Wine Chemistry and Biochemistry

552 C. Santos-Buelga and V. de Freitas

One of the most important properties of tannins, and why tannins are recognised, is

their ability to interact with proteins. In fact, the term “tannin” has been typically

employed to designate the substances of vegetable origin able of transform fresh

skin into leather, thereby interacting and precipitating proteins of the animal skin.

Perhaps the more practical, functional and understandable definition of “tannin” is

that proposed many years ago by Bate-Smith & Swain (1962) as “water soluble

phenolic compounds having molecular weights between 500 and 3000 and, besides

giving the usual phenolic reactions, having special properties such as the ability

to precipitate alkaloids, gelatin and other proteins”. Nowadays, with the relatively

recent advances in the chromatographic andstructural identification techniques of

tannins, it is generally accepted that themolecular weight range proposed by these

authors is very narrow and that tannin structures might reach many thousands of

Dalton.

The general consensus is that proteins and tannins interact via hydrogen bond-

ing and hydrophobic effects (Fig. 9D.11). Ionic interactions do not seem to be

important at neutral or slightly acidic pH, because under these conditions pheno-

lic compounds are not supposed to be charged as they are weak acids (pKa 9–10)

(Oh et al. 1980; Vernhet et al. 1996). Hydrogen bonding can be established between

Hydrophobic Hydrogen bonds

Interactions

Protein Tannin Protein

a) b)

Fig. 9D.11Schematic interaction between tannins and proteins:amain driving forces between
phenolic rings (cross-linkers) of tannins and the amide groups and apolar side chains of amino
acids such as proline;bprotein-tannin aggregates: thegrey“Ss” represent proteins with a number
of tannin binding places, and theblack arrowsrepresent tannins with protein binding sites