**4. Is MAE really green?**

then the design becomes more critical and the use of multiple microwave sources is generally preferable. Control systems should be faster and robust, preferably adaptive, since the load usually is of natural origins, and thus it has a higher variability compared to pre-processed or

**Figure 9.** Specific energy consumption measured during the synthesis of the cobalt aluminate pigment in four different furnaces: conventional—Nannetti, Italy; Multimode1—CEM MAS (USA); Multimode2—Radatherm (Australia); single

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

Last but not least, since during pure (i.e. not hybrid) microwave processing heat is generated in the load and the surrounding environment remains colder, it is important also to provide sufficient thermal insulation to avoid heat losses. This is generally accomplished using insulating materials, but care must be taken when inserting them in the microwave applicator, as they could be (or become) good microwave absorbers, with the result of competing with the load in microwave power absorption and to fail as insulating materials. In this framework, another important component which is usually subjected to high electric field strength is the pressure window: in some installations, where it is required to separate the microwave generation zone from the microwave application one, like in vacuum processing, or even in case of pressurized environments, there is the need for a material able to transmit microwaves and to isolate the two environments. The requirement of materials for pressure windows are very demanding: they should withstand the load (pressure difference), with large safety coefficient, they must hold the pressure and being cleanable to avoid possible overheating of deposited particles of the load, they should present high transmission and low absorption or reflection of microwaves; they should be thermo-mechanically stable and chemical inert; they should withstand the process temperature without carbonizing, they should resist also to thermal shock and the thermal fatigue. Optionally, they should be optically transparent (common devices for arc detection are light sensors that are installed along the microwave transmission line), hence an opaque window would mask the occurrence of an arc. Moreover, for safety reasons, generally it is preferable to install two pressure windows (the second for safety in case of failure of the first one, but also for impedance matching purposes, that is, to minimize reflected power).

synthetic starting materials.

mode—self-assembled. Data from [35].

Even if microwave-assisted extraction techniques are typically considered greener with respect to more conventional extraction processes mainly as a consequence of the enhanced heating rates which are obtainable with the use of microwaves and the resulting reduction in processing time, together with the reduction and sometimes elimination of any solvent, the accurate comparison between the environmental assessments of the different extraction procedures is surprisingly scarce in the scientific literature, making any trustworthy conclusions a sort of act of belief.

Recently, cradle to the grave environmental evaluations of laboratory scale conventional Soxhlet-based extraction as well as of microwave-assisted extraction were performed by applying the life cycle analysis (LCA) methodology [37]. This approach allows considering practically all the fundamentals damage categories that could have an impact on this particular process, thus including, for example, energy consumptions, abiotic resources depletion, transport, laboratory facilities, emissions as well as the disposal of waste [38].

Particularly, that study focused on the natural compound Curcumin that is actually under exponentially increasing investigation for its antioxidant, anti-inflammatory and anticancer properties [39]. In order to realize the most suitable comparison between conventional and MAE procedures, the evaluation of the total potential damage of both approaches was referred to the same mass unit or curcumin molecule (i.e. 1 kg). Both experiments were performed exploiting a similar configuration and the same extractive solvent (i.e. acetone), with the same solid to liquid ratio. In detail, conventional extraction was performed using a conventional Soxhlet apparatus reported on the left of **Figure 10**, while MAE was performed in a TE103 single mode applicator operating at the frequency of 2.45 GHz that is reported on the right of **Figure 10**.

**Figure 10.** Conventional Soxhlet-based apparatus (left) and microwave-assisted extraction applicator (right).

**5.1. Ultrasonic microwave-assisted extraction (UMAE)**

erosion, capillarity, detexturation and sonoporation [41].

mass transfer from the plant to the extracting solvent.

sounds for the extraction from previously dried matrices.

constituted by an organic cation and an organic or inorganic anion [43].

related to their toxicity and the high cost associated to their preparation remain.

**5.2. MAE in ionic liquids (ILs)**

**5.3. Deep eutectic solvents (DESs)**

sure variations.

Similar to MAE techniques, the use of ultrasounds (US) was also reported to tremendously increase extraction yields and reduce extraction time as well as the amounts of solvents needed, as a results of a combination of different extraction mechanisms like fragmentation,

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Independently by the mechanism (or combination of different ones) which are responsible for a given extraction procedure, acoustic cavitation is at the basis of ultrasound-assisted extraction (UAE), since it lead to high shear forces, generation of microjets, shock waves and pres-

Simultaneous irradiation of microwaves and ultrasounds for extraction purposes was firstly reported by Cravotto et al. [42] for the extraction of vegetable oils, resulting in higher yields in comparison with the sole use of microwave energy (employing both open and closed vessels). A dedicated PEEK horn was used in order to avoid arcing and further unwanted phenomena, into the multimode microwave cavity, as a consequence of the use of metallic horns. In all likelihood, the higher extraction yields typically reported when microwave energy is combined with ultrasounds is related to the further contribution offered by ultrasounds to the disruption of plant cell walls (manifested at lower frequencies), thus further enhancing the

A further peculiarity of US is the acceleration in the rehydratation and swelling of the plant, therefore, this can be exploited in the use of a combination of both microwaves and ultra-

The nature of ionic liquids (ILs) makes them suitable candidate solvents to efficiently couple with the electromagnetic energy at the microwave frequencies, mainly due to the ionic losses contribution to the loss factor value. Indeed, ILs are organic salts existing in the liquid state,

Further properties (different from their chemical structure) of ILs that led to intensification of extraction processes from plant materials are their excellent solvating properties, a high thermal stability together with an easy to tailor viscosity. These properties significantly contributed to spread their utilization in extraction of both polar and non-polar natural compounds. Despite the reported advantages, several concerns related to the use of ILs still exist. Indeed, even if considered green solvents as a consequence of their extremely low volatility, issues

In a continuous research for new green solvents for extraction purposes, deep eutectic solvents (DESs) were recently investigated for the extraction of bioactive compounds [44]. Even if they

**Figure 11.** Comparative analysis of curcumin life cycle produced by conventional extraction (CE) and by MAE technique, at a laboratory scale. Adapted from [37].

Conventional Soxhlet extraction was prolonged for 24 h, while the use of microwave energy allowed obtaining comparable extraction yields in only 5 min of heating at reflux conditions. Single score evaluation results are reported in **Figure 11**, where Pt represents the eco-indicator points: the lower its value is the better that particular process will impact on the environment. Particularly, the conventional Soxhlet-based extraction resulted in 135 Pt, while the environmental damage associated with the proposed MAE resulted ca. 75% lower (i.e. 34.4 Pt), thus suggesting a greener character of microwaves with respect to conventional extraction procedures, at least in the considered lab-scale scenario. In both cases the three most impacting categories resulted "non-renewable energy", "global warming" and "respiratory inorganics", primarily as a consequence of consumptions of abiotic resources and emission of inorganic species, which are associated to the processes used to describe the electrical energy demand of the extraction techniques compared.

These results are worth to note also because they were obtained taking into account the poor efficiency a 2.45 GHz microwaves magnetron generators.

### **5. Coupling with further techniques**

In an effort to continuously improve extraction efficiency and contemporary increase the green characteristics of microwave-assisted extraction in a process intensification (PI) perspective [40], several integrating technologies have recently been coupled with microwave heating [15]. The most widely employed ones are summarized in the next sub-sections.

### **5.1. Ultrasonic microwave-assisted extraction (UMAE)**

Similar to MAE techniques, the use of ultrasounds (US) was also reported to tremendously increase extraction yields and reduce extraction time as well as the amounts of solvents needed, as a results of a combination of different extraction mechanisms like fragmentation, erosion, capillarity, detexturation and sonoporation [41].

Independently by the mechanism (or combination of different ones) which are responsible for a given extraction procedure, acoustic cavitation is at the basis of ultrasound-assisted extraction (UAE), since it lead to high shear forces, generation of microjets, shock waves and pressure variations.

Simultaneous irradiation of microwaves and ultrasounds for extraction purposes was firstly reported by Cravotto et al. [42] for the extraction of vegetable oils, resulting in higher yields in comparison with the sole use of microwave energy (employing both open and closed vessels). A dedicated PEEK horn was used in order to avoid arcing and further unwanted phenomena, into the multimode microwave cavity, as a consequence of the use of metallic horns. In all likelihood, the higher extraction yields typically reported when microwave energy is combined with ultrasounds is related to the further contribution offered by ultrasounds to the disruption of plant cell walls (manifested at lower frequencies), thus further enhancing the mass transfer from the plant to the extracting solvent.

A further peculiarity of US is the acceleration in the rehydratation and swelling of the plant, therefore, this can be exploited in the use of a combination of both microwaves and ultrasounds for the extraction from previously dried matrices.

#### **5.2. MAE in ionic liquids (ILs)**

**Figure 11.** Comparative analysis of curcumin life cycle produced by conventional extraction (CE) and by MAE technique,

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

Conventional Soxhlet extraction was prolonged for 24 h, while the use of microwave energy allowed obtaining comparable extraction yields in only 5 min of heating at reflux conditions. Single score evaluation results are reported in **Figure 11**, where Pt represents the eco-indicator points: the lower its value is the better that particular process will impact on the environment. Particularly, the conventional Soxhlet-based extraction resulted in 135 Pt, while the environmental damage associated with the proposed MAE resulted ca. 75% lower (i.e. 34.4 Pt), thus suggesting a greener character of microwaves with respect to conventional extraction procedures, at least in the considered lab-scale scenario. In both cases the three most impacting categories resulted "non-renewable energy", "global warming" and "respiratory inorganics", primarily as a consequence of consumptions of abiotic resources and emission of inorganic species, which are associated to the processes used to describe the electrical energy demand

These results are worth to note also because they were obtained taking into account the poor

In an effort to continuously improve extraction efficiency and contemporary increase the green characteristics of microwave-assisted extraction in a process intensification (PI) perspective [40], several integrating technologies have recently been coupled with microwave heating [15]. The most widely employed ones are summarized in the next sub-sections.

at a laboratory scale. Adapted from [37].

of the extraction techniques compared.

**5. Coupling with further techniques**

efficiency a 2.45 GHz microwaves magnetron generators.

The nature of ionic liquids (ILs) makes them suitable candidate solvents to efficiently couple with the electromagnetic energy at the microwave frequencies, mainly due to the ionic losses contribution to the loss factor value. Indeed, ILs are organic salts existing in the liquid state, constituted by an organic cation and an organic or inorganic anion [43].

Further properties (different from their chemical structure) of ILs that led to intensification of extraction processes from plant materials are their excellent solvating properties, a high thermal stability together with an easy to tailor viscosity. These properties significantly contributed to spread their utilization in extraction of both polar and non-polar natural compounds.

Despite the reported advantages, several concerns related to the use of ILs still exist. Indeed, even if considered green solvents as a consequence of their extremely low volatility, issues related to their toxicity and the high cost associated to their preparation remain.

#### **5.3. Deep eutectic solvents (DESs)**

In a continuous research for new green solvents for extraction purposes, deep eutectic solvents (DESs) were recently investigated for the extraction of bioactive compounds [44]. Even if they can be seen as ILs analogs (since they possess similar physicochemical properties), they have been reported to be biodegradable and to possess lower toxicity and production costs.

the choice of the kind of MAE technique to be applied, cannot discern from a fundamental aspect to be considered, that is represented by the detailed knowledge of the dielectric properties of the matrix as well as the solvent, to be used. They represent important parameters to be used as input information for numerical simulation studies aimed at predict the microwave power distribution

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Numerical simulation is fundamental also since it allows to rapidly select the optimal dimensions of the microwave cavity for a particular load (with particular dielectric properties).

In the same perspective, MAE has been shown to significantly benefit from the coupling of a

However, it has been shown that although MAE can effectively and quantitatively considered greener with respect to conventional soxhlet-based extraction, for example, in the case of lab-scale extraction of curcumin molecule, general conclusions cannot be drawn, since case to case evaluations are mandatory. The use of cradle to the grave LCA evaluation is highly

\*

and Paolo Veronesi<sup>1</sup>

1 Department of Engineering "Enzo Ferrari", University of Modena and Reggio Emilia,

2 Department of Chemical and Geological Sciences, University of Modena and Reggio

[1] Anastas PT, Warner R. Green Chemistry, Theory and Practice. New York: Oxford

[2] Tobiszewski M, Mechlinska A, Namiesnik J. Green analytical chemistry-theory and

[3] Galuszka A, Migaszewski Z, Namiesnik J. The 12 principles of green analytical chemistry and the SIGNIFICANCE mnemonic of green analytical practices. Trends in Analytical

[4] Chemat F, Vian MA, Cravotto G. Green extraction of natural products: Concept and

principles. International Journal of Molecular Sciences. 2012;**13**:8615-8627

and the temperature evolution into the load during the extraction process.

In this view, it can be seen in a process intensification perspective.

different number of integrating technologies and tools.

recommended to reach such assessments.

, Erika Ferrari<sup>2</sup>

University Press; 1998. p. 30

Chemistry. 2013;**20**:78-84

\*Address all correspondence to: paolo.veronesi@unimore.it

practice. Chemical Society Reviews. 2010;**39**:2869-2878

**Author details**

Roberto Rosa<sup>1</sup>

Modena, Italy

**References**

Emilia, Modena, Italy

DESs are systems composed by an eutectic mixture of Lewis or Brønsted acids and bases characterized by melting points much lower with respect to the individual constituents [45].

More recently natural deep eutectic solvents (NADESs) were developed by combining primary metabolites and bio-renewable starting materials, like sugars, amino acids and organic acids [46].

Similar to ILs, their chemical structure (or better in this case the chemical structure of the constituents the eutectic mixture) assure an excellent microwaves absorbing ability, thus making them suitable candidate solvents to be used in MAE as recently proposed, for the extraction of genistin, genistein and apigenin from pigeon pea roots [47]. Efficiency of DESs in coupling with microwave energy, was recently proved to lead to a greater (with respect to more conventional MW-coupling solvents like water and methanol) degree of disruption of cell walls [48].

#### **5.4. Design of experiments (DoE)**

A further tool that can significantly intensify every kind of extraction process (and not only), thus including also microwave-assisted procedures, is represented by design of experiments (DoE). DoE is a well-recognized collection of statistical and mathematical tools that has been widely exploited to improve and optimized extraction processes [49], as well as many other application fields in chemistry and materials science [50–53]. It relies in a statistical-based methodology used to systematically plan all the experiments to perform in order to obtain trustworthy and robust data without loosing extremely useful information on individual variables as well as interaction effects among different variables on the final goal of the study that could be, for example, the optimization of a certain MAE protocol. Indeed, traditionally the optimization of the extraction conditions for a certain technique and a certain matrix, are performed by the one-factor-at-time (OFAT) approach, that is extremely time and energy demanding and moreover, it could lead to the loss of the effects caused by several extraction parameters acting in combination. Therefore, in this perspective DoE approach can be easily considered an important tool to pursue a more rapid green process intensification development.

In a recent work [49], these statistical techniques were applied to the microwave-assisted extraction of phenolic compounds from *Juglans regia* L. fresh male flowers and unripe walnuts seeds, with the aim to investigate the role on the extraction efficiency of temperature, extraction time and number of microwaves heating cycles.
