Mechanistic study of equilibrium and kinetic behaviour of tea cream formation

We report on the formation of tea cream of black tea infusions using turbidimetry and time-resolved (static and dynamic) light scattering. In all cases, the 'equilibrium' phase diagrams (which are to some extent kinetically determined) display similar trends to those of simple mixtures with solubility at high temperatures but separation into immiscible phases below an 'upper critical solution temperature'. Especially in the more concentrated region of the miscibility gap (>2% w/w), demixing (spinodal decomposition) is the predominant mechanism of cream formation, reflecting the substantial insolubility of polyphenols. Time-resolved light scattering results demonstrate that the (narrow) metastable region of the miscibility gap is in general difficult to access even at low concentrations of tea and small temperature quench depths. Only in the case of a more water-soluble decaffeinated tea material did it prove possible to access such a region, with nucleation and growth then having been demonstrated as the important mechanism.