554 C. Santos-Buelga and V. de Freitas
9D.3.3 Tannin Structures and Their Influence in Wine Astringency
Tannins are ubiquitous compounds widespread in the plant kingdom, and are clas-
sically divided into hydrolysable tannins and condensed tannins according to their
chemical structure. Condensed tannins are abundant in the different parts of the
grape (seeds, skins and stems) and areextracted to wine during winemaking.
Hydrolysable tannins are absent in grapes, and it is generally accepted that they pass
to wine from oak during aging in barrels (Clifford and Scalbert 2000). However,
as already referred to above, only a few mg/L of hydrolysable tannins have been
reported in red wines (Fernandez de Simon et al. 2003; Perez-Prieto et al. 2003).
Hydrolysable tannins are composed of polyols (such as glucose and quinic acid)
linked to at least one gallic acid (gallotannins) or one hexahydroxydiphenic acid
(ellagic tannins). Condensed tannins (also known as proanthocyanidins) are poly-
mers of flavan-3-ols. Flavan-3-ol units are usually linked through C-C interflavanol
bonds established between the C4 of one flavan-3-ol unit and the C8 or C6 of another
unit. There are also tannins with an additional ether linkage between the C2 of the
upper unit and the oxygen-bearing C7 or C5 of the lower unit, in addition to the usual
C4-C8 or C4-C6 interflavanol bond. However, only traces of that type of tannins
have been reported in grape and wine (Krueger et al. 2000; Vivas de Gaulejac 2001).
Condensed tannins can be found in grape with different degrees of polymeriza-
tion with two or more units of flavan-3-ols,reaching as high as 80 units of flavanol.
Degrees of polymerization (DP) between 3–83 and 2–16 have been reported in grape
skins and grape seeds, respectively (Souquet et al. 1996). Red wine might contain
up to 4 g/L or even more of condensed tannins and their degree of polymerization
change continuously during aging as a result of the chemical transformations.
Mouth-feel properties of tannins-rich solutions depend on tannin structure and
concentration (Noble 1990; Hagerman et al. 1998; Wroblewski et al. 2001). The
perception of the astringency of condensed tannins increases with tannin size and
degree of galloylation (Vidal et al. 2003a) as does their ability to complex with pro-
teins probably because they have more interaction sites (Bacon and Rhodes 1998;
de Freitas and Mateus 2001; Baxter et al. 1997). However, the influence of the big-
ger structures of condensed tannins in astringency is controversial. Some authors
believe that these complex structures (mean DP>7) are not enough soluble to be
astringent (Lea 1990), while others have worked with highly polymerized struc-
tures of proanthocyanidins (mean DP up to 70) and have observed that they were
soluble in water-alcohol solution and highly astringent (Cheynier et al. 2006;
Vidal et al. 2003a).
Although they were still able to bind proteins efficiently, the bigger struc-
tures of condensed tannins have been shown to have more difficulty to bind pro-
teins, which has been attributed to the decrease in their flexibility (de Freitas and
Mateus 2002; Siebert et al. 1996). Effectively, it has been reported that more flexible
condensed tannins have better ability to bindproteins because they are more efficient
cross linkers (Lea 1992; de Freitas and Mateus 2001; Goldstein and Swain 1963;
Frazier et al. 2003; Okuda et al. 1985).