Colors: Properties and Determination of Natural Pigments

Anthocyanins are water-soluble vacuolar flavonoid pigments. They are responsible for the orange, magenta, violet, and blue color of plant organs, including leaves, stems, roots, flowers, and fruits. Until now, 600 different anthocyanins are known mainly in the form of heterosides. The aglycone form, or anthocyanidin, is a polyhydroxy and polymethoxy derivative of flavylium (2-phenyl-1-benzopyrylium), which is usually bonded to a sugar moiety and can be possibly acylated by a phenolic or aliphatic acid (Figure 1). Among the 23 types of known anthocyanidins, only six are common in higher plants: pelargonidin, cyanidin, peonidin, delphinidin, petunidin, and malvidin. Many factors, that is, pH, heat, light, metals, and copigmentation, may affect their stability, color, hue, and density. Under acidic conditions, the eight conjugated bonds in anthocyanin structures carrying a positive charge on the heterocyclic oxygen ring are responsible for the intense red-orange to blue-violet color. They show a wavelength of maximum adsorption between 465 and 550 nm and a significant absorption in the UV range between 270 and 280 nm. In vitro, under mildly acidic conditions (pH 3–6), the red and stable flavylium cation form predominates, while at pH 6 and above, it changes into the colorless carbinol pseudobase or chalcone forms and finally into the bluer and unstable quinonoidal base form (Figure 2). The larger the number of hydroxyl groups on the B-ring, the bluer the color is. Aromatic acylation causes a blueshift and stabilizes anthocyanins, whereas aliphatic acylation increases the stability and solubility. Selfassociation with flavonoids stabilizes anthocyanins and causes bluing and intensifying of color.