If you drink black tea, you may have noticed a thin, oily film that forms at the top of the drink, especially if you live in a hard water area. Why does this happen? According to new research, the answer lies in its physics, fluid dynamics, and the chemist-tea!

BBC’s Science Focus magazine reports that the oily film on black tea is made from some of the compounds found in tea – particularly, molecules called polyphenols – and from calcium carbonate found in tap water in hard water areas.

Caroline Giacomin, a researcher at ETH Zurich, Switzerland, and co-author on a paper describing the research, published in Physics of Fluids, explained: “Tap water in many regions comes from limestone aquifers, where calcium carbonate, a harmless compound that can make water taste ‘crisper’, is found.”

She said that if you were to make a cup of tea from perfectly pure water, then there would be no film on the beverage.

However, she added that tea made from filtered and pure water will taste quite bitter.

Researchers have known for a while that while calcium carbonate makes the oily film thicker, sugar and lemon juice have the opposite effect and thin it out.

But milk is more complicated: it can thin the film, but small amounts of milk make it thicker.

The researchers studied rheology, or flow, of the film using a technique called bicone interfacial rheometry, to find that there wasn’t a link between the physical thickness of the film and its strength, but there was a correlation between the calcium carbonate concentration and film strength: The higher the concentrate of calcium, the stronger the film.

The researchers say this has implications for the manufacture of different types of tea.

“Conditions forming the strongest film, chemically hardened water, maybe industrially useful in packaged tea beverages for preferable shelf stability,” they write in their paper.

“Conditions forming weakened films, addition of citric acid, may be useful for dried tea mixes.”

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