Characterization of whey protein-polyphenol interactions by dynamic light scattering

Many drinkable dairy products added with green tea polyphenols have been launched by food industries because epidemiological studies suggest an association between the prevention of diseases and the consumption of polyphenol-rich beverages or foods. However, polyphenols have a significant affinity for proteins that leads to formation of soluble complexes, which can grow in size and even form sediments. The aim of this work was to study the binding of green tea polyphenols to β-lactoglobulin, caseinomacropeptide and α-lactalbumin in a pH range between 3.0 and 6.0 by means of dynamic light scattering. Whey proteins and polyphenol aqueous solutions were prepared in citrate buffer (pH 3.0, 0.01M), acetate buffer (pH 4.5, 0.01M) or phosphate buffer (pH 6.0, 0.01M). Dynamic light scattering experiments were carried out in a Zetasizer Nano-Zs (Malvern Instruments, Worcestershire, UK) provided with a He–Ne laser (633 nm) and a digital correlator, Model ZEN3600. Measurements were carried out at a fixed scattering angle of 173o. The assay was carried out in triplicate. The intensity size distributions for all the mixed samples were monomodal and the peaks shifted to higher sizes as compared to pure proteins. It is noticeable that the particle size measured by light scattering changed with pH. Close to the isoelectric point of each protein the aggregation potential of polyphenols and the size of the complexes formed were the highest. Although the initial binding of polyphenols to whey proteins is essentially a non-selective hydrophobically driven interaction, the insolubility of the complex formed is most likely determined by surface charge effects. At the isoelectric point of the proteins, complexation proceeds up to form big particles that can easily precipitate. Controlling the interactions between whey proteins and green tea polyphenols would therefore provide a tool to improve the development new functional foods.