530 C. Santos-Buelga and V. de Freitas
Among polyphenolic compounds, two types of flavonoids, the anthocyanins and
flavanols (i.e., catechins, proanthocyanidins, condensed tannins), are particularly
relevant to the quality of red wines, as they are key compounds for color definition
and astringency. Other flavonoids such as flavonols may have some influence on
color and bitterness, although they are present in red wines in much lower amounts.
Phenolic acids and hydrolysable tannins, released from barrel wood, may also have
an influence on wine taste and color, and hydroxycinnamoyl derivatives from grape
must are involved in the oxidative browning of white wines together with flavanols.
Besides, some of these perceptions may be modified by other sensory characteristics
(e.g. sourness, sweetness) related to other wine components (Preys et al. 2006).
Wine phenolic composition depends on the original grape and on oenological
practices and storage conditions. Anthocyanins and tannins are located in the solid
parts of the cluster from where they are released during winemaking. Further, they
undergo various enzymatic and chemical reactions as the wine is made and aged.
The new compounds formed often exhibit sensory properties different from those
of their precursors, hence modifying thequality of the wines (Cheynier et al. 2006).
In this chapter we will try to revise what is known about relationships between
phenolic composition and sensory properties of red wine, namely color and astrin-
gency, although it is quite possible that in the end more questions than answers will
be found.
9D.2 The Color of Red Wines
9D.2.1 Origin and Components
The initial purple-red color of young red wines arises from the anthocyanins
extracted from grape skins, while during aging this color shifts to a more reddish
brown hue, mainly due to progressive structural changes of anthocyanins. These
changes occur through different mechanisms that have been revised by Monagas
and Bartolome in a previous chapter of this book. Although these chemical trans-
formations and the corresponding wine color changes have been largely studied and
demonstrated to occur during wine life, the interpretation of wine color in terms
of phenolics continues to be a pending matter, as it is affected by the numerous
factors that influence wine phenolic composition, the own reactive nature of phe-
nolic compounds, the anthocyanin equilibria, and the external and internal condi-
tions of the wines (temperature,oxygen access, pH, acetaldehyde, or SO 2 content)
(Monagas et al. 2006).
The anthocyanins are structurally dependent on the conditions and composition
of the media where they are dissolved and suffer interactions among them and
with other compounds that influence their structural equilibria and modify their
color. Anthocyanins are usually represented as their red flavylium cation, but in
aqueous media this form undergoes rapid proton transfer reactions, leading to blue
quinonoidal bases, and hydration, generating colorless hemiketals in equilibrium
with chalcone structures. The proportion of each form is determined by the pH