There’s nothing quite like a good cup of tea. But that cuppa can easily be ruined by the layer of film that can form on the top if the brew sits for more than a minute or two.
Scientists at the Institute of Food, Nutrition and Health in Zurich have discovered why tea has a tendency to develop the scum, and there’s good news: It’s preventable.
“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,” the researchers wrote in the journal Physics of Fluid.
“However, in a lab setting, this film can be carefully controlled.”
They found the film forms when compounds in tea leaves, called polyphenols, bond with the calcium carbonate in tap water.
The higher the amount of minerals like calcium and magnesium in the water, the more likely, and the more thick, the layer of scum.
So the first thing you can do, especially if you live in a hard water area, is use soft or filtered water to make your tea.
The experts advised, however, against using super-pure, deionised water.
“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,” they wrote.
Citrus is a thinning agent, so adding some lemon juice will help expel the scum. If you’re desperate, you could even switch to Earl Grey, because bergamot is naturally acidic.
Those with a sweet tooth will be pleased to learn that while sugar doesn’t thin the film, it doesn’t make it any worse, either.
But even the smallest amount of milk created a much thicker layer of scum than in black tea (it is, of course, full of calcium).
If, however, black isn’t your cup of tea and you don’t fancy shelling out for a filter, you can rest assured the film, while unsightly, is an entirely flavourless and harmless chemical reaction.
The scientists did have one other tip for those wanting a clear cuppa: the film becomes more visible the more the tea cools, so drink it while it’s hot.
There’s also this: “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.”
But that just goes down like a cup of cold tea.