### Innovative and Sustainable Extraction Methods  
#### 2.2.1 Microwave-Assisted Extraction (MAE)  
Microwave-assisted extraction (MAE) is considered a more modern, unconventional, and more sustainable extraction method. This technique provokes the disruption and/or changes of cell structures through the application of nonionizing electromagnetic waves. The frequency of the waves can range from 300 MHz to 300 GHz to a sample matrix; however, it is normally set between 0.915 and 2.45 GHz. The ionic conduction and dipole rotation mechanisms promote the energy transfer during an MAE, thus facilitating the migration of chemical compounds through a solution, and the heat in the media is caused by the resistance of the extraction solvent to the ionic migration when the electromagnetic waves are being applied [49]. It can be described, in a nutshell, as the use of microwave radiation to heat the extraction solvent, this way accelerating the dispersion of the solvent in the sample and assisting the diffusion of the target compounds from the sample into the solvent. This technique is also suitable for the extraction of volatile compounds, considering that the MAE equipment, which is simple and affordable, can be used as a distiller if properly configured with a condenser and a recovery vessel (Figure 1).  
Even though MAE is considered a more modern technique, it has been used by researchers for decades. In 2013, [PERSON] et al. published an investigation on the extraction characteristics and optimal parameters for the MAE of anthocyanin from blueberry powder. In that study, the chosen parameters were extraction temperature, solvent concentration, matrix-solvent ratio, and extraction time. With the optimized parameters, they were able to obtain an extraction rate of anthocyanin of 73.73% within an extraction time of 7 min [50].  
The positive results reported in the literature led to more publications on the use of this technique, some of them focused on the optimization of the method using statistical tools, like the work carried out by [PERSON] et al., where the optimal conditions for the extraction of antioxidant compounds from blackberry were obtained by Box-Behnken design (BBD) of response surface methodology (RSM). The obtained results included a microwave power of 469 W and an extraction time of 4 min yielding 2.18 \(\pm\) 0.06 mg/g, a value 120% higher than the amount of extract obtained by ethanol leaching extraction, according to the authors [51].  
A similar optimization protocol was used more recently to perform a comparison between the MAE and the UAE techniques [52]. The recovery of total phenolic compounds from _Camellia japonica_ var. _Eugenia de Monttijo_ was studied with different parameters for each methodology. For UAE the chosen ones were solvent (acidified ethanol was switched from 0 to 100% \(\sigma/v\)), power (30-80%), and time (5-45 min); meanwhile, the parameters solvent (0 to 100% \(v/v\) acidified ethanol), temperature (50-180 \({}^{\circ}\)C), and time (5-25 min) were optimized for the MAE. Overall, MAE provided better results than UAE at their respective optimum conditions (for MAE: maximum yield: 80% at 180 \({}^{\circ}\)C and 5 min), although a relevant yield was achieved with UAE (56% at 62% amplitude, 8 min, 39%  
Figure 1: Scheme of an apparatus for microwave-assisted extraction (MAE) of volatile and nonvolatile compounds from agri-food byproducts. The matrix to be extracted is dispersed in the most appropriate solvent in a glass vessel, which is placed in the microwave oven. Application of microwaves heats the solvent and induces the extraction of chemical constituents. Volatile compounds are recovered upon distillation through a condenser, while nonvolatile compounds are recovered directly from the vessel.  
acidified ethanol) [52]. Studies using this methodology have evolved during the years, and its use as a pretreatment has yielded some interesting results for the enhancement of nutraceutical oils properties (e.g., [53]).