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Red cabbage (RC) and purple sweet potato (PSP) are naturally rich in acylated cyanidin glycosides that can bind metal ions and develop intramolecular π-stacking interactions between the cyanidin chromophore and the phenolic acyl residues. In this work, a large set of RC and PSP anthocyanins was investigated for its coloring properties in the presence of iron and aluminum ions. Although relatively modest, the structural differences between RC and PSP anthocyanins, i.e., the acylation site at the external glucose of the sophorosyl moiety (C2-OH for RC vs. C6-OH for PSP) and the presence of coordinating acyl groups (caffeoyl) in PSP anthocyanins only, made a large difference in the color expressed by their metal complexes. For instance, the Al3+-induced bathochromic shifts for RC anthocyanins reached ca. 50 nm at pH 6 and pH 7, vs. at best ca. 20 nm for PSP anthocyanins. With Fe2+ (quickly oxidized to Fe3+ in the complexes), the bathochromic shifts for RC anthocyanins were higher, i.e., up to ca. 90 nm at pH 7 and 110 nm at pH 5.7. A kinetic analysis at different metal/ligand molar ratios combined with an investigation by high-resolution mass spectrometry suggested the formation of metal-anthocyanin complexes of 1:1, 1:2, and 1:3 stoichiometries. Contrary to predictions based on steric hindrance, acylation by noncoordinating acyl residues favored metal binding and resulted in complexes having much higher molar absorption coefficients. Moreover, the competition between metal binding and water addition to the free ligands (leading to colorless forms) was less severe, although very dependent on the acylation site(s). Overall, anthocyanins from purple sweet potato, and even more from red cabbage, have a strong potential for development as food colorants expressing red to blue hues depending on pH and metal ion.

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The color of food is critical to the food and beverage industries, as it influences many properties beyond eye-pleasing visuals including flavor, safety, and nutritional value. Blue is one of the rarest colors in nature's food palette-especially a cyan blue-giving scientists few sources for natural blue food colorants. Finding a natural cyan blue dye equivalent to FD&C Blue No. 1 remains an industry-wide challenge and the subject of several research programs worldwide. Computational simulations and large-array spectroscopic techniques were used to determine the 3D chemical structure, color expression, and stability of this previously uncharacterized cyan blue anthocyanin-based colorant. Synthetic biology and computational protein design tools were leveraged to develop an enzymatic transformation of red cabbage anthocyanins into the desired anthocyanin. More broadly, this research demonstrates the power of a multidisciplinary strategy to solve a long-standing challenge in the food industry.

RESUMEN

The main red cabbage anthocyanins (pigments) are cyanidin glycosides bearing one or two acyl groups derived from hydroxycinnamic acids (HCAs). Through π-stacking interactions with the cyanidin chromophore, the HCA residues have a deep influence on the color expressed and its stability. In this work, a series of non-, mono- and diacylated anthocyanins were investigated in neutral solution (pH 7 and 8), where the pigments exhibit purple to blue colors. Under such conditions, the gradual color loss observed is a combination of two distinct processes involving the cyanidin nucleus: reversible water addition and irreversible autoxidation. By acidification to pH < 2, the colorless forms stemming from water addition (hemiketal and chalcones) are converted to the red flavylium ion, thereby permitting the selective monitoring of the irreversible contribution. The kinetics of color loss and of true pigment degradation could thus be recorded for each pigment. The influence of iron - cyanidin binding and of antioxidants (caffeic acid, N-acetylcysteine) was also investigated. A complete kinetic analysis combining the anthocyanin colored and colorless forms and the degradation products is provided. Overall, it appears that acylation is critical to color stability. For instance, the nonacylated pigment is rapidly bleached as a result of fast water addition and its iron complex is too unstable to provide protection. By contrast, the diacylated pigments are efficiently protected against hydration but much more moderately against autoxidation, which on the other hand is inhibited by efficient iron binding and addition of N-acetylcysteine. Finally, the diacylated pigments are much more resistant to bleaching by hydrogen peroxide (possibly produced by cyanidin autoxidation) and bisulfite (a common food preservative).

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Red cabbage anthocyanin extract contains cyanidin derivatives as various non-, mono-, and di-acylated forms capable of expressing various colors from red to blue. The objective of this study was to develop a simple solid phase extraction (SPE) method to fractionate and segregate di-acylated anthocyanins which have greater stability and more desirable blue colorimetric properties. Two different sorbents with different separation chemistries: SCX (Strong Cation Exchange) and C18 sorbents were evaluated. Red cabbage anthocyanins were fractionated and eluted from the SCX cartridge using combinations of different buffers (pH 6-8) and MeOH (20-70%) or eluted from the C18 cartridge using different percentages of acidified (0.01% HCl) MeOH (30-100%) or EtOH (10-100%). With SCX SPE, washing the loaded pigments with a mixture of buffer pH 6 + 20% MeOH followed by elution with buffer pH 8 + 70% MeOH proved most effective in segregating the di-acylated pigments; however, some anthocyanins were permanently retained by the sorbent. When using the C18 cartridge, washing bound pigments with 32% MeOH or 18.5% EtOH and eluting with 100% alcohol respectively resulted in high (> 93%) recovery of di-acylated pigments. The C18 cartridge with MeOH eluent was the most effective for isolating the target di-acylated red cabbage anthocyanins that produced desirable blue colors with increased stability.

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Purple sweet potato, a source of acylated cyanidin and peonidin derivatives, is commercially available as a food colorant. Our objectives were to determine molar absorptivities (ε), spectral and colorimetric properties of purple sweet potato anthocyanins. Anthocyanins were isolated by semi-preparative HPLC, weighed, dried, and redissolved in acidic methanol or water. Anthocyanins were diluted in pH 1-9; ε, spectra, and color were measured on the methanolic and aqueous solutions. Higher ε were obtained in 0.1% HCl methanol (10,797-31,257 L/(mol × cm)) than in aqueous solution pH 1 (8861-24,303 L/(mol × cm)). Peonidin-3-sophoroside-5-glucoside had greatest ε in pH 1, but in alkaline pH, ε of acylated Peonidin-3-sophoroside-5-glucoside derivatives were greatest. Generally monoacylation decreased ε while diacylation increased ε. Location of acylation also affected ε of two Peonidin isomers (pH 1: 15,999 and 21,011 L/(mol × cm)). All anthocyanins expressed red-pink hues (330°-13.2°) in acidic pH and blues (230°-262°) in alkaline pH.

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Anthocyanins express many hues depending on environmental factors and structural aspects, of which aglycone structure and acylation have been considered most impactful. Effects of different glycosylations on anthocyanins' colors are less known. Twelve cyanidin and 3-deoxy-cyanidin (luteolinidin) derivatives were isolated from red cabbage and black carrot hydrolysates and from black sorghum, diluted in pH 1-9, and analyzed by spectrophotometry and colorimetry. Location, number, and structure of glycosylations affected λmax and spectral shape of o-dihydroxylated anthocyanins, playing important roles on color expression. Generally, glycosylation of cyanidin decreased its λmax (≤27 nm), greatest decreases by 3-monoglycosylation. All cyanidin-glycosides appeared red in pH 1-3 and paled in pH 4-6. However, cyanidin-3-glycosides did not decolor completely like 3,5-glycosides. In alkaline pH, glycosylation patterns affected color more greatly: Cy-3-glycosides expressed maroon-purple hues (300-20°), 5-glycosides were green (100-115°), and only 3,5-glycosides expressed blue (240-250°). Luteolinidin derivatives shifted from yellow to red-purple hues with increasing pH.

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The effects of growth temperatures on anthocyanin content and profile were tested on juvenile cabbage and kale plants. The effects of cold storage time were evaluated on both juvenile and mature plants. The anthocyanin content in juvenile plants ranged from 3.82 mg of cyanidin-3,5-diglucoside equivalent (Cy equiv)/g of dry matter (dm) at 25 °C to 10.00 mg of Cy equiv/g of dm at 16 °C, with up to 76% diacylated anthocyanins. Cold storage of juvenile plants decreased the total amount of anthocyanins but increased the diacylated anthocyanin content by 3-5%. In mature plants, cold storage reduced the total anthocyanin content from 22 to 12.23 mg/g after 5 weeks of storage in red cabbage, while the total anthocyanin content increased after 2 weeks of storage from 2.34 to 3.66 mg of Cy equiv/g of dm in kale without having any effect on acylation in either morphotype. The results obtained in this study will be useful for optimizing anthocyanin production.

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Red cabbage extract contains mono and di-acylated cyanidin (Cy) anthocyanins and is often used as food colorants. Our objectives were to determine the molar absorptivity (ε) of different red cabbage Cy-derivatives and to evaluate their spectral behaviors in acidified methanol (MeOH) and buffers pH 1-9. Major red cabbage anthocyanins were isolated using a semi-preparatory HPLC, dried and weighed. Pigments were dissolved in MeOH and diluted with either MeOH (0.1% HCl) or buffers to obtain final concentrations between 5×10(-5) and 1×10(-3) mol/L. Spectra were recorded and ε calculated using Lambert-Beer's law. The ε in acidified MeOH and buffer pH 1 ranged between ~16,000-30,000 and ~13,000-26,000 L/mol cm, respectively. Most pigments showed higher ε in pH 8 than pH 2, and lowest ε between pH 4 and 6. There were bathochromic shifts (81-105 nm) from pH 1 to 8 and hypsochromic shifts from pH 8 to 9 (2-19 nm). Anthocyanins molecular structures and the media were important variables which greatly influenced their ε and spectral behaviors.

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Red cabbage (Brassica oleracea L.) is an excellent source of food colorant. This study aimed to evaluate the anthocyanin pigment contents and profiles from seven red cabbage cultivars at two maturity stages (8 weeks apart) and evaluate their color characteristics and behavior under acidic and neutral pH. Anthocyanin concentrations ranged from 1111 to 1780 mg Cy3G/100 g DM and did not increase with time. Cultivar and maturation affected pigment profile. Some varieties accumulated ≥30% of diacylated pigments, and proportions of monoacylated pigments decreased with time. Extracts from selected varieties at first harvesting time produced colors similar (λmax = 520 nm and ΔE = 6.1-8.8) to FD&C Red No. 3 at pH 3.5. At pH 7, extracts from the second harvest with s higher proportion of diacylation produced λmax ≃ 610 nm, similar to FD&C Blue No. 2. Cultivar selection and maturation affected color and stability of red cabbage extracts at different pH values.

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