**2. The colored world of anthocyanins**

The anthocyanins are the main pigments in many flowers and fruits, e.g. grapes. They are not only responsible for red color of grapes but for a great diversity of the colors (red, violet and blue) found in nature.

Five anthocyanins are particularly abundant in grapes, cyanidin, delphinidin, malvidin, peonidin, and petunidin. Individually, the color of each anthocyanin is determined by its structure, as an example, those anthocyanins with two substituents in the B ring present orange hues, while those with three substituents present more of a red-purple color. Also, the existence of methoxyl groups in the B-ring affects color: the higher the number of methoxyl groups, the higher the displacement toward purple hues (**Figure 1**) [5].

In aqueous solution, the color of anthocyanins is strongly dependent on the pH [6]. At very acidic pH (pH~1), anthocyanins are present in their flavylium cation form which has a red color. When the pH is raised to 3–4, the flavylium cation is

**Figure 1.**

*Chemical structure of the main anthocyanins found in grapes and wines.*

*Anthocyanins and Wine Color: A Complex Story DOI: http://dx.doi.org/10.5772/intechopen.105162*

**Figure 2.** *pH related structural changes in anthocyanins.*

involved in two parallel reactions in equilibrium: deprotonation to form the violet quinonoidal base and hydration at the C-2 position yielding a non-colored hemiketal form. These forms are also in equilibrium with the cis- and trans-chalcone forms which present a yellow color. With a pH rise to values up to 6, the quinonoidal base can be ionized to form the respective blue anionic quinoidal form (**Figure 2**) [6, 7].

Since the flavylium anthocyanin structure is highly unstable due to its deficiency on electrons the free forms of anthocyanins are usually found as glycosylated forms in nature, as these are more stable. Most anthocyanins are 3-O-glycosides and the most common sugar moiety is D-glucose. It has been reported that the stability of anthocyanins increases with the number of methoxyl groups linked to the B-ring, and decreases with the number of hydroxyl groups linked to this ring. Thus, the most stable grape anthocyanidin is malvidin, whereas delphinidin has been shown to be the least stable [8]. Furthermore, the presence of acylation (esterification of the sugar moiety by different organic acids) in the anthocyanin structure also increases its stability and acylated anthocyanins are more resistant against pH changes than the corresponding monoglucosides.

#### **2.1 Localization of anthocyanins in the grape berry and evolution during ripening**

These compounds are located in the grape skins, in the first three or four cell layers of the hypodermis and exceptionally in the pulp, in the "teinturier" grapes [9, 10]. At the subcellular level, these water-soluble molecules are found normally within the vacuoles where they can be accumulated in spherical vesicles called "anthocyanoplasts" or "vacuolar anthocyanin inclusions" [11].

Anthocyanins appear in grapes at veraison and accumulate during maturation, generally reaching their highest value around technological maturity, and then decreasing. This decrease usually coincides with the overripening of the grape [12–14]. This maximum accumulation depends on the environment, climatic conditions and the variety.

The variability in grape anthocyanin composition, due to their structural diversity allows to use anthocyanins for discriminating varieties and species. The composition of anthocyanins has led to grape and wine classifications established according

to their content in acylated and non-acylated anthocyanins. Various authors have statistically established the classification of varieties according to their anthocyanin profile, applying multivariate analysis to verify varietal origin [15–21] or try to objectively distinguish the origin of the wines. Within a variety, anthocyanin levels may vary depending on the temperature, on the insolation conditions, and also on the terroir [21]. Also viticultural practices such as the system of fertilization or irrigation can modify the composition anthocyanin [22–25] but certain features of their anthocyanin profile can be tightly linked to variety.

#### **2.2 Transference of anthocyanins from grape to must-wine**

During the normal fermentation of red grapes, the grapes are destemmed and crushed without breaking the seeds. The must, along with the skins and seeds is inoculated with an active dried yeast culture and left to ferment. At some point (decided by the enologist) the wine is racked off the skins and the skins are pressed to recover the wine captured in the skins.

The anthocyanins that accumulate in the vacuoles are released from the cells when these grape cell walls are mechanically broken during crushing. Anthocyanins are not stable compounds. Right after these compounds are extracted, they start to change, due to the participation of these compounds in various reactions.

The question is, why the aged wines are still red if anthocyanins are such unstable compounds? Which reactions are occurring in the must and wine that assure the red wine color in the long term?
