Let's Geek Out on Tannins

 

Article by Arindam Basu, PhD

What are tannins?

Tannins belong to a group of complex molecules called flavonoids or polyphenols. Plants form tannins as a defense against microbial attacks as well as to prevent pests from eating them. The best known sources of tannin are from tea leaves and rhubarb, whose drying effects on the palate are well known. Other common tannin containing substances are dark chocolate, grape seeds, whole nuts (hazelnut, walnut, almonds), whole spices, and berries.

 
Items containing tannins. Images assembled from Adobe Stock.

Items containing tannins. Images assembled from Adobe Stock.

 

In grapes, tannin polymers are found in the skins, stems, and pips. Extended pressing (or maceration) of grapes, as well as prolonged skin contact between the grape juice and the skins, stems, and pips (the ‘must’) results in higher tannin extraction.

 
Where tannins are located in a grape. Image taken from WSET Level 2 Study Guide.

Where tannins are located in a grape. Image taken from WSET Level 2 Study Guide.

 

As grapes mature and ripen, the “greenness” of the tannins inside decreases, allowing the tannins to become softer and riper. Tannins combine with anthocyanins (pigment compounds) to form polymers. Winemakers often mention short-chain and long-chain tannins, which refers to the degree of polymerization (DP) or the number of tannin subunits joined together. It is believed that shorter chains are more bitter than longer ones. Shorter chains are found primarily in the seeds and the wood, and longer chains are found in the skin of the grapes. The DP of stem tannins ranges from 4 to 28.

 
Molecular tannic structure
 

Tannins produce a sense of astringency in food and drink and form the structure of and contribute to the body of red wine. The term astringency refers to the drying and a puckering sensation in the mouth, and it is a characteristic of mouthfeel of many red wines. Astringency is a tactile sensation (sense of touch), and not a sense of taste, that is brought about by removal of mucins, which lubricate the inner surface of the mouth. Tannins interact with the bitter receptors on the tongue to give a sensation of bitter taste. Tannins reach their most bitter taste at a DP of 4 (four subunits) and become most astringent at DP of 7. Beyond that, they become less astringent. The other structural components of wine such as acidity and alcohol modulate tannin sensation. Higher acid levels in wine increase tannic astringency while higher alcohol levels decrease astringency.

How does saliva and mucins help us perceive tannin?

Saliva contains amongst other substances, two different groups of proteins: proline-rich proteins (PRPs) and histidine-rich proteins (HRPs). These groups of proteins bind to tannins and form precipitates. This aggregation and precipitation of protein-tannin complexes reduce lubricity of saliva and increases friction in the oral cavity – producing the feeling of astringency or dryness. Think of the spit bucket in which wine snobs swirl, swish, and spit wines. The slimy, sticky filamentous strings are tannins that have precipitated after interacting with the proteins in the saliva so that they are not ingested and left to destroy our digestive enzymes.

Now let us put together all the structural components: a higher acidic wine will increase saliva secretion, resulting in more precipitation of tannins, which in turn results in the sense of more drying or puckering (more astringency). Since more tannins are now precipitated, the bitter taste is decreased since there are less tannin polymers to stimulate the bitter receptors on the tongue. Everyone has a different composition, production, and flow-rate of saliva - and to add to the complexity, different hydration levels, emotional states, times of day, and other physiological parameters - that can vastly alter the perception of tannins, aka astringency and mouthfeel, between individuals.

Grapes and tannin levels

Under most circumstances, the amount of astringency depends a lot on the grape varietal.  If you enjoy chewiness, look for higher tannin wines. If you like smoother, easier drinking wines then choose those made from lower tannin grapes.

Varieties with high tannin levels

  • Cabernet Sauvignon

  • Nebbiolo

  • Petit Syrah

  • Petit Verdot

  • Syrah

  • Tannat

Varieties with low tannin levels

  • Pinot Noir

  • Zinfandel

  • Barbera

  • Grenache

  • Gamay

Descriptors used by sommeliers for tannins

More often than not, there are huge differences in opinion as to what should be the proper descriptors for tannins. The first level of differentiation is easy: That is whether the tannin is ripe or unripe (green/stalky). Then comes the next level of complexity. If the tannins are ripe, they can then be classified using words such as velvety, silky, smooth, coarse, and fine-grained. If the tannins are not ripe, they might be described as chewy or grippy. So we not only describe the level (amount) of tannin in the wine, but also the quality or nature of tannin.

This conundrum illustrates well the fact that tannins are a tactile sensation and our sense of touch is much more subjective than objectively knowing how a rose smells or a how a grapefruit tastes, for example.

Tannins and human health

In animals, studies have shown that tannin-rich food is responsible for decreased food intake, digestibility, growth rate, and net metabolizable energy. Certain cancers (such as esophageal cancer) have been reported to be related to consumption of tannin-rich foods such as betel nuts and tea, suggesting that tannins are carcinogenic. However, other studies have shown that the components present in tannins such as polyphenols are responsible for antimutagenic, anti-oxidative effects and scavenging of free radicals. The growth of many fungi, yeasts, bacteria, and viruses has been inhibited by tannins, suggesting antimicrobial properties. These antimicrobial properties are also useful in protecting red wines from bacterial growth.

Tannins have also been reported to exert other physiological effects, such as the acceleration of blood clotting, reducing blood pressure, decreasing serum lipid levels, producing liver necrosis, and modulating immune responses, albeit the dose and the kinds of tannins are extremely critical for these effects. 

References:

  • ‘Tannins and human health: a review’, Chung KT, Wong TY, Wei CI, Huang YW, Lin Y, Crit Rev Food Sci Nutr. 1998; Aug;38(6):421-64. 

  • ‘Phytochemistry of Medicinal Plants’, Saxena M, Saxena J, Nema R, Singh D, Gupta A, J Pharmacognosy Phytochem. 2013; 1(6): 168-182.

  • ‘Wine and Grape Tannin Interactions with Salivary Proteins and Their Impact on Astringency: A Review of Current Research’, McRae JM, Kennedy JA, Molecules 2011; 16,2348-2364; doi:10.3390/molecules16042348

  • ‘Sensorial properties of red wine polyphenols: Astringency and bitterness’, Soares S, Brandão E, Mateus N, de Freitas V, Crit Rev Food Sci Nutr. 2017; Mar 24;57(5):937-948. doi: 10.1080/10408398.2014.946468.

  • The Oxford Companion to Wine, by Jancis Robinson and Julia Harding; 4th ed. Oxford University Press

  • The Science of Wine, by Jamie Goode; 2nd ed. University of California Press

  • Understanding Wine Technology, by David Bird; 3rd ed. DBQA Publishing

 
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