Chapter 9D
Influence of Phenolics on Wine Organoleptic Properties
Celestino Santos-Buelga and Victor de Freitas
Contents
9D.1 Introduction . . . 529
9D.2 The Color of Red Wines . . . 530
9D.2.1 Origin and Components . . . 530
9D.2.2 Copigmentation . . . 531
9D.2.3 Changes During Wine Maturation and Aging . . . 541
9D.3 Astringency . . . 550
9D.3.1 Astringency Perception . . . 550
9D.3.2 Interactions Between Proteins and Tannins . . . 551
9D.3.3 Tannin Structures and Their Influence in Wine Astringency . . . 554
9D.3.4 Protein Structures . . . 555
9D.3.5 Changes in Tannin Composition and Astringency During Wine Aging . . . 556
9D.3.6 Polysaccharides and Astringency . . . 557
9D.3.7 Experimental Studies of the Interactions Between Proteins and Tannins, and Astringency . . . 559
References . . . 560
9D.1 Introduction
In his reference paper about the polymeric nature of wine pigments published in 1971, Somers stated “the chemistry of wine phenolics, which entirely responsible for wine colour and tannin character, is still largely a matter of speculation”. Although relevant advances in the knowledge of the structures of wine pigments and tan- nins and the reactions taking place during aging have been made since then, that statement still applies. Many gaps still exist regarding the precise nature of the compounds and mechanisms that determine red wine color expression and astrin- gency perception and their changes during wine life. These aspects have not been fully elucidated and their control continues basically as an empirical matter and still constitutes “a major challenge in oenological research” (Somers 1971).
C. Santos-Buelga (B)
Unidad de Nutrici´on y Bromatolog´ıa, Facultad de Farmacia, Universidad de Salamanca, Campus Miguel de Unamuno s/n, E-37007 Salamanca, Spain
e-mail: [email protected]
M.V. Moreno-Arribas, M.C. Polo (eds.), Wine Chemistry and Biochemistry,
DOI 10.1007/978-0-387-74118-5 20,C Springer Science+Business Media, LLC 2009
529
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 the quality 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 SO2content) (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