**5. Anthocyanins extraction by SFE**

Anthocyanins are the most abundant flavonoid constituents of fruits and vegetables. Their use into food and/or medical fields has proven to be a technological challenge since these compounds have low stability and are susceptible to degradation through factors such as the presence of light, pH, temperatures usually higher than 60–80°C, the presence of sulfite, ascorbic acid, enzymes (such as glycosidases and phenolases), among other factors [43, 81, 82].

In the literature, the recovery of phytochemicals from solid wastes has been reported using conventional and alternative technologies. According to Paes et al. [44], conventional methods are Soxhlet extraction, maceration extraction, extraction by infusion and vapor distillation. Alternative techniques such as supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE) eventually assisted with ultrasound are also reported.

Supercritical fluids processes have proved to be an excellent alternative to extract natural pigments due to the use of environmentally friendly solvents, such as carbon dioxide. According to Vatai et al. [83], extractions with supercritical CO2 result in non-deteriorated reactions, due to low process temperatures. The CO2 is readily available, relatively cheap, and accepted as a solvent in the food industry. SFE with CO2 is an excellent isolation method for natural materials and gives an alternative to replace the nonpolar organic solvents, which can be a source of food contamination.

Supercritical fluid extraction (SFE) using carbon dioxide (CO<sup>2</sup> ) has been applied for the pretreatment of natural materials, as shown in the works of Paula et al. [84], Ghafoor et al. [85], and Floris et al. [86]. Operating conditions (temperatures varying from 40 to 50°C and pressures above 200 bar) and the use of co-solvents such as ethanol and water were used in their studies as modifiers to obtain the maximum extract yield. According to Seabra et al. [87], even though the choice of the appropriate polar solvent is a key factor for the success of the anthocyanin extraction procedure, its influence on the extract's characteristics is not always clear, due to the diverse structure and composition of plant materials and also the relation material-solvent.
