engleski

Reversible Self-Aggregation of Oxidized Tannins in Model Solutions

Tannins are well-known for their self-aggregation and formation of colloidal complexes with biomacromolecules, e.g. proteins and polysaccharides. These physical-chemical phenomena are conditioned by high sensitivity of tannins to oxidation leading to structural and consequent solvation changes. Oxidation of tannins and resulting chemical and physical evolution are determinant for biological, nutritional and organoleptic properties in tannin-rich foodstuffs. For instance, aggregation and oxidation are both involved in haze formation occurring in some fruit-derived alcoholic beverages (Pommeaux, Port wines, ...). However, mechanisms for these colloidal instabilities are still not elucidated. The present work focuses on the self-aggregation of oxidized tannins, with emphasis on the reversible aggregation. Purified procyanidins from dimer to tetramer were studied in chemically controlled oxidative conditions in hydroalcoholic model solutions. The aggregation / re-dissolution were triggered by (i) temperature drop / jump and (ii) rapid decrease / increase of ethanol fraction in the solvent. Kinetics were monitored by DLS. Pro-colloid fraction (aggregation-prone, T1) and soluble fraction (T2) were separated by ultracentrifugation. The impact of oxidation on T1 and T2 fractions was monitored along the aggregation kinetics by LC-UV-MS coupled to acidolysis in the presence of phloroglucinol. Oxidation markers were followed resolving inter- and intra-molecular oxidation- induced covalent linkages. To investigate the reversibility of the turbidity, the samples were subjected to series of temperature ramp-descend cycles. First significant finding is that the formation of haze is reversible upon heating, revealing a non-covalent nature of the interactions, similary to what was already found in Pommeau samples. This shows that the self- association of oxidized tannins is a sufficient model for comprehension of the hazes in Pommeau. Furthermore, typical kinetics profiles for self- aggregation presents a well distinguished latencyphase, which is interpreted as the “beverage life time in bottle”. We find that the latent phase duration decreases and the self- association rate increases with increasing oxidation degree and polymerisation degree of procyanidins. These results allow highlighting the involvement of oxidized tannins in reversible haze.

procyanidins, oxidation, self-aggregation, haze, DLS

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