Grape- and oak-derived tannins in wine

The quantity and quality of phenolic compounds (especially tannins, anthocyanins, and co-factors) in the wine post-fermentation, and the interaction of those compounds, are critical elements in the postmodern winemaking process as laid out by Clark Smith. Before delving into these phenolic interactions, some background discussion is in order, beginning with tannins in this post and followed by anthocyanins in a subsequent post.

Key tannin properties are:

• Astringency
• Bitterness
• Reaction with ferric chloride
• The ability to bind with protein.

As shown below, flavan-3-ols are the primary tannins in the flavonoid class of phenolics and are derived from the skin, seed and stem while hydrolizable tannins are primarily oak derivatives. There is some small amount of hydrolized tannin in the flavonoid group that is derived from the fleshy part of the fruit but they are bound with other non-phenolic compounds and play no part in tannin-tannin or tannin-anthocyanin interaction. Hence, flavonoid hydrolyzable tannins are not included in the following discussion.

Flavan-3-ols
Grape-derived tannins are primarily monomers and increase in quantity from fruit set through veraison. It is thought that the primary purpose of these compounds in the plant is as a defense against bacteria, viruses, and higher herbivores. The naturally occuring flavan-3-ol compounds are catechin and epicatechin which register at between 10 and 50 mg/L in white wines and 200 mg/L in reds. Catechin and epicatechin are characterized by a single OH group at position 3 of the C ring (shown below). The formation of the compounds gallocatechin and epigallocatechin is signaled by the presence of three OH groups in the B ring. We can also have a gallic acid acylated at position 3 of the C ring to form catechin-3-o-gallate or epicatechin-3-o-gallate.

Source: ergogenics.de

Tannins have the ability to associate (form long chains; also called polymerization) and grape tannin polymers are called proanthocyanidins or condensed tannins. These condensed tannins are unstable and, in the acidic wine environment, are subject to polymerization, hydrolysis, and depolymerization. A limited degree of polymerization occurs during fruit maturation.

If a tannin is hydrolyzed under the acidic conditions in wine, it can break up into shorter lengths, producing one electron-neutral and one positively charged tannin. The positively charged tannin thus released will react with another tannin or with an anthocyanin. In the case of tannin-tannin interaction, a longer, non-colored polymer is formed. This tannin polymerization continues until the chain is end-capped by an anthocyanin molecule.

Increasing polymerization brings increased polymer size which is quantified by a measure called degree of polymerization (DP). DP increases with wine age, yielding greater wine suppleness and a reduction in astringency. Tannin quality is generally considered to be a function of the degree of polymerization and the level of association with other molecules.

Hydrolizyable Tannins
The journey from oak tree to hydrolyzable tannin is shown in the graphic below.

According to Puech, et al., hydrolizyable tannins contain a polyhydric alcohol (more than one hydroxyl group) as the basic structural unit of which the hydroxyl group has been esterified by gallic and hexahydroxydiphenic (HHDP) acid. The bonds between these units can be easily broken -- through enzymatic action or contact with an acid or base -- to produce free gallic acid and HHDP acid, the latter of which spontaneously converts into the lactone ellagic acid by internal condensation. Oak-sourced tannins are classified as gallotannins or ellagitannins depending on the type of acid formed.

Ellagitannins may comprise up to 10% of heartwood. In the plant, ellagitannins are toxic to micro-organisms and provide the oakwood with a defense against fungal degradation. It differs from lignin by its ability to bind with, and precipitate, alkaloids, gelatins, and other proteins. Ellagitannins may be monomeric (one glucose core) or oligomeric with differences based on the position of the couplings. The most frequent ellagitannin monomers extracted from oak are vescalagin and castalagin while the most important oligomers are roburin A and E, both vescalagin or castalagin dimers, or granidin. Fully 50% of the total ellagitannin in heartwood is unextractable.

Ellagitannins influence the structure of phenolic compounds and red wines by speeding up the condensation of procyanidins while limiting their oxidative and precipitatiive degradation.

Bibliography
Augustin Scalbert and LaPierre Catherine, Ellagitannins and Lignin in aging of spirits in oak barrels, Journal of Agriculture and Food Chemistyr, November 1993.
Bruce Zoecklein, Various Enology Notes
Daniel Kuelder, The influence of commercial tannin addition on wine composition and quality, Master of Agricultural Sciences Thesis, Stellenbosch University, 2006.
James A. Kennedy, Grape and Wine Phenolics: Observations and Recent Findings, Ciencia Investigacion, 35(2), 2008.
Puech, et al., The Tannins of Oak Heartwood: Structures, Properties, and their influence on wine flavor.
Zhentian Lee, Monomeric Ellagitannin in Oaks and Sweet Gums, PhD Dissertation, 2002.


©Wine -- Mise en abyme

Myths and legends: the "taste" of "chalky minerality" in wine

In my most recent post, I presented Alex Maltman's arguments against what he perceived as the myth of minerality in wines as evidenced by the frequency of geologic terms in tasting notes. In a comment on my post, reader Themeperks stated "We've all had wines with chalky, flinty, steely, earthy or other "mineral" characteristics. Putting those things in a wine wouldn't make it taste that way, so it's likely those characteristics arose in complex ways, and not necessarily from the soil being chalky, flinty, steely or earthy." The question here is, basically, if these "tastes" are not mineral, what are they and where are they from? The first term mentioned by Themeperks was chalky and that is the concept that I will address in this post.

Based on the work done by an Australian and French research team (S. Vidal et al., Use of an experimental design approach for evaluation of key wine components on mouth-feel perception, Food Quality and Preference 15, 2004) and reported on in Wine Business Monthly (Bibiana Guerra, Key Wine Components in Mouthfeel Perception, November 2011), we learn that chalkiness is an astringency categorization (along with pucker, adhesive, dry, medium-surface smoothness, and coarse-surface smoothness).

According to sensorysociety.org (and Richard Gawel, Secret of the Spit Bucket Revealed, aromadictionary.com), astringency is a tactile sensation, rather than a taste, and is primarily caused by polyphenolic compounds contained in certain foods (including wine) but can also be caused by acids, metal salts (such as alum), and alcohols. A key characteristic of astringency is the fact that it is difficult to clear from the mouth and, as such, builds in intensity on repeated exposure to the source.

The source of astringency in wines is tannins, "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 (Dr. Bruce Zoecklin, Enology Note #16). 

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.

Tannin 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 but then plateau. Seed tannin release is slow, steady, and long and requires alcohol as a solvent. Tannin extraction will continue throughout fermentation with the ratio tilting in favor of seed tannin at some time during the process (James A. Kennedy, Grape and wine phenolics: Observations and recent findings, Ciencia e Investigacion Agraria 35, 2008).

Once grape tannins are in solution, 80% 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 serve 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.

There are two theories as to how astringency presents in the mouth (sensorysociety.org): (i) Polyphenols bind with the proteins in saliva and the resultant proline-rich proteins precipitate out (Gawel sees these precipitates as being reflected in the "stringy" salivary material that we spit into the buckets at wine tastings), thus reducing the ability of saliva to lubricate the mouth (this loss of "lubricity" is perceived as an increase in oral friction). (ii) The astringents directly effect the oral epithelium with a sensation of harshness presenting when the gums brush against the insides of the mouth. According to both Gawel and sensorysociety.org, individuals with high saliva secretion rates will experience lower levels of astringency.

Gawel alludes to three types of astringency:

1. The feeling of having fine particles on the surface of the mouth; referred to by terms such as Powdery, Chalky (our key word), and Grainy
2. Roughness of feeling inside the mouth; referred to by words such as Silky, Emery, Velvety, and Furry
3. Causing the mouth to move; referred to by words such as Pucker, Chewy, Grippy, and Adhesive.

The foregoing has shown a berry rather than a mineral origin for the astringency type which manifests as a chalkiness on the palate. According to Guerra, this chalky sensation will increase with the addition of tannin and in the presence of anthocyanin and will be reduced with an increase in ethanol and in the presence of acidic polysaccharides.

So you are justified in talking about chalkiness in a wine but will be off-base when describing this sensation as "chalky minerality."


©Wine -- Mise en abyme