Anthocyanins are water-soluble antioxidants pigments that belong to the flavonoid family. They have a C6-C3-C6 skeleton
[64]. The flavylium cation gives the red color to the anthocyanins
[65]. Temperature, enzymes, pH, light, metallic ions, oxygen, sulfites, and interaction with flavonoids, phenolics, ascorbic acid, and sugars can affect anthocyanins’ integrity according to their chemical structure and food concentration. Anthocyanins subjected to high temperatures and prolonged heating oxidize. Moreover, the deglycosylation process, nucleophilic attack of water, cleavage, and polymerization determine the structure breakdown. These structural changes decrease anthocyanin content in the final product
[64]. Sulfite adds colorless adducts interrupting the conjugated π-electron system
[65]. Anthocyanin association reactions (e.g., self-association between anthocyanins via hydrophobic interactions, co-pigmentation between anthocyanins and other phenols through van der Waals interactions between the planar polarizable nuclei or binding the sugar and the flavylium nucleus covalently, and metal complexing between anthocyanins and with metals, via their o-hydroxy groups) defend the flavylium chromophore from the water’s nucleophilic bond, enhancing their stability and color
[66]. The anthocyanins can manage and/or prevent chronic degenerative diseases, including cancers, cardiovascular diseases, type 2 diabetes mellitus, dyslipidemias, and neurodegenerative diseases
[67]. Several biochemical parameters involved in the inflammatory responses (e.g., interleukins (ILs), tumor necrosis factor-alpha (TNF-α), nuclear factor-kappa B (NF-kB), and cyclooxygenase 2), which are able to improve malonaldehyde and reactive oxygen species, and to reduce the activity/expression of antioxidant enzymes (e.g., catalase and superoxide dismutase), have been related to the prevention or development of these diseases
[68][69][70]. The anthocyanins improve the sensitivity, secretion, and lipid profile of insulin by inhibiting the limiting enzyme of cholesterol synthesis (3-hydroxy-3-methylglutaryl-coenzyme A) or adipose triglyceride lipase engaged in triglyceride breakdown in diabetics and prediabetic subjects
[71][72]. Moreover, anthocyanins regulate the expression of adipokines, thereby enabling the avoidance of insulin resistance and the progression of type 2 diabetes mellitus
[73]. Anthocyanins can reduce triglycerides, cholesterol, LDL-cholesterol, and inflammatory biomarkers regulating the expression/activity of pro-inflammatory cytokines (e.g., IL-6, TNF-α, and IL-1A), pro-inflammatory enzymes (e.g., COX-2), and NF-κB signaling pathways, decreasing the production of pro-inflammatory molecules (e.g., C-reactive protein), and improving SOD and proliferator-activated receptor-γ expression
[74][75]. Finally, anthocyanin supplementation can enhance cardiovascular function
[76][77], regulate gut microbiota composition
[78][79], improve exercise recovery effectiveness
[80], decrease ulcerative colitis symptoms
[81], reduce ocular fatigue
[82], and promote healthy facial skin conditions
[80]. Some innovative processes are proposed for the extraction of anthocyanin from biowastes, such as the extractions assisted by a pulsed electric field, microwave, and ultrasound technologies. The enhancement of the extraction of anthocyanin, helped by pulsed electric field technology, is related to permanent (irreversible) or temporary (reversible) pores in the cell membranes, which facilitate the anthocyanins release into the medium
[64]. The target compounds’ extraction does not constantly improve with the increasing of the electric field strength, specific energy input, treatment time, temperature, and pulse number
[83][84]. For example, studies on blueberry extraction showed an improvement (+75%) in the extraction of anthocyanin when the specific energy input was increased
[85]. Instead, studies showed that improving electric field intensity and specific energy input does not increase the anthocyanins content extracted from the sweet cherry byproduct
[86]. The application of high intensity can lead to anthocyanin degradation. instead, the application of low/moderate-intensity enhanced anthocyanin recovery without anthocyanin’s degradation/modification. The pulsed electric field technology allows the selective extraction of the single anthocyanin classes
[87]. The pulsed electric field technology improves the monoglucoside anthocyanin extraction compared to acylated glucoside anthocyanins from grape pomace
[88] and the extraction of cyanidin, delphinidin, and petunidin glycosides from blueberry byproducts
[89]. The anthocyanin recovery from grape pomace enhances when the microwave and irradiation time improve
[90], and longer irradiation times enhance anthocyanin recovery from wine lees
[90], sour cherry pomace
[91], and saffron floral bio-residues
[92]. Nevertheless, the excessive intensification of MW extraction process parameters can decrease the extraction of anthocyanin from biowaste due to their degradation (anthocyanins are thermolabile compounds)
[93]. Thus, it is recommended to use extraction temperatures below 60 °C to minimize the anthocyanin’s losses. The extraction time also impacts MW extraction. The excess time causes the degradation of anthocyanin due to higher exposure to microwave powers and high temperatures
[94]. The high microwave power determines internal overheating, leading to carbonization and isomerization, and/or degradation of molecules
[95]. According to some authors, anthocyanins’ thermal degradation determines the loss of sugar moieties, the formation of a carbinol pseudo base, and chalcone by hydrolysis of the remaining sugar moiety and cut between C2 and C3
[84]. According to others, the degradation of anthocyanin is due to decomposition reactions of water molecules and the production of reactive oxygen species
[96]. Studies on anthocyanin recovery from eggplant peel and fig peel showed that the recovery of anthocyanin decreased when the microwave powers and irradiation times improved
[97]. Studies on grape pomace
[87], blackcurrant bagasse
[98], blueberry peel
[99], black rice bran
[100], and corn husk
[101] showed that the extraction of anthocyanin decreased when the irradiation times were long. Finally, anthocyanins’ structures impact their recovery from bio matrices. For example, anthocyanin analogs that are unsubstituted at C3 of the C-ring are more stable to MW treatment than other anthocyanins
[102], as well as acylated anthocyanins than non-acylated ones
[103]. Another technique used to improve the extraction of anthocyanin is ultrasound. Acoustic cavitation can determine the thermal and chemical degradations of the anthocyanins since the acoustic cavitation phenomenon can determine thermal stress and free radical formation
[104]. Long processing times can cause severe degradations in anthocyanins
[105]. For example, the anthocyanin extracted from black chokeberry wastes degrades when an ultrasound water bath (30.8 kHz) for 60 min at 70 °C, with a nominal ultrasound power of 100 W, and 50% ethanol in water are used
[106]. Using enzymes (pectinase compound and pectinase) combined with ultrasound can improve the extraction technique’s performance
[105].