Discovery of a natural cyan blue: A unique food-sourced anthocyanin could replace synthetic brilliant blue

Abstract

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.

Acylated Anthocyanins from Red Cabbage and Purple Sweet Potato Can Bind Metal Ions and Produce Stable Blue Colors

“…Hence, it can be proposed that C7-OH in the complexes was not dissociated at pH 6, and lost its proton when the pH is raised to 8 ( Scheme 2 ). This proposal is consistent with the theoretical visible spectrum calculated on the Al(PB) 3 complex, which, while involving 3 ligands undissociated at C7-OH, were found fully consistent with the experimental spectrum [ 8 ]. Moreover, comparing the neutral base of the 7-O-β-D-glucosyloxy-4′-hydroxyflavylium ion (proton loss from C4′-OH) with that of its 4′-O-β-D-glucosyloxy-7-hydroxyflavylium regioisomer (proton loss from C7-OH) also showed that the former displayed a λ max value that was 20 nm higher [ 14 ] and a molar absorption coefficient almost 3 times as large.…”

“…The flexible acyl residue of P4′ is probably much less apt to develop π-stacking interactions with the cyanidin nucleus than the more rigid sinapoyl residue of PB. Consistently, it has been demonstrated that RC anthocyanins having a single HCA residue at R 1 (P1–P3) are much more susceptible to water addition than PB [ 8 ] as a consequence of the latter adopting folded conformations in which the cyanidin and HCA moieties are in molecular contact. However, PSP pigments have a specific advantage over RC pigments: the presence of caffeoyl residues at Glc-1 and/or Glc-2, which themselves can bind metal ions.…”

“…The λ max values of the complexes were even higher than that of the anionic base. Thus, even if proton loss from C7-OH was not complete at pH 7 for the Al 3+ complexes, the strong π-stacking interactions occurring between the cyanidin nucleus and R 2 = sinapoyl and the concomitant torsion imposed between the B- and C-rings [ 8 ] effectively turned the color to an intense cyan hue ( Figure 1 ) close to the one expressed by the major synthetic blue food colorants Brilliant Blue and indigotine.…”

“…The colorimetric data of the PSP cyanidin glycosides and their Al 3+ complexes ( Table S2 ) provide additional evidence of the bluing effect induced by raising the pH from 6 to 8, or adding increasing Al 3+ concentrations at a given pH. For comparison, a hue angle of 207.3 was recorded for the PB-Al 3+ complex at pH 7, i.e., a close match for that of the synthetic triarylcarbonium colorant Brilliant Blue (h 0 = 209.4), regarded as a reference for a vibrant cyan hue in the confectionary industry [ 8 ]. No such match was observed with the PSP anthocyanins and the best result recorded, i.e., the P6′-Al 3+ complex at pH 8 (h 0 = 221.7) remained off target.…”

“…Indeed, phenolic acyl groups stacked onto the cyanidin nucleus could either directly participate in metal binding (e.g., caffeic acid residues through their catechol ring) or at least strengthen metal binding by building a hydrophobic pocket around the metal–cyanidin complex. Recently, a remarkable RC anthocyanin (called pigment B or PB), displaying a single ideally located sinapoyl residue, was shown to form an aluminum(III) complex of 1:3 stoichiometry in which the three PB ligands in octahedral coordination to Al 3+ adopt a chiral arrangement, causing an intense positive Cotton effect in the visible part of its circular dichroism spectrum [ 8 ]. The Al(PB) 3 complex is strongly stabilized by the π-stacking interactions taking place between each cyanidin chromophore and the sinapoyl residue of an adjacent ligand.…”