PH effect on colloidal characteristics of micro-nano particles in lapsang souchong black tea infusion

Abstract

Tea infusion is a colloidal system. As a first attempt to systematically characterize tea micro-nanoparticles, the effect of pH on colloidal properties of Lapsang souchong black tea infusion was investigated in this study. At the native pH 5 of black tea infusion, micro/nanoparticles were revealed by Cryo-TEM (Cryo-transmission electron microscopy) to be of a structure consisting of dominantly spherical polyphenol oxidase (PPO)-polyphenols complexes and surrounding linear polysaccharide chains with an average hydrodynamic diameter ( D h ) of 128 nm and ζ-potential of −23.7 mV. It is the first time to demonstrate the black tea micelle consists of spherical and fibrous nanostructures, possibly responding to protein and polysaccharides. Increasing pH resulted in a decrease in polyphenols content and the disassembling of the micro/nanostructure. On the other hand, the decrease of pH through their respective isoelectric points led to the formation of PPO-polyphenols complexes, polysaccharide aggregation, and further aggregations among different molecules and their complexes and a drastic increase in the precipitation when pH was lower than pH 5. The predominant effect of pH on colloidal properties of black tea infusion reveals an electrostatic nature of intramolecular and intermolecular interactions among PPO, polysaccharides and polyphenols. Those results provide new clues to a practical and effective quality control of black tea products as well as an insight into health benefits of black tea. • The black tea contains negatively charged NPs (Dh = 128 nm) at its native pH of 5. • Increasing pH induced the disassembly of the micro/nanostructure. • pH < 5, formation of complexes and precipitates was promoted. • Electrostatic interaction primarily defines the nanostructure.