Better Black Tea

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Physicists from the Department of Health Science and Technology in the Institute of Food Nutrition and Health at the ETH Zürich have applied the science of rheology to the seemingly quaint purpose of improving the quality of a cup of black tea.
Black tea brewed in tap water; whereas tea can visibly generate a film where it interfaces with air, its more acidic counterpart, coffee cannot. Image credit: Giacomin & Fischer, doi: 10.1063/5.0059760.
“An interfacial phenomenon can be visually observed in a cup of tea: the surface film that forms is brittle and mildly iridescent, and it cracks like ice floes when disturbed,” said Dr. Caroline Giacomina and Dr. Peter Fischera.
“Anyone can produce a cup of tea with just water and tea leaves, but the film may seem to randomly form in eyes of the everyday tea drinker.”
“However, in a lab setting, this film can be carefully controlled.”
Using interfacial rheology, the researchers investigated the properties of tea film with varied calcium carbonate concentrations and the effect of additions provoked by some cultural tea practices.
“In interfacial rheology, experiments performed involve a metal device placed at the surface of the tea,” Dr. Giacomin explained.
“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.”
The researchers found 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 for dried tea mixes.
“Tea film thickness does not directly correlate with film strength,” the scientists said.
“In our oscillatory interfacial shear strain sweeps, tea films break at shear strains of about 1%.”
“Citric acid can be added to tea to soften tea films and make them less brittle, breaking only at an interfacial shear strain of about 2%.”
“Adding sugar does not generate a more-glassy film than tea from hard water alone, leaving the strain at break unaffected.”
“Conditions forming the strongest film may be industrially useful in packaged tea beverages for preferable shelf stability and for emulsion stabilization of milk tea products.”
The authors suggest making tea with hard tap water and without additives to best observe this interfacial phenomenon in the kitchen.
“Removing the tea leaves once brewing is complete affords best visibility,” they said.
“To view the phenomena without hard water, one can also neglect to wash the teacup and let the dissolvable minerals build up over a few uses.”
The team’s results were published in the journal Physics of Fluids.
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Caroline E. Giacomin & Peter Fischer. 2021. Black tea interfacial rheology and calcium carbonate. Physics of Fluids 33, 092105; doi: 10.1063/5.0059760

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