Kinetics and thermodynamics of tea cream formation: A colloidal approach

Abstract

We report on the effect of decaffeination and pH on the kinetics and thermodynamics of tea cream formation using turbidimetry and time-resolved (static and dynamic) light scattering.

Decaffeination enhances the solubility of tea solids (polyphenols) in black tea infusions, resulting in a shift of the location of the phase diagram to lower temperatures in comparison to “standard” black tea material. The phase diagrams for the studied tea samples displays similar trends to those of (classical) simple mixtures which dissolve at higher temperatures but separate into immiscible phases below the (upper) critical solution temperature. In the case of decaffeinated tea it did prove possible to access the metastable region of the miscibility gap with nucleation and growth being the important mechanism in the dilute part of the phase diagram. More often than not, however, the mechanism responsible for the formation of tea cream is governed by demixing through spinodal decomposition caused by the increased insolubility of polyphenols.

On lowering the pH, the location of the phase diagram is shifted to higher temperatures and larger particles of associated (polyphenol and polyphenol/caffeine) structures are identified, reflecting the decrease in solubility of tea solids in black tea infusions as a result of decreased electrostatic interactions. At natural pH of 4.8 the electrostatic repulsion between the charged droplet surfaces protects the droplets from coagulation, whereas at pH=2.0 close to the iso-electric point coagulation of tea cream particles takes place resulting in a rapid settling out of tea solids, as observed previously by Harbron. Results at pH=3.0 and c=0.3 wt%, however, do not indicate aggregation/coagulation between different particles, which means that under these conditions there is apparently enough electrostatic repulsion preventing cream particles from coagulation.