Tea interfacial rheology

Abstract

Tea is consumed daily worldwide and is present in many culturally significant activities. This includes, among others, British afternoon tea, yerba mate rituals of South America, and traditional tea ceremonies in Japan or China. When tea is left to steep, a thin film at the air-water interface can form. In certain conditions, this film is observable by naked eye and, when disturbed, visibly cracks like sea ice. The properties of this interfacial film are assessed using interfacial shear rheology and surface tension measurements. Layer properties are distinguished between tea varieties and water conditions. Water hardness, acidity, presence of sugar or milk, tea concentration, and brewing temperature all affect the formation of this layer. Interfaces formed in hard water (200 mg/L CaCO3) exhibit increased elastic modulus when compared to those in both moderately hard water (100 mg/L CaCO3) and soft water (0 mg/L CaCO3). All films formed in chemically hardened water exhibit yielding point behavior in an amplitude sweep. Conditions forming strongest layer may be industrially useful in packaged tea beverages for preferable shelf life stability and for milk tea beverages, emulsion stabilization.