Flavan-3-ol aggregation in model ethanolic solutions: Incidence of polyphenol structure, concentration, ethanol content, and ionic strength

Colloidal interactions involving polyphenols play a crucial part in wine stability, clarification, and taste. Though there is some evidence that polyphenolic compounds form stacks and aggregates in hydroalcoholic solutions, only little is known about their colloidal behavior. The aim of this study was, thus, to investigate in model ethanolic solutions the colloidal aggregation of flavan-3-ol monomers and polymer fractions extracted from grape seeds as well as apple and pear parenchyma. Aggregation was studied by means of phase diagrams, and aggregates were characterized by dynamic light scattering and cryo-transmission electron microscopy. Several parameters were studied: (i) the incidence of the tannin structure (mean degree of polymerization, mDP, and percentage of galloylation) and concentration (between 10(-2) and 5 g L-1) and (ii) the incidence of the ethanol content (from 2 to 20%) and ionic strength (from 10(-3) to 10(-1) M) of the solvent. Regarding the tannin structure, galloylation enhanced the formation of aggregates as far as monomers were concerned, but this could not be confirmed with polymers. The mDP had a complex effect: aggregation increased first with mDP for relatively low molecular weight polymers but decreased again for higher molecular weight fractions. This suggests that the higher molecular weight polymers can adopt a conformation in solution that enhances their solubility. Increasing the ionic strength resulted in a much lower tannin solubility and, when soluble, in the formation of bigger and much more polydisperse particles (salting-out effect). The ethanol content of the solvent also had a strong incidence on self-aggregation: increasing the ethanol content resulted in higher tannin solubilities and smaller and less polydisperse colloidal aggregates. This could be linked to the superficial tension properties of solvents containing various amounts of ethanol and confirmed the determinant part played by lyophobic interactions in flavan-3-ol aggregation.