## 4 Valorization of Agricultural By-Products Using DESs  
As it has been indicated in the introduction section, agricultural by-products have surged as an important source of bioactive compounds which bring about the possibility of valorization of these products to be applied in the food industry, for cosmetic, nutraceutical or pharmacological applications [28]. In this context, research has been done in order to develop suitable procedures to effectively extract the desired bioactive compounds from these sources. However, most of these procedures involve conventional extractions in which high quantities of toxic solvents are used. For this reason, important effort is directed towards the design and development of greener methodologies that not only contribute with sustainability from a wide perspective but also that do not compromise the suitability of the obtained products to be subsequently used in the mentioned sectors by the use of toxic materials that can risk the safety of consumers [29; 30]. Different action lines based on the principles of green chemistry have been carried out. Among them, the application of DESs as innovative green solvents has sharply increased in recent years as a consequence of the already mentioned advantages of the use of DESs as extraction solvents. As can be seen in Table 2, in which some examples of recent applications of DESs for the extraction of bioactive compounds from agricultural by-products have been compiled, most applications developed so far are based on NADESs due to the lower or absent toxicity and higher biocompatibility of this group of DESs. The applications have been focused on the extraction of phenolic compounds [30; 31; 32; 33; 34; 35; 36], although others such as proteins, polysaccharides, flavonoids, carotenoids, lignans and organic acids have been also investigated [30; 31; 32; 33; 35; 37; 38] in vegetable, fruit and plant by-products. Considering the solid nature of most samples evaluated, solid-liquid extraction (SLE) has been the preferred procedure, although the assistance of ultrasounds (UAE), or microwaves, MAE, were necessary in most cases in order to favor the interaction of analytes-solvent. As an example, [PERSON] et al. [38] proposed the application of UAE using ChCl-based NADESs for the effective extraction of anthocyanins from wine lees providing a reduction of energy consumption, higher yields, and lower time than conventional nonassisted extraction. In addition, the authors considered the addition of water and the increase of the temperature in order to decrease NADES viscosity and favor the extraction process since the high viscosity difficult cavitation process in UAE resulted in a lower extraction efficiency [33]. In this case the temperature was set at 35 \({}^{\circ}\)C to avoid analytes degradation whereas the water content was 50% since it enhances the extraction efficiency, however, higher amount resulted to be unfavorable due to the weakening of the solvent stability. Apart from the advantages of using these solvents instead of conventional toxic ones, NADESs also have shown to provide a positive effect on the application of the assisted procedure in comparison with commonly used solvents. It is the case of the work developed by [PERSON] et al. [36] in which the extraction of polyphenols from grapefruit peels was investigated using high voltage electrical discharges (HVED) followed by SLE at 50 \({}^{\circ}\)C using NADESs and conventional solvents, prior to determination by HPLC-DAD. Results showed that, among the different solvents evaluated (i.e., water, water/ethanol, glycerol, water/glycerol and NADESs based on ChCl, lactic and tartaric acids, glycine, glucose and sodium and ammonium acetates) when glycerol or lactic acid:glucose in a molar ratio (5:1) were applied, the energy applied at the HVED could be reduced up to six times, obtaining the same extraction kinetic and yields, which clearly favor the green character of the methodology. \begin{table}
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