492 N. Terrier et al.
flavanol monomers with proteins follows the same order as their partition coeffi-
cients between octanol and water, meaning that it increases with hydrophobicity of
the phenolic compound (Poncet-Legrand et al. 2007). Phenolic oxidation, gener-
ating polymeric species, resulted in enhanced protein interactions as evidenced by
higher inhibition of enzymes (Guyot et al.1996a), or changes in casein adsorption
properties at the air/liquid interface (Sausse et al. 2003). As mentioned above, higher
molecular weight flavanols are also selectively precipitated out by proteins. More-
over, within gelatins (Maury et al. 2001) or glutens (Maury et al. 2003), smaller
molecular weight proteins appeared more selective than larger ones. The interaction
mechanism also depends on protein concentration. At low concentration, it occurs
in three stages as the polyphenol/protein ratio increases, as described above: sat-
uration of the interaction sites, formation of metastable colloids, and aggregation
leading to haze. At high protein concentration, direct bridging occurs, resulting in
lower aggregation and turbidity thresholds. Interactions of flavonols with proteins
(Dufour and Dangles 2005) as well as their adsorption on PVPP is much more effi-
cient with aglycones as the sugar residue on the glycosides weakens the driving
forces (Laborde et al. 2006). Finally, other parameters such as the solvent character-
istics, the presence of other solutes and the temperature influence protein/flavanol
association and the properties of resulting complexes. Thus the affinity between
tannins and PRPs is lower at higher temperatures (Charlton et al. 2002a). The
presence of polysaccharides prevents coprecipitation of tannins and proteins (Luck
et al. 1994; Cheynier et al. 2006). Ionic strength and pH affect proteins solubility.
Precipitation of tannin protein complexes is highest at the protein pHi as electro-
static repulsions are minimal (Calderon et al. 1968; Perez-Maldonado et al. 1995;
Charlton et al. 2002a; Kawamoto and Nakatsubo 1997). The effect of ionic strength
and ethanol content on the interactions of epigallocatechin gallate with a salivary
PRP was investigated (Pascal et al. 2006). Increasing the ionic strength with sodium
chloride or tartrate ions resulted in an increased stability of the aggregates, mean-
ing that aggregation was not driven by repulsive electrostatic interactions. In 12%
ethanol, the protein was not fully dissolved and aggregation with epigallocatechin
gallate required much higher concentrations of the latter, confirming the role of
hydrophobic interactions.
9B.4.3.2 Interactionswith Polysaccharides
Actors
Major wine polysaccharides, including mannoproteins originating from yeasts and
plant cell wall constituents (e.g. arabinogalactan proteins (AGP) and rhamnogalac-
turonan II (RGII)), have been shown to interact with flavanols (Riou et al. 2002).
Besides, arabic gum (a mixture of arabinogalactans and arabinogalactan proteins)
can be added as a protecting colloid to limit or prevent aggregation, flocculation and
precipitation of tannins and tannin-protein complexes (Pellerin and Cabanis 1998).