Anthocyanins are natural pigments belonging to the flavonoid family and are responsible for a wide range of colors in fruits, vegetables and flowers.
They are polyhydroxy or polymethoxy derivatives of 2-phenyl-benzopyrylium. Many are present in plants attached to sugars as mono, di or triglycosides by α- or β-linkages (Marta et al., 2009).
There are more than 500 anthocyanins in nature, and the most common anthocyanins in plants are cyanidin (Cn), pelargonidin (Pg), peonidin (Pn), delphinidin (Dp), petunidin (Pt) and malvidin (Mv), which have a 3-glycoside structure (Ali et al., 2016).
There are four species of anthocyanin. The quinoidal base is blue, the flavylium cation is red, the pseudobase or carbitol is colourless, and the chalcone is also colourless. These four species exist in equilibrium, and the colour of the anthocyanin compound depends on the ratio of these species in solution. In acid solutions they are red, but the intensity of the colour decreases with the rise in pH. Freshly made neutral or alkali anthocyanin solutions are violet or blue, but their colour fades slowly. An increase in the amount of the pseudobase or the chalcone species gives rise to colourless solutions (Torskangerpoll and Andersen, 2005).
Due to the urgent need for natural and healthy pigments in the food industry, the exploration of anthocyanins from edible plants is receiving increasing attention (Araceli et al., 2009).
Stability Of Anthocyanins
Temperature degrades the colour of anthocyanins by favouring formation of the chalcone species. Structural features can reverse this change in equilibrium, but usually heating results in the colour of anthocyanins fading, or going colourless, due to the change in equilibrium towards chalcone formation, at the expense of the other three species (Brouillard, 1982).
Ali, H.M., Almagribi, W., Alrashidi, M.N. (2016) Antiradical and reductant activities of anthocyanidins and anthocyanins, structure-activity relationship and synthesis. Food Chemistry. 194 pp. 1275–1282. doi: 10.1016/j.foodchem.2015.09.003.
Araceli C, Madelourdes PH, Maelena P, Joséa R, Carlosandrés G. Chemical studies of anthocyanins: A review. Food Chemistry. 113 pp. 859–871. doi: 10.1016/j.foodchem.2008.09.001
Brouillard, R. (1982) Chemical structure of anthocyanins. In: P. Markakis, Editor, Anthocyanins as food colours. Food science and technology, Academic Press, London (1982), pp. 1–40.
Marta, Á., et al. (2009) Bioconversion of anthocyanin glycosides by Bifidobacteria and Lactobacillus. Food Research International. 42 pp. 1453–1461. doi: 10.1016/j.foodres.2009.07.026.
Torskangerpoll, K. and Andersen, O. (2005) Colour stability of anthocyanins in aqueous solutions at various pH values. Food Chemistry, 89 (3), pp. 427-440