The development of tannins diminishes the astringency of wines.
HOW DOES WINE AGE? Gourmets have long complained of the lack of scientific interest in the role of tannins in the development of red wines. Tannins are astringent substances that are abundant in young wines and that change as the wine ages, giving it an adobe color, smooth taste, and powerful aroma. Over time, tannins are said to soften up. With the aid of modern analysis techniques, Joseph Vercauteren and Laurence Balas at the University of Bordeaux revived a topic of research that had largely been abandoned since the work of Yves Glories in 1976 and elucidated several chemical transformations whose existence had previously been suspected by oenologists.
Tannins are what create the sensation of astringency when one chews rose petals, for example, or drinks a wine that is too young. By forming complexes with the lubricating proteins in saliva they prevent these proteins from playing their natural role, so that the mouth feels dry. Tannins are found in the woody parts of plants as well as fruits. The alcoholic solution that is formed in the process of making red wines, in particular, extracts tannins from the pip, skin, and stalk of the grape.
Thirteen years after Glories wrote his thesis on the “coloring matter of wines,” in 1989, Vercauteren and Balas analyzed the behavior of tannins in the course of vinification. Every other day they took samples from two red wines (Château Cérons and Château Baron Philippe de Mouton Rothschild) in order to monitor variations in tannic concentration. Using ethyl acetate to extract the tannins from these wines, they obtained paradoxical results. Whereas certain oenologists believed that maceration must be prolonged beyond two weeks in order for tannins to accumulate, giving the wine body and a chewy sensation, the Bordeaux chemists discovered that the mass of recovered tannins for the three grapes tested (Merlot, Cabernet Sauvignon, and Cabernet Franc) was greatest around the tenth day of vinification. Why this contradiction? Was it because tannins, being reactive molecules, are transformed into compounds that are difficult to extract when they are put into an ethyl acetate solution?
This initial finding made it clear that a fresh start had to be made, synthesizing the constituent elements of tannins and studying their chemistry in the hope of being able to identify them in the wines. Vercauteren and Balas therefore sought to develop a method for hemisynthesizing condensed tannins, which is to say for creating flavonol-based compounds derived from the parent ring structure of flavan that contain several hydroxyl (–OH) groups. Taxifoliol, extracted from the bark of the Douglas fir, contains several preponderant compounds whose chemical structure was subjected to analysis.
The question needing to be answered, then, was how flavonols bond with condensed tannins. The simple case of flavonolic dimers, formed from the combination of only two flavonols, was already difficult enough, for its two subunits can bond in two different ways. The Bordeaux chemists compared the structure of the synthesized compounds and various natural tannins by nuclear magnetic resonance imaging, but this method did not disclose the two types of bonds. By contrast, transforming the tannins into peracetates by replacing –OH hydroxyl groups with –OOCCH3 acetate groups yielded a general method for determining the structure of condensed tannins.
Armed with this result, Vercauteren and Balas sought next to determine whether flavonols and glycosylated tannins (tannins bonded with sugars) were present in the wines they had selected for examination. They knew that the glycosylation of certain hydroxyl functions stabilizes polyphenols (the class of molecules to which tannins belong), but condensed tannins had never been observed in glycosylated form in either grapes or wine. Nonetheless, they suspected that the diminishing astringency of wines over time resulted from an intensification of tannin–sugar bonds. Resorting once again to a method that had served them well in the past, they synthesized glycosylated tannins, analyzed their characteristics, and then sought to identify these molecules in the wines.
Because the compounds extracted by means of ethyl acetate were rich in two simple flavonols, catechin and epicatechin, the chemists studied its glycosylation using glucose, the sugar found in grape juice. Four glycosylated flavonols were shown to be highly insoluble in ethyl acetate, which corroborated the initial hypothesis: The reason that flavonol and glycosylated tannins had not been found in either the grape or the wine was that the extractive solvents were unsuitable to the task. Subsequent analysis of these hard-to-extract compounds indicated that at least three glycosylated tannins are present in both the grape and the wine, thus establishing that the polymerization and glycosylation of tannins are two of the mechanisms responsible for the aging of wines.