Colloidal dispersions of tannins in water-ethanol solutions.

The molecular interactions of grape-seed tannins dissolved in water-ethanol solutions have been studied through small angle neutron scattering, light scattering, and physical separation techniques. Through selective precipitation in different solvent mixtures, three populations of tannin macromolecules have been identified: T1 (2% of the total tannin), which forms colloidal particles when the ethanol content of the solvent is brought below phiA = 0.6; T2a (33% of the tannin), which phase-separates below phiA = 0.08 in ionic conditions that are typical of wine; and T2b (65% of the tannin), which remains in solution regardless of ethanol content. Each population remains molecularly dissolved (i.e., it does not form any association structures such as stacks or micelles) until the threshold where dense colloidal particles are formed through nucleation and growth. The colloidal dispersions are metastable, due to the adsorption of organic acids on the particle surfaces; increasing ionic strength and reducing ethanol content cause the loss of this metastability and the aggregation of the particles.

Polar interactions in flavan-3-ol adsorption on solid surfaces.

The adsorption of flavan-3-ol monomers and grape seed procyanidin fractions of different mean degrees of polymerization was studied on three surfaces by means of adsorption isotherms. Reversibility upon dilution was also investigated. These surfaces were three polymeric microfiltration membranes, presenting close Lifshitz-van der Waals components of their surface tension but differing in their surface polar properties. The electron-donor character of the surface was of primary importance for the adsorption of nongalloylated monomers. Increasing the number of phenolic rings above two (galloylated monomers and procyanidins) sharply enhanced flavan-3-ol affinity for surfaces whatever their polarity. However, maximum adsorbed amounts were always much higher on the most polar material. The general trend was a partial reversibility with monomers, whereas an irreversible process was evidenced from the lowest molecular weight tannin fractions. This indicated the formation of multiple bonds with surfaces, in accordance with the high affinity type isotherms. The whole results indicated very different mechanisms in the buildup of the adsorbed layers when the surface electron-donor character varied.

Changes in proanthocyanidin chain length in winelike model solutions.

Reactions of seed and skin proanthocyanidins in the presence or absence of (-)-epicatechin were followed in winelike solutions over 53 days at 30 degrees C. Proanthocyanidins were separated from flavanol monomers by sequential elution from a Sep Pak cartridge, and changes in proanthocyanidin composition were monitored by thiolysis analysis of the proanthocyanidin fraction. In solutions containing no free (-)-epicatechin, trace amounts of monomers were released and important losses of proanthocyanidins were measured, but their average composition and mean degree of polymerization (mDP) were hardly modified. In the presence of (-)-epicatechin, the mDP value decreased and oligomeric proanthocyanidins accumulated throughout the incubation while losses of total units were dramatically reduced. Our data indicate that interflavanic bond cleavage of proanthocyanidins occurred under mild acidic conditions such as encountered in wine and that the resulting carbocation proceeded to unknown species. The latter reaction did not take place in the presence of (-)-epicatechin. Epicatechin added to the intermediate carbocation, thus being incorporated as the end unit of a shorter proanthocyanidin chain. The results of this study are discussed in relation to the loss of astringency reported during wine aging.