**2. Type and structure of anthocyanins**

Anthocyanins are kinds of water-soluble natural pigment of flavonoids, and its molecular glycosylation type, position, and hydroxyl number are the main basis for distinguishing different anthocyanins. Right now, there are more than 20 kinds of anthocyanins known in plants [3], whose basic carbon skeleton is C6-C3-C6 with cationic structure of 3,5,7-trihydroxy-2-phenyl benzopyrane (**Figure 1**). Six anthocyanins of them are the most common, including *Centaurea cyanus* L. *anthocyanin*, *Consolida ajacis* (L.) Schur *anthocyanin, Petunia hybrida* (J.D. Hooker) Vilmorin *anthocyanin, Pelargonium hortorum Anthocyanin*, *Paenonia lactiflora Pall anthocyanin*, *and Malva sinensis* Cavan *anthocyanin* (**Figure 2**). The hydroxyl group in anthocyanins exists in the form of cationic ions in the cell solution in a lower pH value and shows a strong antioxidant capacity [4, 5]. Anthocyanins are highly contained in a lot of plants and natural medicinal materials, including grapes, apples, hawthorn, tea, peanuts, purple potatoes, and ginkgo. In addition, proanthocyanidins are also found in grape juice, red wine, chocolate, and beer [6]. Proanthocyanidins can convert into the anthocyanins by heating in an acidic medium. Under natural conditions, free anthocyanins are extremely rare and often exist in combination with a single or multiple glucose, rhamnose, or galactose to form glucoside, namely, anthocyanins [7]. It is in fact that anthocyanins are the primary pigment group of plant color, which produces a wider range of colors, ranging from light yellow to violet [8]. The double bonds and polyhydroxyl structure of anthocyanins are the foundation of their antioxidant activity.

**Figure 1.** *The basic structure of anthocyanin.*
