Kinetics of the formation of β-casein/tannin mixed micelles

The complexation kinetics of β-casein with tannins were investigated by means of stopped flow and small-angle X-ray scattering (SAXS). Several small plant tannins have been considered: epigallocatechin-gallate (EGCG) from green tea and a set of oligomeric tannins from apples. We show that the kinetics are composed of two processes. The first process is a rapid uptake of tannins by the β-casein micelles over 40-100 ms and the second process is a slow reorganization of the tannin-dressed proteins into stable heavier micelles over a period of up to 200 s. In the first process, the protein segments in the cores of the micelles are rapidly coated by tannins. Detailed analysis of the SAXS profiles during the slow dynamics reveals that the system remains composed of micelles whose structural attributes evolve smoothly toward equilibrium values. The quantity of the bound tannins remains constant during the whole slow evolution of the system. We conclude that the dominant elementary events that drive the slow kinetics are the exchange processes of tannin-dressed proteins from one micelle to another.