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Is It Safe To Drink Tea With a Film on Top?


After brewing a cup of black tea, I like to take in the aroma for a few seconds before the first sip. Often when I do this, I’ll notice a thin layer of film on the surface of the tea, which I tend to shrug off. However, new research offers some answers as to what this film (also called “tea scum”) is, and how to prevent it.

What is the film on my tea?

A recent study published in Physics of Fluids looked at some of the factors that cause this film formation. Researchers focused on black tea for this study because it’s the most popular type of tea: 63 percent of tea-drinkers prefer it. Key factor studied were the hardness of the water used, acidity, whether sugar or milk was added, tea concentration, and brewing temperature. Researchers noted that as soon as hot black tea starts to cool down, the film begins to form. If you stir or try to dissolve the film, it “cracks visibly like sea ice.”

Tea scum film

The scientists experimented with various factors to see if they could stop the tea scum from forming. Woman’s World talked to lead study author Caroline E. Giacomin, who told us that using hard tap water to make black tea was the most common reason for a strong, visible film to form on the surface. Hard water contains 180 mg. of calcium carbonate (CaCO3), which is a chemical compound that might react with the tea leaves and other ingredients, causing the film to form.

Study authors added that while add-ins such as milk and sugar might cause a thick film on the tea, it might not be visible enough to notice.

Is it safe to drink tea scum?

Luckily, this film, or “tea scum,” is completely safe to drink. Whether you choose to drink it or not, however, is a personal preference. “For those not used to seeing the film, it can be a bit aesthetically displeasing. But I find the film to be rather beautiful,” Giacomin says.

If you’re not a fan of the film and would prefer to sip a completely clear mug of tea, you’re in luck: There are some simple ways to prevent tea scum.

How do you prevent tea scum?

Researchers found that adding lemon juice to black tea reduced the thickness of the film. Why? The acid in the lemon reacts to the calcium ions from tea made with hard water, causing the film to dissolve. You’ll also want to take the tea bag out as soon as your tea is brewed to your liking, as overbrewing was another cause of tea scum.

If you’re open to switching up your tea ritual, Giacomin says going for rooibos tea instead of black tea is another way to avoid tea scum. Rooibos won’t form a film regardless of water hardness. Plus, it’s caffeine-free, so you can enjoy an extra cup without any jitters. And green tea lovers can rejoice, as she notes that while green tea forms a film similar to black tea, it tends to break more easily, making for a clearer appearance.

If you’re bothered by a film on your tea, keep these things in mind the next time you’re brewing a cup!

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