Minerals: Revising the Current Literature

Citation: Cunha, R.; Trigueiro, P.; Orta Cuevas, M.d.M.; Medina-Carrasco, S.; Duarte, T.M.; Honório, L.M.d.C.; Damacena, D.H.L.; Fonseca, M.G.; da Silva-Filho, E.C.; Osajima, J.A. The Stability of Anthocyanins and Their Derivatives through Clay Minerals: Revising the Current Literature. Minerals 2023,13, 268. https://doi.org/10.3390/ min13020268 Academic Editor: Andrey G. Kalinichev Received: 20 December 2022 Revised: 9 February 2023 Accepted: 10 February 2023 Published: 14 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Review The Stability of Anthocyanins and Their Derivatives through Clay Minerals: Revising the Current Literature Robson Cunha 1, Pollyana Trigueiro 1,2, María del Mar Orta Cuevas 3, Santiago Medina-Carrasco 4, Thiago M. Duarte 1, Luzia M. de C. Honório 5, Dihêgo H. L. Damacena 1, Maria Gardennia Fonseca 6,* , Edson C. da Silva-Filho 1and Josy A. Osajima 1,* 1Interdisciplinary Laboratory for Advanced Materials (LIMAV), UFPI, Teresina 64049-550, PI, Brazil 2Materials Science and Engineering Postgraduate Program–PPGCM/CCSST, UFMA, Imperatriz 65900-410, MA, Brazil 3Departamento de Química Analítica, Facultad de Farmacia, Universidad de Sevilla, C/Profesor García, González 2, 41012 Sevilla, Spain 4Laboratorio de Rayos-X (CITIUS), Universidad de Sevilla, Avenida Reina Mercedes 4B, 41012 Sevilla, Spain 5Department of Chemistry and Physics, Center for Agrarian Sciences, UFPB, Areia 58397-000, PB, Brazil 6 Research and Extension Center-Fuel and Materials Laboratory (NPE–LACOM), Federal University of Paraíba, João Pessoa 58051-970, PB, Brazil *Correspondence: [email protected] (M.G.F.); [email protected] (J.A.O.) Abstract: In recent years, anthocyanins, natural dyes, have promoted great scientific and technological interest. Their intrinsic antioxidant properties and health benefits make them ideal representatives of natural dyes as replacements for synthetic dyes. However, their instability can limit their use. A promising strategy to increase the color stability of anthocyanins is their interaction with clay minerals structures. Clay minerals have characteristics favorable to the stabilization of natural dyes, such as adsorption capacity, good surface chemistry, ion exchange capacity, abundance, non-toxicity, and environmental compatibility. This review summarizes relevant works that present different clays or clay minerals as robust inorganic matrices for incorporation, improved stability, and increased resistance against the thermal- and photodegradation of anthocyanins. In addition, several authors propose different applications for the formed anthocyanin-based hybrids. Keywords: clay minerals; natural dyes; bioactive molecules; fading; protection 1. Introduction Anthocyanins are natural dyes belonging to the group of flavonoids widely found in fruits and vegetables. They have strong absorption in the visible, ultraviolet region of the electromagnetic spectrum and are determinants in red-blue colors and their derivatives in the plant kingdom. These characteristics place them in second place in importance, immediately after chlorophyll pigments [ 1 ]. Anthocyanins play an essential role in seed dispersal, pollination, and the development of plant organs. However, they also adapt to pathogenic attacks, drought, lack of nutrients, and high-intensity light [ 2 ]. Due to their chemical structure, with a central core of 2-phenylbenzopyrylium or flavylium cation, anthocyanins can be classified as polyphenols and secondary metabolites [3]. Anthocyanins can carry out interactions with other compounds [ 4 ]. A class of compounds comprises more than 700 different species of anthocyanins. Only six are abundant in nature and represent about 90% of all the identified anthocyanins—for example, cyanidin, pelargonidin, delphinidin, peonidin, petunidin, and malvidin [5]. Figure 1shows some foods that contain anthocyanin. They are complex and have different functions in plants and the human body, mainly due to interactions with several chemical compounds. The hues of these dyes vary between red, blue, and violet colors [ 6 – 8 ]. In addition to providing color in the food industry, anthocyanins also add functionality to Minerals 2023,13, 268. https://doi.org/10.3390/min13020268 https://www.mdpi.com/journal/minerals Minerals 2023,13, 268 2 of 20 foods. These compounds have therapeutic and medicinal features associated with health maintenance, such as antioxidant, antimicrobial, and anti-inflammatory properties [9–12] . Furthermore, regularly consuming anthocyanins can help prevent cardiovascular, endocrine, immunological, and degenerative diseases and reduce cancer incidence [ 13 ]. Due to these properties, anthocyanins are attractive compounds emerging for use in further research and industrial sectors. Molecular modifications can increase the stability of these bioactive compounds to develop new functionalized products which can serve the areas of cosmetics, food, nutraceuticals, molecular biology, and biotechnology [14]. Minerals 2022, 12, x FOR PEER REVIEW 2 of 22 cyanidin, pelargonidin, delphinidin, peonidin, petunidin, and malvidin [5]. Figure 1 shows some foods that contain anthocyanin. They are complex and have different functions in plants and the human body, mainly due to interactions with several chemical compounds. The hues of these dyes vary between red, blue, and violet colors [6–8]. In addition to providing color in the food industry, anthocyanins also add functionality to foods. These compounds have therapeutic and medicinal features associated with health maintenance, such as antioxidant, antimicrobial, and anti-inflammatory properties [9–12]. Furthermore, regularly consuming anthocyanins can help prevent cardiovascular, endocrine, immunological, and degenerative diseases and reduce cancer incidence [13]. Due to these properties, anthocyanins are attractive compounds emerging for use in further research and industrial sectors. Molecular modifications can increase the stability of these bioactive compounds to develop new functionalized products which can serve the areas of cosmetics, food, nutraceuticals, molecular biology, and biotechnology [14]. Figure 1. Main sources of anthocyanin dyes. Anthocyanins are natural water-soluble dyes with the most diverse colors and hues: red, blue, and violet. Under acidic conditions, they show red colors due to the presence of the flavylium ion and change to violet and blue as the pH of the medium increases. These dyes have a complex chemical balance, and colored shapes are susceptible to fading by adding water and auto-oxidation reactions [15]. Several studies, mainly in vitro, have demonstrated a variety of biological properties of anthocyanins, including antioxidant activities [16], anti-inflammatory [17], antibacterial [18] and anti-carcinogenic effects [19], improved vision [20], and cardioprotective effects [21]. Therefore, these bioactive molecules have essential properties and several health benefits, such as preventing cardiovascular and neurodegenerative diseases, diabetes, cataracts, and cancer; strengthening of the immune and circulatory systems; preventing premature aging and cellular oxidative stress; and assisting in vitamin C absorption by the organism. Anthocyanins belong to the flavonoid class, a subset of the polyphenol class. They are compounds found in nature as glycosides of polyhydroxy and poly ethoxy derivatives of flavylium or phenyl lbenzopyrilium salts [22]. Although they have a positive charge on the C-ring oxygen atom of the basic flavonoid structure, also called flavylium Figure 1. Main sources of anthocyanin dyes. Anthocyanins are natural water-soluble dyes with the most diverse colors and hues: red, blue, and violet. Under acidic conditions, they show red colors due to the presence of the flavylium ion and change to violet and blue as the pH of the medium increases. These dyes have a complex chemical balance, and colored shapes are susceptible to fading by adding water and auto-oxidation reactions [15]. Several studies, mainly in vitro , have demonstrated a variety of biological properties of anthocyanins, including antioxidant activities [ 16 ], anti-inflammatory [ 17 ], antibacterial [ 18 ] and anti-carcinogenic effects [ 19 ], improved vision [ 20 ], and cardioprotective effects [ 21 ]. Therefore, these bioactive molecules have essential properties and several health benefits, such as preventing cardiovascular and neurodegenerative diseases, diabetes, cataracts, and cancer; strengthening of the immune and circulatory systems; preventing premature aging and cellular oxidative stress; and assisting in vitamin C absorption by the organism. Anthocyanins belong to the flavonoid class, a subset of the polyphenol class. They are compounds found in nature as glycosides of polyhydroxy and poly ethoxy derivatives of flavylium or phenyl lbenzopyrilium salts [ 22 ]. Although they have a positive charge on the C-ring oxygen atom of the basic flavonoid structure, also called flavylium ion (2-phenyl choro menilyl), they contain a backbone of glycosylated anthocyanidin, which can also be esterified with aliphatic acids and aromatic compounds (Figure 2). The structure of different anthocyanins varies in the number and position of the hydroxyl groups, the degree of methylation of the hydroxyl, the groups attached to the phenolic molecule, and the nature of any aliphatic or aromatic acids attached to the sugar groups [23,24]. Minerals 2023,13, 268 3 of 20 Minerals 2022, 12, x FOR PEER REVIEW 3 of 22 ion (2-phenyl choro menilyl), they contain a backbone of glycosylated anthocyanidin, which can also be esterified with aliphatic acids and aromatic compounds (Figure 2). The structure of different anthocyanins varies in the number and position of the hydroxyl groups, the degree of methylation of the hydroxyl, the groups attached to the phenolic molecule, and the nature of any aliphatic or aromatic acids attached to the sugar groups [23,24]. O+ R7 R6 R5 R3 R5' R4' R3' 8 7 6 54 3 2 11' 6' 5' 4' 3' 2' Figure 2. Anthocyanin structure. Anthocyanidins are grouped into 3-hydroxy anthocyanidins, 3-deoxyanocyanidins, and O-methylated anthocyanidins, while anthocyanins are in the glycoside forms of anthocyanidins and acylated anthocyanins. The most common anthocyanidins are cyanidin, delphinidin, pelargonidin, peonidin, malvidin, and petunidin (Table 1). Acylated anthocyanins are also detected in plants, and acylated anthocyanin is further divided in- to acrylate, co-arylated, caffeoylated, and malonylate [23,25]. Table 1. Main types of anthocyanidins. Name R7 R 6 R 5 R 3 R 5’ R 4’ R 3’ Cyanidin OH H OH OH OH OH H Delphinidin OH H OH OH OH OH OH Pelargonidin OH H OH OH H OH H Peonidin OH H OH OH H OH OCH3 Malvidin OH H OH OH OCH3 OH OCH3 Petunidin OH H OH OH OCH3 OH OH The classification and color diversity of anthocyanins are mainly related to the number and position of the hydroxyl and methoxy groups in their structure and the type and position of sugar groups and acetylating agents [26]. Cyanidin is a reddish-purple dye in fruit berries and reddish colors in vegetables such as red sweet potato and purple corn. Delphinidin has a chemical characteristic similar to most anthocyanidins. It appears as a reddish-blue or purple dye in the plant. The blue hue of flowers is due to the delphinidin dye. Pelargonidin differs from most anthocyanidins in nature; it appears as a red colorant that gives an orange hue to flowers and red to some fruits and berries. Methylated anthocyanidins, such as peonidin, are another type of anthocyanidin found abundantly in plants. They have a visible magenta color, and are widely found in fruits, grapes, and red wines. Malvidin is another O-methylated anthocyanidin, has a visible purple color, is abundant in blue flowers, is the primary red dye in red wine, and appears as a darker "dusty" red. Petunidin is a methylated anthocyanidin present as a dark red or purple dye that is water-soluble and has been detected in blackcurrants and purple flower petals [27–36]. Figure 2. Anthocyanin structure. Anthocyanidins are grouped into 3-hydroxy anthocyanidins, 3-deoxyanocyanidins, and O-methylated anthocyanidins, while anthocyanins are in the glycoside forms of anthocyanidins and acylated anthocyanins. The most common anthocyanidins are cyanidin, delphinidin, pelargonidin, peonidin, malvidin, and petunidin (Table 1). Acylated anthocyanins are also detected in plants, and acylated anthocyanin is further divided into acrylate, co-arylated, caffeoylated, and malonylate [23,25]. Table 1. Main types of anthocyanidins. Name R7R6R5R3R50R40R30 Cyanidin OH H OH OH OH OH H Delphinidin OH H OH OH OH OH OH Pelargonidin OH H OH OH H OH H Peonidin OH H OH OH H OH OCH3 Malvidin OH H OH OH OCH3OH OCH3 Petunidin OH H OH OH OCH3OH OH The classification and color diversity of anthocyanins are mainly related to the number and position of the hydroxyl and methoxy groups in their structure and the type and position of sugar groups and acetylating agents [26]. Cyanidin is a reddish-purple dye in fruit berries and reddish colors in vegetables such as red sweet potato and purple corn. Delphinidin has a chemical characteristic similar to most anthocyanidins. It appears as a reddish-blue or purple dye in the plant. The blue hue of flowers is due to the delphinidin dye. Pelargonidin differs from most anthocyanidins in nature; it appears as a red colorant that gives an orange hue to flowers and red to some fruits and berries. Methylated anthocyanidins, such as peonidin, are another type of anthocyanidin found abundantly in plants. They have a visible magenta color, and are widely found in fruits, grapes, and red wines. Malvidin is another O-methylated anthocyanidin, has a visible purple color, is abundant in blue flowers, is the primary red dye in red wine, and appears as a darker "dusty" red. Petunidin is a methylated anthocyanidin present as a dark red or purple dye that is water-soluble and has been detected in blackcurrants and purple flower petals [27–36]. Legal restrictions on synthetic dyes have driven a growing interest in natural dyes. The side effects of synthetic dyes cause people to be concerned about the risks to their health and the environment [ 37 , 38 ]. Moreover, substituting synthetic dyes with natural ones implies some challenges due to problems related to stability, especially in processing and storage conditions, causing loss of color and biological activity in foods and beverages. Among the effective parameters that can reduce the degradation of dyes such as anthocyanins are oxygen, temperature, light, pH, and composition of the food matrix; for example, the presence of vitamins, proteins, acids, and sugars [ 39 – 42 ]. Several studies have Minerals 2023,13, 268 4 of 20 been conducted involving clays and clay minerals as inorganic host materials to improve anthocyanin stability [43–48]. Clays are compounds consisting mostly of fine-grained minerals: the clay minerals. Clay minerals are phyllosilicates characterized by a layered structure with a dimension on the nanometric scale. The layers can be of the 1:1 type (TO), where a tetrahedral sheet is bonded to an octahedral sheet, or of the 2:1 type (TOT), where an octahedral sheet is located between two tetrahedral sheets, as observed in Figure 3a. Each tetrahedron consists of a central cation of Si 4+ or Al 3+ , coordinated to four oxygens and related to the neighboring tetrahedra through the basal oxygens forming a hexagonal symmetry in the continuous tetrahedral sheet. Each octahedron consists of a central cation of Al 3+ , Fe 3+ , Mg 2+ , or Fe 2+ , linked to six oxygens. The octahedrons connect to the neighboring octahedra through the edges forming the octahedral sheet and presenting hexagonal symmetry [49–51]. Minerals 2022, 12, x FOR PEER REVIEW 5 of 22 Figure 3. (a) Representative models of tetrahedral and octahedral sheets forming the 1:1 (TO) and 2:1 (TOT) layer structures and (b) 2:1 layer with hydrated interlayer cations. Different clay minerals have different structures and physicochemical properties. Therefore, there are apparent differences in the interaction mechanism, carrying capacity, and environmental stability of natural pigment/clay mineral hybrid materials [63–65]. Montmorillonite, palygorskite, sepiolite, and saponite were highlighted among the clay minerals studied. These minerals have different structures and specific physicochemical properties. Therefore, there are some differences in the interaction mechanism with organic molecules, load capacity, and environmental stability of hybrid materials of natural dyes/clay minerals. Compared with synthetic substrates, clay minerals have advantages in forming hybrid pigments: low cost, abundance in nature, ecologically correct, chemical stability, large surface area, and significant adsorption capacity [56,57,66,67]. When dispersed in water, clay minerals, such as smectite or vermiculite, can be delaminated or exfoliated, facilitating the intercalation of organic molecules. Adsorption sites are located on clay minerals' outer and inner surfaces. These materials show a strong affinity for the adsorp- Figure 3. ( a ) Representative models of tetrahedral and octahedral sheets forming the 1:1 (TO) and 2:1 (TOT) layer structures and (b) 2:1 layer with hydrated interlayer cations. Minerals 2023,13, 268 5 of 20 When tetrahedral and octahedral sheets form lamellae, the result can be an electrically neutral lamella or a negatively charged lamella. The negative charge of the lamella is the result of isomorphic substitutions of the tetrahedral (Al3+ for Si4+) or octahedral (divalent cations for trivalent cations) sites. The excess negative charge is balanced by the presence of different cations in the interlamellar spacings, where Na + , K + , Ca 2+ , and Mg 2+ are the most found, as observed in Figure 3b. In the case of clay minerals of the 1:1 type, the lamellar charge is generally close to zero, and in the case of clay minerals of the 2:1 type , the charge is calculated based on the chemical formula. It can vary from 0.2 to 2.0 for montmorillonites and brittle micas, respectively. This configuration influences these materials’ degree of hydration and cation exchange capacity [ 52 , 53 ]. Porosity arises from intraparticle interactions, the interlamellar region, or channels. It is a characteristic closely related to the water content and adsorption capacity of lamellar and fibrous clay minerals [54]. Clay minerals can currently be classified as planar or non-planar. The planars are subdivided into seven groups according to the layer design and interlayer material, such as kaolinite, smectite, and vermiculite. The non-planar clay minerals are subdivided according to the modulated structures. For example, they include tubular halloysite, sepiolite, and palygorskite. In addition, phyllosilicates have varied particle morphologies, such as plates, laths, discs, or fibers [50]. The interaction of clay minerals with different organic molecules depends on factors such as the pH of the medium, molecular charge, structure, and size of the molecule. For lamellar clay minerals, such as montmorillonites, interactions can occur by intercalation, ion exchange, complexation, or solvation. For fibrous clay minerals, these interactions occur mainly on the external surface. However, penetration into the internal cavities of sepiolite or palygorskite may occur. The interaction with dyes and clay minerals has active sites on the external and internal surfaces that facilitate the incorporation of chromophore groups [ 55 – 57 ]. Moreover, in the case of anionic dyes, electrostatic forces govern the interactions, while in the case of anionic dyes, interactions are possible from changes in clay minerals forming organo-clay minerals. These properties, combined with features such as abundance, affordable cost, biocompatibility, surface reactivity, and the possibility of structural modifications, make clay minerals promising materials for various applications and mainly for use in the adsorption processes of organic compounds [58–62]. Different clay minerals have different structures and physicochemical properties. Therefore, there are apparent differences in the interaction mechanism, carrying capacity, and environmental stability of natural pigment/clay mineral hybrid materials [63–65]. Montmorillonite, palygorskite, sepiolite, and saponite were highlighted among the clay minerals studied. These minerals have different structures and specific physicochemical properties. Therefore, there are some differences in the interaction mechanism with organic molecules, load capacity, and environmental stability of hybrid materials of natural dyes/clay minerals. Compared with synthetic substrates, clay minerals have advantages in forming hybrid pigments: low cost, abundance in nature, ecologically correct, chemical stability, large surface area, and significant adsorption capacity [ 56 , 57 , 66 , 67 ]. When dispersed in water, clay minerals, such as smectite or vermiculite, can be delaminated or exfoliated, facilitating the intercalation of organic molecules. Adsorption sites are located on clay minerals’ outer and inner surfaces. These materials show a strong affinity for the adsorption of cationic and anionic dyes from specific chemical modifications. Consequently, dyes, in general, are rapidly adsorbed by clay minerals, and the different species formed—after adsorption—are easily detected by spectroscopic techniques. The degradation process under light occurs when a molecule absorbs energy, promoting radical formations. Through this process, photoproducts and loss of color can occur. In addition, in some cases, they are toxic. In photochemical reactions, compounds derived from clay minerals have been successfully reported in the literature [68–71]. Both the functionalized compound and the type of light source used are decisive in the oxida- Minerals 2023,13, 268 6 of 20 tion of a given product and forming reactive intermediates oxidized by reactive oxygen species (ROS) [ 72 , 73 ]. The use of clay minerals to promote photostability is an alternative. One of the photostability mechanisms is the interaction promoted by the substrate and the clay mineral. Accordingly, to reduce the degradation of natural dyes, further studies are essential in searching for alternatives that enable new sources of stability. Thus, obtaining a final product that is easy to handle, store, and mainly preserve, especially against agents such as light and temperature, assumes great importance in this production context. Due to the growing scientific and technological interest in using natural dyes, the present review aims to investigate the primary sources of degradation and potential mechanisms of chemical/photochemical stabilization of anthocyanin, mainly using clays or clay minerals. 2. Methodology The resulting data for this analysis were obtained from Web of Science, Scopus, and Scielo. The search was done in January of 2023 from 2010. For the systematic search, the following filter topics were used: article title, abstract, keywords, and publication period, and the results are summarized in Table 2. The reference themes used for keywords were anthocyanin, stability, degradation, temperature, pH, protein, light, ascorbic acid, vitamin, oxygen, glucose, clay, and fading. The most diverse combinations with the keywords were made to filter the documents to expand the research field. In addition, this method can lead to a smaller number of specific articles, thus promoting greater relevance on search bases. Table 2. The publications involving the study of anthocyanins. Keywords Web of Science Scopus Scielo Anthocyanin 29,667 30,432 345 Anthocyanin AND degradation 2336 1776 33 Anthocyanin AND stability 3835 2634 43 Anthocyanin AND protection 900 1400 4 Anthocyanin AND degradation AND temperature AND fading 13 8 0 Anthocyanin AND degradation AND pH AND fading 14 13 0 Anthocyanin AND degradation AND protein AND fading 11 6 0 Anthocyanin AND degradation AND light AND fading 15 5 0 Anthocyanin AND degradation AND ascorbic acid AND fading 670 Anthocyanin AND degradation AND vitamin AND fading 4 3 0 Anthocyanin AND degradation AND oxygen AND fading 3 1 0 Anthocyanin AND degradation AND glucose AND fading 1 0 0 Anthocyanin AND clay 65 56 1 Anthocyanin AND clay AND protection 4 1 0 Anthocyanin AND clay AND degradation 3 3 0 Anthocyanin AND degradation AND clay AND fading 0 0 0 The keywords and word combinations used in the search were: anthocyanin, anthocyanin AND degradation, anthocyanin AND stability, anthocyanin AND protection, anthocyanin AND degradation AND temperature AND fading, anthocyanin AND degradation AND pH AND fading, anthocyanin AND degradation AND protein AND fading, anthocyanin AND degradation AND light AND fading, anthocyanin AND degradation AND ascorbic acid AND fading, anthocyanin AND degradation AND vitamin AND fading, anthocyanin AND degradation AND oxygen AND fading, anthocyanin AND degradation AND glucose AND fading, anthocyanin AND clay, anthocyanin AND clay AND protection, anthocyanin AND clay AND degradation, anthocyanin AND degradation AND clay AND fading. 3. Results and Discussion Many articles used only the word “anthocyanin,” and fewer publications were found when only two or three keywords were combined. In contrast, a small number of stud- Minerals 2023,13, 268 7 of 20 ies were obtained by typing combined words in the specific search, including keywords corresponding to the sources of degradation. The keywords “anthocyanin, degradation, protection and fading” were repeated in most combinations. When associated with the keywords “temperature, pH, protein, light, and ascorbic acid,” we obtained a more significant number of publications than when combined with the keywords "vitamin, oxygen, and glucose." Thus, no published articles were found in specific combinations of the Scopus database and Scielo. By combining the words clay and anthocyanin, 65 and 56 articles were found in the Web of Science and Scopus databases. Among the articles, 26 had in common the study of anthocyanin stabilization using clay or clay mineral. The other articles were not related to the purpose of this review. Instead, their study focused on the effects of soil and climate on growing food, the use of mineral fertilizers, improvements in herbicides, and evaluation of soil and plant behavior after adding natural nutrients, processes, and materials used in winemaking. There was a limited number of works involving clay minerals in studies of degradation, fading, or protection of anthocyanin, signaling that the study of clays or clay minerals as a source of protection or stability mechanism of this compound still needs to be explored. According to the publication dates, it was evident that studies containing anthocyanin degradation mechanisms concerning color fading are relatively recent, as publications between 2010 and 2022 accounted for about 80% of the articles (compared to a survey fast, regardless of the publication time restriction). The papers detailed in the survey were classified by the type of degradation source or stabilization mechanism, described in the keywords: temperature, pH, protein, light, ascorbic acid, vitamin, oxygen, glucose, and clay. The following characteristics were highlighted during the classification of the works found in the research: source of anthocyanin, extracting solvent, purification model, characterization techniques, degradation sources, and stability mechanisms. However, the prospective study and the literature review containing relevant studies on anthocyanin-type dyes mainly involved stabilization strategies and trend applications. Table 3shows the studies that investigated the methods for obtaining anthocyanin. The keywords column refers to the sources of degradation used in the combinations used in the systematic search for the papers. The anthocyanin extraction process was described in only some of the publications presented. In some situations, the anthocyanin powder previously obtained was used, which justifies the absence of the extracting solvent and purification in some works [12,74–79]. The most used method for separating and purifying anthocyanins was high-performance liquid chromatography (HPLC). This technique simultaneously separates, identifies, and quantifies anthocyanin dyes without requiring excessive purification of the extracts. Solidphase extraction in C18 cartridges, presented in only one study, is a widespread technique in laboratories due to the relative ease of eliminating impurities, polar and non-phenolic substances, where first, more polar substances than anthocyanins are eluted, such as sugars, acids, and soluble substances, and afterwards the anthocyanin pigments are eluted [ 80 ]. The purple carrot was the source of anthocyanin most used for the experiments. It is preferred for studies of anthocyanin degeneration in beverages [12,74–77]. For extracting solvent, acidic solvents were confirmed, since anthocyanin is dissolved in the cell sap. Its extraction involves using these solvents to break the tissue cell membrane and dissolve the dyes [ 81 ]. The extraction method can influence the amount and safety of extracted anthocyanin, antioxidant activity, and biological potential [82]. The combination of keywords with Oxygen (O) or Glucose (G) had a small number of published articles, and the study could be expanded because it is emerging research. On the other hand, temperature (T), Light (L), and pH (pH) were the most used keywords in the articles because of the greater appeal for the food and beverage industry of the search for a stability mechanism that fights the degeneration of anthocyanins during the manufacture and storage of products [12,77]. Table 4summarizes the main characterization techniques for anthocyanin-type dyes. Minerals 2023,13, 268 8 of 20 Table 3. Summary of anthocyanin extraction and purification methods. Ref. Keywords Anthocyanin Source Extractant Solvent Separation Method [74] T; V; P; pH; AA Purple carrot - - [75] pH; L; AA Purple carrot - - [76] V; P; AA Purple carrot extract - - [83] pH; O; L - - [84] P; pH Grape skin 5 mL of 1% formic acid (v/v) in methanol and sonicated for 3 min Liquid Chromatography HPLC [80] T; L Vanda ‘Sansai Blue’ orchid flower petals Acidified methanol (containing 0.01% HCl) and acidified water (containing 0.01% HCl) C18 cartridge (Water Sep-Pak) [77] V; pH; AA Black carrots, grape juice, and purple sweet potatoes - - [12] T; pH Purple carrot - - [78] L Narcissus pseudonarcissus plant - - [81] pH Purple sweet potato from Vietnam Ethanol solution - [85] pH Purple sweet potato Methanol containing 0.01% HCl Liquid Chromatography HPLC [86] L Crabapple flowers (Malus orientalis) Phenolic extraction solution Containing 70% methanol and 2% formic acid Liquid Chromatography HPLC [79] L; G Jujuba fruit (Ziziphus jujuba Mill.) - - [87] T; pH; L Strawberry juice Citric acid - [88] P; L ‘Red Bartlett’ and ‘Starkrimson’ pears HCl-methanol solution - [89] P; L Pear leaves and fruits 1% HCl-methanol solution (5 mL) - [36] T Chili paprika Acetic Acid 10% (v/v)Liquid Chromatography HPLC Temperature = T, pH, protein = P, luz = L, ascorbic acid = AA, vitamin = V, oxygen = O, glucose = G. Table 4. Summary of characterization methods and stability mechanisms. Ref. Characterization Techniques Source of Degradation [74]UV-visible spectrophotometer; colorimeter; Fluorescence extinction analysis pH, ascorbic acid, temperature, light [75]UV-visible spectrophotometer; colorimeter; Fluorescence measurements Temperature, pH, light, ascorbic acid [76] UV-visible spectrophotometer, colorimetry, visual appearance through digital photography and fluorescence measurements Ascorbic acid, temperature, pH and light [83] pH, oxygen, light [84] Fluorescence spectroscopy, Synchronous Fluorescence Measurement (SF), FTIR Spectroscopy Circular dichroism (CD) spectroscopy, thermal stability test, Spectrophotometer for color analysis pH, Temperature, Light and H2O2 Minerals 2023,13, 268 9 of 20 Table 4. Cont. Ref. Characterization Techniques Source of Degradation [80] HPLC Ethylene, peroxidase enzymes (POD), temperature and light [77] UV-visible spectrophotometer Ascorbic acid, temperature, pH and light [12] Instrumental colorimeter; dynamic shear rheometer; UV-visible spectrophotometer; optical and confocal scanning laser microscopy pH, temperature [81] UV-visible spectrophotometer pH [85] UV-visible spectrophotometer, HPLC pH, temperature [86] UV-visible spectrophotometer, HPLC H2O2 [79] Analysis of staining mechanisms using metabolomics and transcriptomics approaches Light, temperature [87]UV-visible spectrophotometer Temperature, pH and light [88]UV-visible spectrophotometer Light, LAC and POD degrading enzymes [89]UV-visible spectrophotometer Light, COP1 type protein (Photomorphogenic Constituents: PbCOP1.1 and PbCOP1.2) LAC = Lactase Enzyme; POD = Peroxidase Enzymes; PbCOP1.1 = Protein; PbCOP1.2 = Protein. UV-Vis spectrophotometry was the most used characterization tool among the papers researched in analyzing anthocyanins, especially when coupled with techniques such as highperformance liquid chromatography (HPLC), a complementary tool for qualitative analysis. However, UV-Vis spectrophotometry is more useful in quantitative analysis [77,81,85,86]. As previously mentioned, anthocyanins have different extraction methods, based mainly on their polar nature, using different solvents such as ethanol, methanol, and water. Therefore, chromatographic techniques are the most used for purification processes. Additionally, the characterization step of anthocyanins and derivatives is essential for stability studies of these substances. Thus, chromatographic and spectroscopic methods are the most commonly used for the structural identification of anthocyanins. 3.1. Anthocyanin Degradation Mechanisms Anthocyanins are highly susceptible to chemical degradation in aqueous solutions, decreasing their color and biological activities. Their chemical stability is compromised by oxygen, temperature, light composition, enzymes, pH, and food matrices; for example, carbohydrates, proteins, acids, salts, sugars, and minerals [ 90 ]. Anthocyanin degeneration is also influenced by the B ring in the anthocyanidin structure and the presence of hydroxyl or methoxyl groups [ 91 ]. Anthocyanins are sensitive to chemical degradation and color fading in the presence of vitamin C. The potential of three amino acids (L-phenylalanine, L-tyrosine, L-tryptophan) and a polypeptide (e-poly-L-lysine) decreases the degeneration of color in purple carrot anthocyanin extract in beverages. L-ascorbic acid and citric acid with pH 3.0, stored at a high temperature of 40 ◦ C for seven days, were examined. Without amino acids or peptides, anthocyanin degraded at the first-order reaction rate. Adding amino acids or peptides (0.1%) increased the color stability of anthocyanins, with a more significant improvement observed for L-tryptophan [74]. Another study on the effects of ascorbic acid had the presence of flavonoids and sugars in the thermal degradation of anthocyanins in orange juice. Ascorbic acid and sugars significantly accelerated the degradation of anthocyanins, while flavonoids had a protective Minerals 2023,13, 268 16 of 20 Author Contributions: R.C.: methodology and writing—original draft; P.T.: conceptualization, writing—review and editing; T.M.D. methodology; L.M.d.C.H.: methodology; D.H.L.D.: writing—review ; M.G.F.: supervision; S.M.-C.: writing—review; M.d.M.O.C.: writing—review ; E.C.d.S.-F.: writing—review ; J.A.O.: supervision, project administration, and writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by CAPES and CNPq Fellowship. Data Availability Statement: Not applicable. Acknowledgments: The authors thank UFPI and IFPI. Conflicts of Interest: The authors declare no conflict of interest. References 1. Sinopoli, A.; Calogero, G.; Bartolotta, A. Computational Aspects of Anthocyanidins and Anthocyanins: A Review. Food Chem. 2019,297, 124898. [CrossRef] [PubMed] 2. Šamec, D.; Karalija, E.; Šola, I.; Vujˇci´c Bok, V.; Salopek-Sondi, B. The Role of Polyphenols in Abiotic Stress Response: The Influence of Molecular Structure. Plants 2021,10, 118. [CrossRef] [PubMed] 3. Brooks, M.S.-L.; Celli, G.B. (Eds.) Anthocyanins from Natural Sources: Exploiting Targeted Delivery for Improved Health; Royal Society of Chemistry: London, UK, 2019. 4. Fernandes, A.; Brás, N.F.; Mateus, N.; De Freitas, V. A Study of Anthocyanin Self-Association by NMR Spectroscopy. New J. Chem. 2015,39, 2602–2611. [CrossRef] 5. Wallace, T.C.; Giusti, M.M. 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