Polymeric tannins

Polymeric tannins are complex molecules comprising flavan-3-ols or catechins, and called "procyanidins" 48, 49a-d ... 

The conversion of animal hides into leather by treatment with water-soluble plant extractives has been practiced since antiquity. This process became known as tanning and obviously involved the reaction of a naturally occurring extractive, tannin, with the protein in the hide. We now know, of course, that tannins comprise a whole spectrum of chemical compounds, but generally they are polyphenolic and polymeric. Tannins have been isolated from a wide variety of raw materials, including insect galls, fruit skins, seed hulls, leaves, bark, and heartwood. Indeed, tannins are of nearly ubiquitous occurrence in higher orders... 

Aldehyde-tannin and aldehyde-anthocyanin condensation reactions result in polymer formation (Figure 1). These polymers may be responsible for haze formation in wine and the polymers may eventually precipitate out of solution (26). The polymerized tannins have different flavor properties than the monomeric starting units (21-29) and formation of anthocyanin polymers affects wine color. In addition, these reactions may result in a reduction of aldehyde flavors in the wine. These condensation reactions are discussed more fully in other chapters of this volume. The formation of strong covalent bonds between the aldehyde and the tannin or anthocyanin makes recovery of the bound aldehydes difficult. 

Polymeric tannins have more complicated structures including esterified oligosaccharide chains for the hydrolyzable ones or further polymerization of the flavonoid units for condensed ones. Because of their diversity including small molecules and polymers, characterization of natural tannins is not simple. The first step in their characterization is isolation. This has been done using several procedures such as ultrafiltration and gel permeation chromatography [2]. 

In many European wine growing areas where large quantities of tannins are extracted during the winemaking process, the wines are turbid, unrefined, and aggressive, requiring several years aging to clarify and acquire finesse. This is the only case in which precipitation of polymerized tannins makes the wine softer. 

A long period of bottle aging involves a set of stripping reactions that continue until the wine has finished developing. These reactions canse the polymerization of tannins and anthocyanins (Section 6.3). It is also possible to envisage associations in the form of micelles that become hydrophobic and precipitate, even if the polymerized tannin molecnles are smaller than 100 A. 

In practice, these various conditions must be taken into account in aging red wines, and modulated according to the type of wine. Aeration is desirable at the beginning, to degas the wine and promote the stable, purplish-colored combinations with ethanal that develop from only slightly polymerized tannins. Aeration should then be reduced to maintain a high oxidation-reduction potential, as this is favorable to the evolution of tannins. Aeration should... 

At the phylogenetic level of the plants, it appears to catalyze the formation of intermediates in biosynthetic systems which produce the flower pigments and related flavonoids, the lacs and lacquers, the simple and polymeric tannins and their esters, the phenolic alkaloids, the quinones, tropolones and simple plant melanins, and the lignins. At higher phylogenetic levels the phenolase complex catalyzes intermediate phases in the... 

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