tannin

tannin is a critical structural element in fine red wines and in barrel-aged spirits. It is derived from barrels and any other wood that comes into prolonged contact with a spirit, as well as from wood elements in wine-based spirits such as brandies and pomace brandies, including grape skins, seeds, and stems.

Tannin is a type of polyphenol, and like other polyphenols, it tends to be highly reactive in the mouth and body, as well as in solutions of alcohol. See phenols. When tasted, it creates both bitterness and astringency. Bitterness is a basic taste—the body has its own physiochemical receptors for it, just as it does for sweet, sour, salt, and umami. Astringency is at least partially based upon tannin’s ability to bind with saliva’s proteins, leading to dryness and even increased friction in the mouth, all of which add to a spirit’s perceived weight.

Polyphenolic reactivity also manifests in tannin’s complex bonding and re-bonding with various other compounds. While humans are capable of discerning sensory/tactile differences between these differing compounds, the science is still nascent in being able to predict and define the sensory character of them. Tasters will describe tannins as “sharp” or “fruity” or “sandpaper” or “blocky,” among many other descriptors, and there are numerous theories as to how and why certain tannins might be so perceived, but there is no consensus. Researchers will talk about tannins, catechins, epicatechins, and the like, and undoubtedly each of those elements contributes to mouthfeel. But discussions of “short-chain tannins” (which were once thought to be more intense but now are believed to leave a softer impression) and “long chain tannins” (which also have flip-flopped their reputation) are still more hypothesis than fact. Says Steven Price of ETS Laboratories, “I don’t think we’ll ever really be able to analyze tannins in the laboratory as well as we do in our mouths.” Discussion of the “number” of tannins seems to be pointless as well; there may be billions of structural isomers possible for a single tannin consisting of thirty monomers, according to wine chemistry.

Nonetheless, spirits producers will choose barrels, wood products, and practices based upon their perception of “which” tannins each will impart to their beverage. Whether or not there is a solid scientific basis for these views, most tasters find at least moderate agreement in describing the character imparted by each choice. American oak barrels are often seen to have more aggressive tannins than European oak, but this is far too general a view to be particularly useful in the distillery or warehouse.

For one, while Quercus alba is the primary oak used for American barrel production, there are others, such as Oregon’s Quercus garryana, that offer a differing tannin “profile”. European forests host several species of oak trees: Quercus robur is the most common for French oak production, but Quercus petraea, with its notably finer grain, seems to provide less overt tannins. And specific forests, by providing divergent growing conditions, may exhibit tighter or looser grains and are perceived to express stronger or smoother tannins as a result.

Barrels from the Alliers, Tronçais, or Vosges forests are most often directed toward wine production. Oaks from the Limousin or Monleuzon forests, both showing looser grains, are more commonly discussed in regard to their appropriateness for brandy (cognac and Armagnac, respectively). Moreover, cognac producers will alternate between barrels sourced from the Limousin and Tronçais forests as they seek to either bolster their brandy for the long haul (with Limousin) or create a softer, perhaps earlier-drinking cognac (with Tronçais).

Tannin uptake is also based upon warehouse conditions (particularly humidity and heat) as well as the size and age of the barrels. American oak tends to give up its tannin more quickly than European oak (though this, too, is a rough generalization). And for coopers, a hotter, faster “toast” of the wood will tend to generate more prominent, grittier tannins, while a slower toast may offer gentler tannins.

With every spirit that is barrel aged, there is a point after which the tannins from the barrel eclipse the base material from which the spirit was distilled, and another beyond which all but the most motivated or tannin-insensitive drinkers will find the spirit too astringent to drink. The warehouse manager must recognize when the first point is approaching and also know just how long it will then take before the spirit hits the second. Right now, this remains an art.

Buxton, Ian, and Paul S. Hughes. The Science and Commerce of Whisky. London: Royal Society of Chemistry, 2015.

Jackson, Ronald. Wine Science, Principles and Applications, 4th ed. Amsterdam: Academic Press, 2014.

Jeffrey, David W., Gavin Sacks, and Andrew Waterhouse. Understanding Wine Chemistry. Chichester, UK: Wiley, 2016

By: Doug Frost