Key tannin properties are:
- Reaction with ferric chloride
- The ability to bind with protein.
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.
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.
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.
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.
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