**2. Chemical structure of anthocyanins**

Anthocyanins occur naturally in fruits and vegetables as glycosides, having one or more sugar attached to an aglycone nucleus (anthocyanidin). Their aglycones share a C6-C3-C6 carbon skeleton, characterized by the presence of two benzyl rings (A and B) and a heterocyclic ring (C) [24].

**Figure 1.** Chemical structure of the six most common anthocyanidins.

According to hydroxylation and methylations on the different positions of the rings, there are close to 25 different aglycones [25]. They exist in natural products, mainly in a form combined with glucose, galactose, and rhamnose, the more common sugar moieties attached to the aglycone but others sugars are also frequently found, and can be divided into at least six common types, such as pelargonidin, cyanidin, delphinidin, peonidin, petunidin, and malvidin, according to the different substituent groups on flavylium B-ring (**Figure 1**) [26, 27]. The sugars attached to the aglycone may in turn be further linked to other sugars through glyosidic bonds or acylated with organic aromatic or aliphatic acids [28]. One of the most striking properties linked to their chemical structure is that their color changes depending on the pH. They are natural pH indicators; they appear pink at low pH, purple in neutral conditions, and greenish-yellow in basics but the most stable form dominates at low pH [26].
