Unveiling the mysteries of tea foam and scum. No cause for alarm

Does the foam visible after brewing tea signal something is wrong with the beverage? And that oily film (a scum) that shimmers on the surface of the brew like gasoline spilt in a puddle, should it raise our doubts?

Wondering why your tea foams? Where does the scum come from? The answer is simpler than you think. Most importantly, these phenomena should not cause concern. Here's why.

Suppose a foam has formed on the surface of tea just after being poured with boiling water, and it can be confidently stated that it is not a residue from not thoroughly rinsed washing-up liquid. In that case, the foaming is most likely due to saponins in the tea. These compounds benefit our health, and there is no need to be concerned about them or the foam that forms because of their presence.

The method of pouring can also influence whether foam will form. Pouring it from a height in a strong stream introduces more air into the beverage, which then escapes during the brewing process in the form of foam (sometimes such an effect is intentionally achieved, as in the case of Moroccan tea, which is poured from a great height). The type of tea leaves can have a similar impact; smaller, loose leaves contain more air, which "escapes" after the water is poured, creating foam.

The scum that forms on tea is quite different from the skin that forms on heated milk but rather creates an effect similar to spilt and shimmering colours of gasoline. This scum is the result of a reaction between polyphenols in tea and calcium carbonate present in water (particularly hard water).

Therefore, the harder the water, the more pronounced the scum formation on the surface of the tea will be. Again, there's nothing harmful about it. Filtering the water is recommended to enjoy tea without the visible oily film. Adding a slice of lemon can also help - it will either prevent the scum from forming or make it less visible.