One of a family of water-soluble plant pigments that give red, purple and blue to berries, red cabbage, red grapes and many flowers, and that change colour with acidity.
Flavonoid pigments stored in the cell vacuole. In strong acid they exist as the red flavylium cation; as acidity falls, blue quinoidal forms and colourless forms appear, and in alkaline conditions the pigment turns blue-green and degrades. Stability also depends on temperature, light, oxygen and metal ions.
The same pigment can be red, purple or blue. The colour of an anthocyanin-rich food says as much about its acidity as about which pigment it contains.
Red cabbage turns pink with vinegar and blue-green with bicarbonate of soda.
It explains why berry and red-wine colours shift in cooking, baking and ageing, and why natural red and purple food colours are harder to keep stable than synthetic ones.
Anthocyanins are measured by absorbance in a water-based extract, and the reading means little without the pH, because the molecule changes form with acidity. In strong acid it is the red flavylium cation. As acidity falls, blue quinoidal forms and colourless forms take over, and in alkaline conditions the pigment degrades. Wine laboratories use this deliberately: at a typical wine pH only about a fifth to a quarter of the anthocyanin is in the coloured form, so the standard colour intensity of a red wine is read on the wine as it is, as the sum of its absorbances at 420, 520 and 620 nm through 1 cm.
Blueberries, red cabbage or blackcurrants are cooked with bicarbonate of soda, or baked in a batter that is too alkaline.
The pigment shifts from red-purple to blue-green and then degrades towards a dull yellow-brown. A little acid, such as lemon juice or buttermilk, keeps the colour red.
A manufacturer replaces a synthetic red with an anthocyanin extract in a drink or a sweet.
The colour now depends on the product's pH and fades with heat, light and oxygen, and metal ions can shift it towards blue. The recipe and the packaging both have to be redesigned round the pigment.