Rheology and Mobility at Interfaces

Artificial tap water and interfacial rheology of tea varieties

October 13, 2021 (Wednesday) 2:45

Track 5 / Ballroom 6

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  1. Giacomin, Caroline E. (ETH Zurich, Health Science and Technology)
  2. Chen, Rebecca Yun (ETH Zurich, Health Science and Technology)
  3. Fischer, Peter (ETH Zurich)

(in printed abstract book)
Caroline E. Giacomin, Rebecca Yun Chen and Peter Fischer
ETH Zurich, Zurich 8092, Switzerland

Giacomin, Caroline E.

experimental methods; interfacial rheology

Tap water is known to contain ions including calcium, magnesium, sodium, sulfate, and chloride [1]. While water composition has been shown to be important to the flavanol content of brewed tea [2], most everyday tea drinkers use their local tap water. For tea brewed in tap water, an interfacial phenomenon can be observed. The surface film that forms is brittle and mildly iridescent, it cracks like ice floes when disturbed. Ion content of the water influences the mass of tea film generated such that deionized water forms no collectable film [3]. This ion content, as well as pH, has been shown to have an effect on theaflavin extraction from tea leaves [4]. Additions of common tea add-ins, such as lemon, milk, or sugar effect the physical thickness of the tea film but physical thickness does not correlate directly with the film rheological properties [3]. The interfacial film rheological properties are assessed using interfacial rheometry. All films formed in artificial tap water (ATW) exhibit yielding point behavior in interfacial oscillatory shear. Green and black tea are produced from the same plant, Camellia sinensis (L.), and form visible films, while rooibos tea forms no film, visibly or rheologically. Green tea films are less brittle than black tea films. Interfacial moduli of green tea are less affected by ions than black tea. When the effect of milk components is studied, β-caseins extend the yielding interfacial shear strain by two orders of magnitude; β-lactoglobulins extend this same metric by only 0.4 %. Addition of salt in ion rich ATW brews reduced interfacial viscous and elastic moduli equally by 0.8 Pa m. Conditions forming the weakest film may be useful in producing dried tea mixtures to reduce film appearance in regions with ion-heavy tap water.

[1] C.E. Boyd, Hydrobiologia 80 (1981) 91
[2] M. Spiro, P.L.L. Lam, Food Chem. 54 (1995) 393
[3] C.E. Giacomin, P. Fischer, Phys. Fluids. Submitted (2021)
[4] W.E. Price, M. Spiro, J. Sci. Food Agric. 24 (1987) 51