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Put down the filter: Tea made with impure water tastes better © Getty Images

Put down the filter: Tea made with impure water tastes better

Published: 08th September, 2021 at 10:58
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Calcium carbonate in tap water makes a cup of tea taste crisper, researchers say.

Chemical impurities in tap water can cause a thin film to form on the surface of a cup of tea, and these make it taste better than a drink made with pure water, according to researchers at ETH Zurich in Switzerland.


Pour yourself a cup of tea and leave it to cool slightly, and you may see a film on the surface that cracks like sea ice when you disturb the cup. There are many factors that affect the formation of this film, the researchers say, but the primary one is calcium carbonate in the water. When tap water contains a high amount of minerals such as calcium carbonate, it is called hard water.

“Tap water in many regions comes from limestone aquifers, where calcium carbonate, a harmless compound that can make water taste ‘crisper’, is found,” said Caroline Giacomin, co-author of the paper. “Many homes in the US Midwest have water softeners to reduce this in their water supply to prevent build-up on faucets.”

Other factors that affect the formation of this film include milk, sugar or lemon added to the tea, the brewing temperature, and the concentration of the tea.

The team studied how the strength of the film changed with water hardness by placing a metal device on the surface of the tea and rotating it. “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,” said Giacomin.

They found that the more calcium carbonate in the water, the stronger the film would be. “If you were to make a cup of tea in perfectly pure water, it would not form a film at all, but the tea would taste quite bitter,” Giacomin explained.


The team’s findings could be useful in industrial settings, where creating conditions to form a strong film could improve the shelf life in packaged tea drinks.

Is it possible to drink too much tea?

Tea contains natural antioxidants called polyphenols, which have many positive effects on the body. For instance, drinking more than three cups a day has been found to reduce the risk of a heart attack.

The 40mg of caffeine per cup (roughly half as much as coffee) won’t have any impact on your health until you get to at least eight cups per day – so you should probably stop at that point. But it’s possible to take anything to extremes. In 2013, a US woman lost all her teeth due to fluoride poisoning from tea. She was drinking a pitcher of iced tea every day for 17 years, and each pitcher was reportedly brewed with 100 to 150 tea bags!

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.


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