Black tea interfacial rheology and calcium carbonate

Abstract

An interfacial phenomenon can be observed in the kitchen in a cup of black tea. When tea is left to cool after steeping, a thin film at the air–water interface can form. In certain conditions, this film is observable by naked eye and, when disturbed, cracks visibly like sea ice. The mechanical properties of this interfacial film are assessed using bicone interfacial rheometry. Water hardness, acidity, the presence of sugar or milk, tea concentration, and brewing temperature all affect the formation of this film. Interfaces formed in hard water (200 mg CaCO3/L) exhibit increased elastic modulus vs those in moderately hard water (100 mg CaCO3/L), soft water (50 mg CaCO3/L), and Milli-Q water. All films formed in chemically hardened water exhibit yielding point behavior in the interfacial oscillatory shear. Film physical thickness shows no correlation with measured physical strength. Conditions forming the strongest film, chemically hardened water, may be industrially useful in packaged tea beverages for preferable shelf stability and for emulsion stabilization of milk tea products. Conditions forming weakened films, addition of citric acid, may be useful for dried tea mixes. In lab conditions, the film visibility is obscured due to purity of tea ingredients and careful washing. However, the film physically forms and can still be measured through interfacial rheometry.

Tea film formation in artificial tap water

“…Total protein content was 3.3% for the milk used in prior work. 32 Cow milk protein consists of 30% β-casein and 10% β-lactoglobulin. 44,45 Consequently, 44.6 mg β-casein and 14.9 mg β-lactoglobulin were added to the 150 mL black tea infusion.…”

“…A concentration of 48 g citric acid per L was used. 32,42 Into the 150 mL tea infusion, 4.5 mL of this citric acid solution was added. Isolated effects of two milk proteins were analysed: b-casein (498%, Sigma Aldrich, USA) and b-lactoglobulin (97% provided by the Food and Bioprocess Engineering group at the Technical University of Munich 43 ).…”

“…19 Another important finding was that no tea film formed on tea brewed with distilled water. 19,[31][32][33] Therefore, the authors concluded that ions present in tap water are crucial for the formation of this surface film.…”

“…This was followed by an amplitude sweep with increasing shear strain from 0.01-100% at 1 rad s À1 to determine the interfacial shear strain breaking point of the film. Subphase viscosity was adjusted for in RheoCompass software (Anton Paar, Graz, Austria) with a constant tea subphase viscosity of 2.4 mPa s. 32,46,47 A representative sample was chosen for each measurement.…”

“…Water hardness (defined as dissolved CaCO 3 concentration in Milli-Q water) was found to be a significant factor for film strength. 32 Milli-Q water is water purified with ion exchange cartridges with the resulting water having a conductivity of 18.2 MΩ cm −1 . Tea brewed with pure Milli-Q water showed no tea film formation, while the strongest tea film formed on tea brewed with hard water containing 200 mg CaCO 3 per L. Citric acid significantly weakens the film.…”