What are Tannins?

There are three articles in the two trade journals, Practical Winery & Vineyard and Wines & Vines that we received this month, that focus on tannins. Anna Katherine Mansfield wrote a really good article in the January 2015 issue of Wines & Vines, called A Few Truths About Phenolics which makes the subject of tannins readable and relatively approachable.
Tannins
Tannins are responsible for the astringency we experience because they bind to the proteins located on our tongue. They are also responsible for the mouthfeel associated with high-quality red wines therefore, the understanding of what a tannin is, is important if making quality red wines is the goal.
Tannins are part of a class of molecules that contain a phenolic group, so they may also be referred to as a polyphenolic or also as a flavonoid, specifically a member of the flavan-3-ol class of molecules, due to their chemical composition. The flavan-3-ol molecules found in grapes include (+)-catechin, (-)-epicatechin,
(-)-epigallocatechin and (-)-epicatechin gallate.

From a winemaker’s point of view, there are two types of tannin relevant to winemaking:

• Condensed
• Hydrolysable

The condensed tannins are ogliomers or polymers (short or long chains) of flavan-3-ols. Tannins originate in the skins and seeds of grapes. Although seeds have a higher tannin concentration, they are less extractable, so skin tannin predominates in most wines. This is an important distinction because seed derived tannins are shorter polymers of flavan-3-ols (4-20 subunits) and tastes bitter and skin derived tannins are longer polymers of flavan-3-ols (25-100+ subunits) and contributes to the sensation of astringency.
Hydrolysable tannins are wood-derived and historically found only in barrel-aged wines. Hydrolyzed tannins will be less likely to stabilize color but may promote fuller mid-palate and a softer mouthfeel due to lower, slower protein interaction.

Condensed Tannins	Hydrolyzable Tannins
Derived from mainly from grape skin and some from grape seeds	Derived from wood and histroically found only in barrel-aged wines
Subunits include (+)-catechin, (-)-epi-catechin, (-)-epigallocatechin and (-)-epicatechin gallate	Major subunits include gallic and elagic acids
Binds with protein at a ratio of 20:1	Binds with protein at a ratio of 40:1
Aids in color stabilization by binding with anthocyanins	Less likely to stabilize color
Higher proportion of skin-derived tannins has been found to enhance perception of red wine quality	May promote fuller mid-palate and a softer mouthfeel due to lower, slower protein interaction

The current state of tannin research is complicated. Although some chemical information about tannins is known, the impact of tannins on taste is affected by the wine matrix and it's resultant impact on sensory perception. Acidity in wine increases both the bitterness and astringency, an increase in alcohol concentration reduces the perceived astringency and increases bitterness and residual sugar decreases the sensation of astringency.

References:
1. Anna Katherine Mansfield, A Few Truths About Phenolics, Wines & Vines, January 2015.
2. Douglas Adams, VI257, Lesson 8, pg. 50.
3. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008.
4. Matthew R. Revelette, Ralph S. Yacco and James A. Kennedy, STICKINESS New concept in grape and wine tannin analysis, Practical Winery & Vineyard, January 2015, pg. 32-37.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.