Why Does Tea Get That Film on Top? Understanding Tea Scum

Tea scum can be one of the more frustrating aspects of a daily tea routine.

That thin film that sometimes forms on the surface of your tea isn’t just unpleasant—it can also raise questions about what’s actually happening in your cup.

As it turns out, there are two main types of “tea scum”:

• Scum caused by the tea itself
• Scum caused by added milk

Let’s take a closer look at both.

Scum Caused by the Tea

At one time, it was believed that the film on tea came from a waxy coating on the leaves that melted during brewing. However, research has shown otherwise.

Chemical analysis revealed that tea scum is largely made up of calcium compounds—particularly calcium carbonate—combined with other substances found in tea.

This means that hard water, which contains higher levels of calcium, plays a major role in scum formation.

Tea itself contributes additional compounds, especially polyphenols, which react with minerals in the water to form that visible film.

While this might not look appealing, the amount formed is extremely small and is not considered a health concern.

How to Reduce Tea Scum

There are several simple ways to minimize or prevent this type of scum:

• Use filtered or bottled water
• Add a slice of lemon (the acidity helps reduce buildup)
• Brew a slightly stronger tea, which can offset mineral effects

All of these approaches address the underlying mineral interaction that causes the film.

What About Tea Stains?

The brown stains that build up inside teacups and teapots are sometimes confused with tea scum—but they are different.

These stains are caused by pigments formed during the processing of tea leaves, particularly in black tea. As the tea dries or evaporates, these pigments are left behind.

To clean them, a simple method works well:

• Use a damp cloth
• Add a small amount of baking soda
• Scrub gently but thoroughly

Scum Caused by Milk

If you add milk to your tea and let it sit too long, you may notice a different kind of film forming on the surface.

This “milk scum” can be just as unappealing.

There are two main theories about its cause:

• Fat globules rising and clumping together when heated
• Proteins coagulating and forming a thin layer on the surface

Either way, heat plays a central role in the process.

For example:

• Lower-fat milk (like skim milk) may reduce or prevent the effect
• Extended warming—such as using a cup warmer—can increase the likelihood

Stirring may help reduce surface formation, but in practical terms, most people would rather just drink the tea before it becomes an issue.

Bottom Line

Tea scum may be unpleasant, but it’s usually harmless and fairly easy to manage.

For tea-related scum: adjust your water or add acidity (like lemon).

For milk-related scum: drink your tea sooner or adjust how it’s heated.

In the end, it’s a small inconvenience—one that can usually be solved with simple adjustments.

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