People are only just realising why 'scum' often forms on tea

People in the UK drink 164 million cups a day, so it's safe to say most Brits know what goes into making a good brew - even if we do argue about the 'correct way'. No matter how much tea you drink and how much you feel like you've perfected it, you may have noticed a nasty layer of scum floating on the surface from time to time.

It's actually nothing to do with how you're brewing it - for example whether you put milk or hot water first, or how long you have the tea bag in. Instead, the so-called "tea scum", which looks like an oily or waxy layer that forms on the surface of the tea, comes from a chemical reaction.

It turns out many tea fans have been living in the dark when it comes to knowing why this scum keeps forming on their brews. The topic recently came up on the Lunchbox Envy podcast.

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In the podcast, QI researchers Jack Chambers and Manu Henriot are joined by chef and food writer Rosie MacKean to speak about all things food and drink. In a clip shared to TikTok, Manu asked: "Do you guys know why scum can form on top of your tea sometimes when it's brewing?"

QI Elf Alex Bell, who also appears on the podcast, joked: "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 isn't the case. Menu explained: "Luckily, you're wrong. In 2021 researchers at the Institute of Food and Nutrition in Zurich found out that it was to do with the way that the polyphenols in the tea were reacting with the minerals in the hard water." Speaking of how to get rid of the scum, Manu suggested adding "some lemon juice".

Scientists found that the harder the water, the stronger the film, or scum. They found using soft water, which has a lower mineral content, stopped the scum forming, but also warned 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."

The prevent the scum 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 turns out 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."