Wine Chemistry and Biochemistry

(Steven Felgate) #1

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

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