464 JOURNAL OF COSMETIC SCIENCE However, these stabilization methods were confine to particular cases (e.g., showing sta- bility at a specific low pH or temperature) rather than general cases. Beetroot Red. Beetroot red is obtained from the roots of red beets (Beta vulgaris). Its coloring compo- nents are heterocyclic ring betalains consisting of betacyanins (red-purple) and betaxan- thins (yellow) (Figure 17) (92). Betacyanins and betaxanthins are distinguished according to their chemical structure. The most well-known betacyanin is a betanin 9 that is a betanidin 5-O-β-glucoside (Figure 18). Beetroot red is obtained from the pressing of crushed beets or by aqueous or alcoholic extraction. Direct extraction includes a high- sugar content with a low betalain content. Consequently, the fermentation process may be included to reduce the sugar content and improve the color intensity (56). Betanin is considered the main coloring component, and the raw material should contain more than 0.4% of betanin in the EU. In Korea, isobetanine and betanine are considered as the main coloring components and should contain more than the indicated content in raw materials. Two countries have set limits on nitrate which are high concentrated in beetroots. Beetroot, like other natural colorants, is affected by several external factors: heat, pH, moisture, and metal ions (77,93). Hydrolysis–is known to be affected by several factors such as moisture, heat, and pH–is the most important factor affecting the color shift from red to yellow (77). Betanins (iminium ion) are readily hydrolyzed to betalamic acid (carbonyl compound) and cyclo-Dopa 5-O-β-glucoside (amine) to lose their color under the increased temperature and alkaline conditions (Figure 18) (92). Figure 17. Structures of betalains. Adapted from Rodriquez-Amaya et al. (92). Figure 18. Hydrolysis of betanin. Adapted from Rodriquez-Amaya et al. (92).
465 COSMETIC COLORATION: A REVIEW The fact that betanins are stable between pH 3 and 7 provides a strong advantage ­ compared to anthocyanins (94). To protect betanins from external factors, encapsula- tion methods are being used. Maltodextrin and gum are used as encapsulating agents in freeze- or spray-drying techniques (95,96). Curcumin. Curcumin is obtained from the solvent extraction of turmeric (rhizomes of Curcuma longa L.). Curcumin produces bright yellow color close to Yellow No. 5. The major coloring components are curcumin, desmethoxy- and bisdemethoxy curcumin (56). Curcumin consists of a mixture of tautomers: the enol form (more stable) and the keto form (Figure 19). In the EU’s specification, the content of curcumins should be more than 90%. To gain a concentrated content of curcumins, the extract may be purified further by crystallization. Residual solvents are limited in the EU and Korea. Curcumin has low water solubility however, it dissolves in organic solvents such as acetone, methanol, and dichlorometh- ane. It decomposes rapidly in alkaline solutions (pH 7), although it shows stability in acidic conditions. In a previous report, 90% of curcumin decomposed within 30 min at pH 7.2 (98). The exact degradation mechanism has not been elucidated, however, curcumin is thought to degrade through an oxidation mechanism rather than hydrolysis (99). Moreover, curcumin has been found to decompose from visible light in both its solid and solution states. In a previous study, curcumin powder decomposed after exposure to sunlight for 120 h to colorless compounds: vanillin (34%), ferulic aldehyde, ferulic acid, and vanillic acid (0.5% each) (97). Again, to enhance curcumins’ stability, encapsulation by freeze-drying and spray drying is the most used technique (100,101). In summary, this report discussed colorants from natural origins that are included on the positive lists of cosmetic regulation. There is no widely accepted definition of natural colorants. The use of natural colorants has increased as the clean beauty trend has become more popular. Customers believe that compared to synthetic colorants, natural colorants have greater potential benefit to public health. However, the most important problem and main drawback of natural colorants are their instability issues. To overcome these limitations, the encapsulation technique using spray- or freeze-drying methods has been studied. Natural colorants are defined ambiguously and are relatively loosely regulated and studied compared to synthetic colorants. First, a correct definition of each natural colorant is required (e.g., Cu-chlorophyllin, anthocyanins). Second, permitted origins of Figure 19. Tautomerization of curcumin. Adapted from Jankun et al. (97) and Delgado-Vargas et al. (56).
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