Sunday, March 11, 2012

The role of tannins in wine balance

Wine quality, as perceived by the consumer, has a number of elements, an important one of which is wine balance.  I have defined wine balance in a previous post but will reproduce Dr. Bruce Zoecklein's (Virginia Tech Enologist) Palate Balance Equation here to highlight the included elements.

              SweetAcid + Phenolics (Astringency and Bitterness).

I have treated the sweet (alcohol) and acid elements of this equation in prior posts and will focus on the phenolics element in this post.  The placement of phenolics (tannin) and wine balance in the quality assessment framework is indicated by the arrow in the figure below.

So what are tannins?  According to the indefatigable Dr. Zoecklein (Enology Note #116), tannins are "a heterogeneous group of phenolic compounds" with properties to include: astringency (caused when the tannin binds with protein in saliva; evidenced by mouth pucker and a bitter aftertaste); bitterness; the ability to react with ferric chloride; and the ability to bind with proteins.  A key characteristic of phenols, according to Dr. Zoecklein, is the ability to associate with (polymerize, in scientific lingo) themselves and other compounds thus yielding larger molecules.  The degree of polymerization (an actual metric) tends to increase with the passage of time.

There are two main types of tannins: hydrolizable and condensed (more properly called proanthocyanidins).  Hydrolizable tannins are found in the bark of oak and other plants and are formed in the growing tree for the purpose of food storage. These tannins are called hydrolizable because they can be broken down into smaller components in the presence of an acid or water.  Condensed tannins are insoluble and are found in tea, pomegranates, and the seed, skins, and stems of grapes.

Grape tannins are a combination of compounds (cathecin, epicathecin, epigallocathecin) which link up in chains and of which at least two need to be present for the compound to be termed a tannin. Seed tannins weigh, on average, 3.5 - 5mg per berry while skin tannins weigh in between 0.5 and 0.9 mg.  Seed tannin polymers are shorter than skin tannin polymers (the longer the tannin chain the higher the astringency) yet seed tannins are perceived by winemakers to be harsher, greener, and more astringent than skin tannins and that is evidenced in the way that the berry is handled once it enters the winery.  Oak tannins are astringent in tree matter and need to be seasoned and toasted -- as a part of the barrel treatment -- in order to increase their usefulness.

Grape tannins accumulate during the first period of berry growth with skin tannin synthesis beginning earlier than seed tannin synthesis and then ending with the conclusion of the first phase of growth. Seed tannin synthesis continues into the early period of berry ripening before concluding.  Both skin and seed tannins continue to mature during the berry ripening phase.

Tannins release is a function of how the grapes are handled in the winery.  Tannins are not desirable in white wines so white wine grapes are pressed lightly and there is no contact between the juice and skin.  In the case of red wines, tannins are desirable for color, mouthfeel, and aging and there is extensive skin contact. The amount of tannins released are a function of skin thickness (Cabernet Sauvignon, Nebbiolo, and Syrah are thick-skinned and thus release more tannins than do thin-skinned varieties like Pinot Noir, Gamay, and Cabernet Franc), the length of maceration, and the number of cap punch-downs to which the must is subjected.  Skin tannins release early and easily (as they are water soluble) but then plateau.  Seed tannin release is slow, steady, and long and requires alcohol as a solvent.  Once grape tannins are in solution, 80% of them undergo one or the other of the structural changes listed below:
  • Tannin-tannin linkages
  • Oxidative change
  • Acid-catalyzed reactions
  • Bind with anthocyanins (color compounds) to form polymeric pigments that ensure long-term color stability
  • Formations that provide structure and mouthfeel to wines.
Grape tannins provide color, flavor, structure, and texture to the wine and serves a preservative function.  Oak tannins play an essential role in wine maturation in that they: (i) promote oxidation products (react with oxygen in the presence of a transitional metal to release activated oxygen which, in turn, oxidizes alcohol to acetaldehyde);  (ii) produce astringency; and (iii) aid in the removal of off-notes.  Tannin-anthocyanin complexes sediment out of wines as they age resulting in browner, less tannic wines.

Tannin affects wine balance in the following ways:
  • The lower the tannin levels, the greater the amount of acidity the wine can support; conversely, the higher the tannin levels, the lower should be the acidity
    • High-acid, high-tannin wines tend towards astringency
  • Too much tannin results in wines that are heavy on the palate, lacking in finesse, and possessing a rough finish
  • Increasing alcohol content increases the intensity of bitterness and decreases the sensation of astringency
  • Low alcohol levels will result in dominant acidity and astringency and harsh, thin wines
  • Lowering wine pH increases the astringency of the tannins.
The winemaker has a number of tools at his/her disposal to increase/reduce tannin levels as required and I will discuss those tools in a follow-up post.

© Wine -- The View From Orlando

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