The life and times of a tannin molecule

“By contrast, for adapted species, tannins can actually act as feeding stimulants.” (Berbehenn 2011)

They’re an integral part of red wines and even present in some white wines, and if they’re not there when we expect them we complain the wine is flaccid or weak. If they’re too forthcoming we wish we had left the bottle in the cellar a few years more As with every element of the biological product that is wine, tannins have a complex life... and one could even say there’s meaning to that life, starting with their biological purpose in the plant and ending with the purpose we intend them to have in wine aging.

Both of those purposes hinge on (what else?) the chemical structure of the tannin molecule. Tannins are actually a large and diverse family of phenolic compounds with varying, sometimes ambiguous structures. They’re divided into two main types:
- hydrolyzable tannins (smaller, not found so much in grapes, though often in oak)
- condensed tannins (large polymers, common in grapes and in red wine)

Structure of condensed tannins. From Barbehenn (2011)

 

Most of what we discuss in this post will refer to condensed tannins, which are the main contributor to the astringency of red wine—although some hydrolyzable tannins can come from oak barrels, and can contribute bitterness or interact with pigments in their own way.

Condensed tannins are polymers of flavan-3-ol (see the figure to the left, from Barbehenn 2011). The monomer is colorless but polymers are dark-colored. 

 

Tannin-protein interaction. From Santos-Buelga and Freitas (2009)

In short, the chemical effect of tannins is to cause proteins to clump together and fall out of solution.

In slightly more fleshed-out chemical detail:
The key things to keep in mind about this structure are that 1) it’s very large (up to hundreds of kDa in molecular weight); and 2) it has a large number of hydroxyl groups; and 3) it consists of many large hydrophobic rings. The hydroxyl groups are excellent at forming hydrogen bonds with proteins; the hydrophobic nature of the benzene rings interact especially tightly with proline residues in proteins; and because a single tannin molecule is so large, it can condense many proteins together, causing the whole mass to precipitate. (See the figure at right, from Santos-Buelga and Freitas 2009.)

 

A condensed tannin molecule from a wine grape, over the course of its lifetime, plays a variety of different roles: plant protector, wine character enhancer, anti-aging supplement...

 

1) In the plant

Grapes produce tannins not because they have the dimensionality and longevity of our wines in mind, but as an act of self-defense. Tannins are known to be a very important defense system used by many plants against herbivory. In many woody plants, 5-10% of the dry tissue weight can be tannins (Barbehenn); in wine grape skins, tannin levels can vary between 1.76g/kg of berries for (Mourvedre) to almost 3.15 for (Muscat de Hambourg) (Terrier). Some trees have been found to produce more tannins in response to being chewed upon by insects—though this isn’t true for all trees, and I’m not aware of how much we know about grapes in this respect.

There has been some debate about how exactly it is that tannins are bad for insects; for a long time it was thought that the tannins bound up proteins to make them undigestible, but now it’s thought that they may be toxic because of the oxidative stress that they cause—or they may simply taste bad. Anyway, the chemical properties of tannins that make it a good caterpillar deterrent are the same properties that make them essential to good red wine.

 

2) From grape into wine

In the grape, tannins are found in the stems, leaves, seeds, and grape skins—pretty much everywhere except the berry pulp. Tannins take energy for the plant to manufacture, and can get in the way of the plant’s own metabolic processes, so it makes sense to put them only in the parts of the plant that would be the first to be attacked by predators. Consequently, they are only introduced into the wine through pressing and fermenting in contact with those parts—and therefore are relatively less prominent, if present at all, in white wines.

 

3) In your mouth

As mentioned above, tannins bind proteins ferociously, especially those rich in proline. As luck would have it, some 70% of the proteins in your stimulated saliva are in a family called Proline-Rich Proteins. Predictably, they bind to tannins tightly enough that they become unable to do their job, which is to lubricate your mouth. Your tongue is left feeling rough and irritated, a sensation we describe as astringency. Somehow, wine drinkers have come to expect and even desire this sensation, despite the fact that is is probably a remnant of the grape’s effort to prevent animals from consuming it (see the quote from Berbehenn at the head of this article—which I believe was meant to describe insects, but seems to fit humans pretty well).

There are actually two kinds of salivary glands and many kinds of proteins involved, and if you’re interested in finding out all about salivary lubrication and the role of tannins in astringency perception, I recommend reading Gawel’s 1998 review paper (cited at the bottom of this post).

Not all astringency is created equal. Your perception is affected by your own personal salivary system, as well as the pH of your mouth, and of course the exact kinds of tannins in the wine you’re drinking. There is a vocabulary for describing the tannins, some of which you learned on Thursday. In an effort for greater depth, Gawel in 2000 created a “Mouth-feel wheel”—a nod to Ann Noble’s now-ubiquitous Aroma Wheel—though, as Santos-Buelga writes, “this new vocabulary seems to be too extensive, which hampers their implementation in a common language.” As if there were a limit to the extent of description wine drinkers are willing to lavish on their wines...

 

4) In old wine

You may have heard that red wines high in tannins are likely to age well. You may also have seen, on a day you had reason to open a bottle a decade old or older, some dark crusty-looking gunk at the bottom of the bottle. And you may have noticed that the old wine was a sort of a brick red and a shade paler than the inky purple of its younger relatives. These phenomena are all related.

During the process of aging—in the barrel or in the bottle—a little contact with oxygen will lead to the transformation of some ethanol into acetaldehyde. Acetaldehyde can react with flavanols (the monomers of tannins) and with anthocyanins (molecules that are similar in structure but important mainly because they’re responsible for the red color in wine) to form large flavanol-anthocyanin complexes that eventually become so big they precipitate out. This smoothes out the astringency of a young wine, and also reduces the intensity of its red.

 

More fun facts about tannins

The interaction between tannins and proteins that occurs on your tongue also occurs during the tanning of leather. And in fact many tan-like phrases-- leather tanning, the color “tan,” the Tanoak tree, Tannenbaum (the German word for fir)—all arise, originally, from the process by which tannins were extracted from trees and used to cure leather to a light brown color.

Other common tannin-containing foods include tea, some nuts, some legumes, and many fruits (particularly underripe persimmons). Acorns, a staple food for many native Americans in northern California, contain such high levels of tannins that they had to undergo an extensive leaching process before they were edible.

 

References

Barbehenn, Raymond V., and C. Peter Constabel. 2011. “Tannins in Plant–herbivore Interactions.” Phytochemistry 72 (13): 1551–65.

Gawel, Richard. 1998. “Red Wine Astringency: A Review.” Australian Journal of Grape and Wine Research 4 (2): 74–95.

Gawel, Richard, A. Oberholster, and I. Leigh Francis. 2000. “A ‘Mouth-Feel Wheel’: Terminology for Communicating the Mouth-Feel Characteristics of Red Wine.” Australian Journal of Grape and Wine Research 6 (3): 203–7.

Santos-Buelga, Celestino, and Victor de Freitas. 2009. “Influence of Phenolics on Wine Organoleptic Properties.” In Wine Chemistry and Biochemistry, edited by M. Victoria Moreno-Arribas and M. Carmen Polo, 529–70. Springer New York.

Moreno, Juan, and Rafael Peinado, ed. 2012. “Chapter 21 - Chemical Aging.” In Enological Chemistry, 375–88. San Diego: Academic Press.

“Tannin.” 2014. Wikipedia, the Free Encyclopedia. http://en.wikipedia.org/w/index.php?title=Tannin&oldid=593125727.

Terrier, Nancy, Céline Poncet-Legrand, and Véronique Cheynier. 2009. “Flavanols, Flavonols and Dihydroflavonols.” In Wine Chemistry and Biochemistry, edited by M. Victoria Moreno-Arribas and M. Carmen Polo, 463–507. Springer New York.