispersing lipophilic molecules in water, this facilitating the subsequent separation and purification of EOs [54, 55]. This technique was developed in 1950 [56]. It has been used to extract many EOs especially from flowers, leaves, or seeds [32, 55]. As known disadvantages, it requires filtration steps, and possible degradation of compounds at high frequencies occurs (**Figure 9**) [57].

Microwave-assisted extraction (MAE) is a process of using microwave energy to heat the solvent in contact with a sample in order to partition analytes from the sample into the solvent. The ability to rapidly heat the sample solvent mixture is inherent to MAE and is the main advantage of this technique [59]. It is a recent green technology broadly used to extract various EOs from plant. It has been established as an alternative method to conventional heating because it allows gain of time, volume of solvent used, and amount of biomass needed while increasing

*2.3.1.2.3 Microwave-assisted hydrodistillation*

*2.3.1.2.4 Ultrasound-assisted extraction*

*2.3.1.2.5 The microwave-assisted extraction*

*Schematic and picture of MAHD apparatus [52, 53].*

**82**

**Figure 8.**

Solvent-free microwave extraction (SFME) is proposed as a method for "green" extraction of edible EOs from fresh plant material, at atmospheric pressure without addition of water or organic solvent [61]. The SFME apparatus (**Figure 3**) is an original combination of microwave heating and dry distillation at atmospheric pressure. Based on a relatively simple principle, this method involves placing the plant material in a microwave reactor, without adding any solvent or water. The internal heating of the in situ water within the fresh plant material distends the plant cells and leads to the rupture of the glands and oleiferous receptacles. This process thus free EO which is evaporated by in situ water of the plant material. A cooling system outside the microwave oven condensed the distillate continuously. The excess of water is refluxed to the extraction vessel in order to restore in situ water to the plant material. At the end, EO is removed from the aqueous extract by simple decantation. SFME is neither a modified microwave-assisted extraction (MAE) which uses organic solvents nor a modified hydrodistillation process which uses a large amount of water; it can be consider as a dry distillation process, with water coming from the fresh plant material [62–64]. As advantages, the SFME

**Figure 9.** *Ultrasound-assisted extraction (UAE): from laboratory (a) to pilot scale (b) [58].*

**Figure 10.** *Picture and schematic diagram of the microwave oven adaptation to perform MAE [60].*

method increases the EO yield, ameliorate the EO composition, eliminate the waste of water treatment, and also contributes to limited time, and lower an energy consumption (**Figure 11**) [62].
