Grapevine & Wine 02/03/2009

Why is wine so annoyingly astringent? It’s the tannins' fault

They are natural constituents of the grape and they represent a stabilizing element for the color and average lifetime of great red wines. The work in the cellar is the key

Tannins are phenolic compounds characterized by highly complex structures commonly found in vegetables. They are a natural compound of grapes too, that they protect from phytophagi and pathogenic agents.
Tannins are extracted from the parings and the grape seeds during the fermentation, but these compounds can be found in the wood, so that the wine increase its tannins content while in barrels or barriques too.
The principal function of tannins in wine is related with color. Indeed, it is the presence of tannins and antocians that either confers the delicate rosé nuances or the stronger colors of the garnet reds.
Another function of tannins consists in contributing to the conservation of the wine, acting as antibacterials and protecting the wine, guaranteeing its longevity.
Not all tannins have a positive effect on wine, though. Grape seeds tannins, for instance, have a bitter taste, so that pressing them without tearing them is recommended.
The concentration of tannins in red wines varies from 1‰ up to 5 ‰, in white wines, that remain in contact with vinasse for a short time, it varies from 0.1‰ and 0.4‰.

Organoleptic characteristics
Tannins have an astringent effect on mucosae, hence they affect not only the color of a wine, but its taste too and its general balance.
They are responsible for the astringency sensations, that hit gingiva. The sommelier design this sensation as “allege”, “tannic”, “little tannic”, lacking. The intensity of these sensation depends on the vineyard of origin: certain varieties of grapes, normally those of lesser quality, have acid, aggressive, sour tannins, that negatively affect the organoleptic features of wines. Noble tannins characterize instead the better productions, being among the most important elements for refining the taste of a wine. The higher the concentration of tannins, the stronger the sourness and astringency of the taste. These sensations evolve during time, in the case of the best tannins, bestowing the right balance on a wine, that is to say less astringency and a full flavor, while maintaining a vivid color in red wines for maturation.

Wood and tannins
The stabilization of a color is traditionally related to the permanence of a wine in a wood container, for times as longer as bigger is the container. It is well known that this process is due to the reactions that occur between the phenolic fraction of a wine and the oxygen that penetrates through the walls of barrels or barriques (micro-oxygenation) or following technical practices such as decanting, topping, ecc.
The barrique, small wooden barrel with a capacity of 225 liters, is becoming more and more popular in the production of great wines over the years. The origins of the pairing wine-wood are extremely ancient and during the centuries, the barrel turned from a means of transportation into an important instrument for the vinification and refining of the wine.
Red wines in general remain in wooden containers for a minimum of six months and up to a maximum of three years (large barrels) or 18 months (barrique). White wines are generally vinified directly into the barrique, where they remain for 6 to 12 months.
The type of wood into which a barrel is built is essential: nowadays, all large barrels and barriques too, are produced in oak wood, but until not long ago, chestnut wood, acacia wood, cherry-wood and ash wood were prevalently used, thanks to their availability and also because the role of certain compounds present in the wood in the evolution and characterization of a wine was not clear. More detailed studies on the influence of gallium tannins in the refinement of wines have shown how important the organoleptic properties of oak tannins are and their low influence on the bitterness.
Another significant factor is the roasting of woods, that consists in a quick burning of the barrique walls, with the purpose of fixating the aromatic substances that will be discharged into the wine. A wine from a barrique can assume extremely pleasant aromas if well calibrated, annoying and excessive in case of a too long refinement by contact with the wood or in those cases when the wine does not have sufficient organoleptic characteristics to balance. Under such circumstances, the final result will be a wine where the wood smells will cover the wine aromas.
These are the reasons why the use of barriques should always be well balanced and require wines deriving from quality grapes, after an excellent and high quality vinification, whose peculiarities could be emphasized by the wood, in order to avoid what a famous winemaker, buddy of mine, calls the tragic “cappuccino effect”, due to the mix between the coffee of the wood and the lactic of the wine.

Enological technology
The use of tannins is possible in any phase of the vinification/maturation process. Undoubtedly, one of the advantages of using exogenous compounds (“in the box”, ready for use), consists in more versatility. In particular, during the maceration, adding tannins protects the anthocians from oxidation during the early extraction phases, subsequently triggering polymerization and stabilization reactions. Experimental trials have demonstrated that , in the case of wines with good structure and suited for long refinements, the treatment by tannins increases the color intensity, which does not occur if not couple to oxygenations, though. Still, it is important to stress that these treatments are of no usefulness for the correction of poor wines, not suited for long maturations.

Wine and cork
The only constituents of cork that can directly interact with wine are the tannins which are partially extracted during the boiling of plance and can be partially released by the cork into the wine.
Cork tannins are hydrolysable, can be extracted in a hydro-alcoholic solution, have a characteristic taste (tannic, astringent), able to affect the organoleptic expression of certain wines.
Nowadays, the study of macromolecular components of cork is still under way, but the detailed knowledge of the different polymeric fractions is a hardly attainable goal, because of the chemical inertia of cork and the high variability of its composition

Tannins and health
It has been recently discovered that tannins have a medical value too, thanks to their excellent cardio-protective qualities. They also have an anti-oxidant effect and can rapidly induce precipitation of lipoproteins which carry cholesterol. This does not mean that wine reduces cholesterol, of course, but it contributes to increase the so-called “good cholesterol”

di Graziano Alderighi