1 of 3
Do you view the colorful film on the surface of your morning cup scum or a colorful sheen?
2 of 3
A Rheometer is used to measure characteristics such as elasticity of interfacial surface film that forms as tea cools.
3 of 3
Researchers at ETH Zurich compare cups of tea in the lab for a paper published in the Physics of Fluids by AIP Publishing.
Once thought to be primarily composed of waxy compounds in tea leaves, it is now clear that the colorful film that forms on top of a black cup of tea is due to the reaction of oxygen, tea polyphenols, and carbonates in the water.
The sheen, which can resemble arctic ice floes, indicates well-made tea, conclude researchers. And for those who wish to prevent the film from forming, researchers calculated the influence of water hardness, acidity, sugar or milk, tea concentration, and brewing temperature.
Co-author Caroline Giacomin, a physicist at ETH Zurich, explains the experiments performed involve a metal device placed at the surface of the tea. “The rotation of that device is carefully controlled, and the resistance to rotation that the film applies is what allows us to determine its strength,” she said.
Interfacial rheology is the study of weird fluids: think about substances like oobleck, the cornstarch, and water mixture that kids like to play with famously known as slime, Giacomin told STiR tea and coffee.
“I'm following the work of a researcher in England in the 1990s who published a 14-section paper of which 6 or 7 sections sought to explain tea film," she said. “He was studying the components and proved that carbonate contributes the greatest amount to compounds that react with the tea. We decided to study the physics,” she said.
Researchers determined the film is an interfacial reaction of polyphenols and other components in the tea that bond with ions in the water. She explains that there's no visible film when you make tea with perfectly pure water, "what we call Milli Q water. The film does not float like wax and does not form on green tea. Its elasticity and mass depend on the concentration of carbonates in the water, generally indicating good tea.
“The film is quite beautiful,” she says with enthusiasm, “I'm a scientist describing tea scum as beautiful, especially when you don't add milk.”
Adding milk quite often results in a film that is not particularly visually pleasing, she explains. “In fact, it kind of looks gross, but those two films are actually made of very different components. Which is why in my research, we weren't able to measure the resistance of the milk film because there's too much oil and fat in it for it to be measured by our device, it caused too much slipping, essentially,” she said.
The two films are different. So, if you're making tea, where you would like to have milk in it, I suggest that you make that tea with water that's been filtered or if you're living in a place where the water isn't particularly hard anyways, that it shouldn't matter too much, you're not going to have much of a film.
Both films are harmless.
Conditions contributing to the formation of the strongest film, namely chemically hardened water, may be industrially useful in packaged tea beverages for prolonged shelf life and for emulsion stabilization of milk tea products. Conversely, conditions forming weakened films may be useful in dried tea mixes, according to a press release announcing the findings.
For example, “The addition of an acidic component, like citrus, in a dried tea blend will reduce the visibility of the film and add flavor.”
Reference
Giacomin CE, Fischer P. Black tea interfacial rheology and calcium carbonate. Physics of Fluids. 2021;33(9):092105. doi:10.1063/5.0059760