**2. Microwave-assisted extraction (MAE): state of the art**

Extraction of natural occurring chemical compounds is only one of the plenty food-processing technologies in which the use of microwave energy demonstrated significant advantages and unique possibilities [14]. The existing microwave-based techniques have been extensively reviewed [15–18], thus the present section is dedicated only at presenting the principles at the basis of the mostly employed ones, essentially presenting a major classification among those based on the use of a solvent and those operating in solvent-free conditions.

#### **2.1. Microwave-assisted solvent extraction (MASE)**

The first use of microwave energy for extraction purposes [19] was practically reported simultaneously with the first experiments performed in organic synthesis field which led to tremendous increase in yields and reduction in reaction times [20, 21]. Similarly, the use of water, methanol or a mixture of both solvents resulted in higher extraction yields, as a consequence of the polarity of the employed solvents, thus of their ability to efficiently be heated under a microwave field. Indeed, most of the solvents recommended by both green chemistry and green extraction principles can be efficiently heated when exposed to electromagnetic fields at the microwave frequencies. Therefore, necessarily the first use of microwave energy in the extraction of naturally occurring chemical compounds involved the use of solvent, leading to the group of experimental techniques called microwave-assisted solvent extraction (MASE) [22].

The extraction mechanism when using microwaves absorbing solvents is fundamentally similar with those of conventional extraction procedures. Nevertheless, the possibility of microwave to instantly and precisely furnish energy to the extraction mixture led first of all to advantages in terms of extraction times.

A further possibility is represented by the use of non-polar microwave-transparent solvents, like hexane and toluene, for example, even if not properly recommended in a green chemistry perspective [23]. In this latter case, the plant matrix needs to be wet in order to well couple with microwave energy. In doing so, the analytes of interest are then released in the solvent. The reported advantages of this latter approach in the green chemistry perspective are the typical less amount of solvent needed when compared to extraction technique based on a conventional mechanism.

Independently by the solvent employed, MASE techniques can be performed by employing both closed and open vessels, each of which possesses the proper advantages. When closed vessels are employed, temperature higher with respect to the boiling point of the used solvent can easily be reached, therefore typically reaching higher extraction yields. This could potentially be performed also by exploiting conventional heating strategies (e.g. by using an autoclave), but it would, however, need longer times to reach the selected extraction temperature.

In case of open vessels, the peculiar heating mechanism of microwaves together with its volumetric character is considered responsible for significant reduction in the processing time.

#### **2.2. Solvent-free microwave extraction (SFME)**

heating of vegetal matrixes, with the ideal result of being able to achieve greener extraction processes, with almost no preparatory or separation steps, literally "from the field to the

Section 4 is devoted to the environmental assessment evaluation of MAE with respect to traditional Soxhlet-based extraction, presenting a cradle to the grave life cycle assessment (LCA)

In Section 5, advantages deriving from the coupling of microwaves with further innovative techniques/extraction systems will be reviewed, highlighting their consequences in a process

Extraction of natural occurring chemical compounds is only one of the plenty food-processing technologies in which the use of microwave energy demonstrated significant advantages and unique possibilities [14]. The existing microwave-based techniques have been extensively reviewed [15–18], thus the present section is dedicated only at presenting the principles at the basis of the mostly employed ones, essentially presenting a major classification among those

The first use of microwave energy for extraction purposes [19] was practically reported simultaneously with the first experiments performed in organic synthesis field which led to tremendous increase in yields and reduction in reaction times [20, 21]. Similarly, the use of water, methanol or a mixture of both solvents resulted in higher extraction yields, as a consequence of the polarity of the employed solvents, thus of their ability to efficiently be heated under a microwave field. Indeed, most of the solvents recommended by both green chemistry and green extraction principles can be efficiently heated when exposed to electromagnetic fields at the microwave frequencies. Therefore, necessarily the first use of microwave energy in the extraction of naturally occurring chemical compounds involved the use of solvent, leading to the group of experimental techniques called microwave-assisted solvent extraction (MASE) [22]. The extraction mechanism when using microwaves absorbing solvents is fundamentally similar with those of conventional extraction procedures. Nevertheless, the possibility of microwave to instantly and precisely furnish energy to the extraction mixture led first of all to

A further possibility is represented by the use of non-polar microwave-transparent solvents, like hexane and toluene, for example, even if not properly recommended in a green chemistry perspective [23]. In this latter case, the plant matrix needs to be wet in order to well couple with microwave energy. In doing so, the analytes of interest are then released in the solvent. The reported advantages of this latter approach in the green chemistry perspective are the typical less amount of solvent needed when compared to extraction technique based on a

study for the case of curcumin molecule extraction at a laboratory scale.

182 Emerging Microwave Technologies in Industrial, Agricultural, Medical and Food Processing

**2. Microwave-assisted extraction (MAE): state of the art**

based on the use of a solvent and those operating in solvent-free conditions.

**2.1. Microwave-assisted solvent extraction (MASE)**

advantages in terms of extraction times.

conventional mechanism.

shelf". This is indeed the main focus of Section 3.

intensification perspective.

The unique possibility offered by microwave energy to directly interact with the plant cells containing the target chemicals together with the possibility to realize a heat transport occurring in the same direction as the mass transport (see **Figure 1**), led to the development of extraction technique characterized by the absence of any extraction solvent.

Indeed, solvent-free microwave extraction (SFME) techniques, invented in 2004 [24, 25] needs to be performed on fresh plant matrices, since the water contained in it is directly heated by interaction with microwaves, breaking the plant gland and releasing, for example, the essential oil. The water steam generating from the plant drives the essential oil recovered outside the microwave furnace, where a condenser is employed in order to cool down the distillate. The plants that have been efficiently exposed to SFME of essential oils have been reviewed in [26].

A further extraction technique employing microwave energy and solvent-free conditions is the so-called microwave hydrodiffusion and gravity (MHG) patented by Chemat et al. in 2008 [27]. In this configuration, the effect of the gravity is exploited synergically to the warming of the water (and other polar molecules) contained in the plant matrix, the latter leading to the disruption of the cell walls and the releasing of these substances (hydrodiffusion) outside the plant matrix. The extract is recovered at the bottom, outside the microwave furnace, where a refrigerator is placed.
