Anthocyanins (from Greek: (anthos) = flower + (kyanos) = blue) are water-soluble vacuolar flavonoid pigments that appear red to blue, according to pH. They are synthesized exclusively by organisms of the plant kingdom, and have been observed to occur in all tissues of higher plants, providing color in leaves, stems, roots, flowers, and fruits.

Not all land plants contain anthocyanin and in the Caryophyllales, Cactus and Galium mollugo they are replaced by betacyanins.

Anatomically, anthocyanins are found mostly in flowers and fruits but also in leaves, stems, and roots. In these parts they are found predominantly in outer cell layers such as the epidermis and peripheral mesophyll cells. The amounts are relatively large: one kilogram of blackberry for example contains approximately 1.15 gram, and red and black legumes can contain 20 gram per 1 kg. Other plants rich in anthocyanins are chokeberry, cherry, eggplant, blue grape, Vaccinium and red cabbage and also the Usambara-violet. Anthocyanins are less abundant in banana, asparagus, pea, fennel, pear and potato. Most frequent in nature are the glycosides of cyanidin, delphinidin, malvidin, pelargonidin, peonidin and petunidin. Roughly 2% of all hydrocarbons fixated in photosynthesis are converted into flavonoids and their derivatives such as the anthocyanins. This is no less than 10⁹ tons per year.



In plants anthocyanins are present together with other natural pigments like the closely chemically related flavonoids, carotenoids, anthoxanthins and betacyanins.

In still relatively young plants or new growth, where chlorophyll and wax production has not yet begun and which would be unprotected from UV light, anthocyanin production increases. Parts or even the whole plant are colored by these "juvenile anthocyanins," and thereby protected from damage. As soon as chlorophyll production begins, the production of the anthocyanin dye is reduced. The build-up of anthocyanin in plants is specific to the plant type, since it depends on the soil conditions, light, warmth and plant type and/or sort. Plants that have only a single anthocyanin as pigment is extremely rare, but occurs nevertheless. The absence or particularly strong prevalence for a certain anthocyanin in a plant is due to genetic circumstances.

The pigment components of the anthocyanidins, the sugar-free anthocyanins, can be identified based on the structure of a large group of polymethine dye, as salt derivative of benzopyrylium (a cationic form of benzopyran, i.e an oxygen-containing heterocycle pyran fused to a benzene ring). The pyran ring in anthocyanin is connected to a phenyl group at the 2-position, which can carry different substituents. The counterion for the cationic oxygen in the pyran ring is mostly chloride. With this positive charge the anthocyanins differ from other flavonoids. The anthocyanins, anthocyanidins with sugar group, are mostly 3-glucosides of the anthocyanidins. In red wine, especially after maturation, many derivatives of the anthocyanins can be found.

The anthocyanins are subdivided into the sugar-free anthocyanidine aglycons and the anthocyanin glycosides. They are considered secondary metabolites and allowed as a food additive with E number 163. As of 2003 more than 400 anthocyanins had been reported while more recent literature (early 2006), puts the number at more than 550 different anthocyanins. The difference in chemical structure that occurs in response to changes in pH is the reason why anthocyanins are often used as pH indicator, as they change from red in acids to blue in bases.