**Author details**

can be seen as ILs analogs (since they possess similar physicochemical properties), they have

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

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].

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.

tion time and number of microwaves heating cycles.

**6. Conclusions**

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, extrac-

Microwave-assisted extraction (MAE) of naturally occurring chemical compounds is an extremely efficient extraction technique mainly as a consequence of the unique heating mechanism that is able to generate peculiar and otherwise impossible to reach extraction mechanisms. Different variants of MAE can be though according to the system matrix-solvent to be employed. Indeed,

been reported to be biodegradable and to possess lower toxicity and production costs.

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

acids [46].

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

Roberto Rosa<sup>1</sup> , Erika Ferrari<sup>2</sup> and Paolo Veronesi<sup>1</sup> \*

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

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

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