Tea fans are only just realising why nasty ‘scum’ often forms on brew

Brits are known for their love of a good cuppa, with 164 million cups being consumed daily across the UK. However, even the most seasoned tea drinkers may have noticed an unpleasant layer of scum floating on the surface of their brew from time to time.

This has nothing to do with your brewing method – whether you add milk or hot water first, or how long you steep the tea bag. The so-called “tea scum”, which appears as an oily or waxy layer on the surface of the tea, is actually the result of a chemical reaction.

Many tea enthusiasts have been left in the dark about why this scum keeps forming on their beloved brews. This topic was recently discussed on the Lunchbox Envy podcast.

On the podcast, QI researchers Jack Chambers and Manu Henriot were joined by chef and food writer Rosie MacKean to discuss all things related to food and drink. In a clip shared on TikTok, Manu posed the question: “Do you guys know why scum can form on top of your tea sometimes when it’s brewing?”.

QI Elf Alex Bell, who also features on the podcast, humorously suggested: “I thought it was because we live in London and the water has been recycled through 100 disgusting different things and I just have to close my eyes and try not to think about what I’m drinking.”

However, this is not the case. Menu clarified: “Luckily, you’re wrong. In 2021 researchers at the Institute of Food and Nutrition in Zurich discovered that it was due to the way that the polyphenols in the tea were reacting with the minerals in the hard water.”

To eliminate the scum, Manu suggested adding “some lemon juice”.

The harder the water, the thicker the film or scum, according to scientists. They found that using soft water, which has a lower mineral content, prevented the scum from forming, but also cautioned that using super-pure forms of water could make the tea taste bitter.

The study, published in the Physics of Fluids journal, explained: “Anyone can produce a cup of tea with just water and tea leaves, but the film may seem to form randomly in the eyes of the everyday tea drinker. However, in a lab setting, this film can be carefully controlled.”

To prevent the scum from forming, they recommended using purified water, or using a filter for tap water in households in hard water areas. Lemon juice can also lead to an “even thinner film”, of scum.

And it appears the QI elves weren’t the only ones who didn’t know the reason why. “I was told it was dust on the teabag and if you give it a couple of shakes before putting in cup it gets rid of it,” said one person.

“Hilarious! A university spent time doing that,” added another. A third said: “I had it so bought an under sink filter that removes limescale. It got rid of it and improved the taste.”