WHY does tea leave a stain on the cup?

Tea leaves contain phenolic compounds, tannins, giving the strong drink an astringent taste, and alkaloids, primarily thein. When the tea is brewed, these come out in solution and react with each other. As it cools, the solubility of this compound drops and it is partially released on the surface of the liquid as a film.

When the cup is tilted and sipped from it, this film ends up on the walls and dries out. Plaque is not harmful to health, but its regular occurrence is a reason to wonder if the tea you drink is too strong.

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