Why Does My Tea Foam? The Science Behind Unexpected Bubbles
No, your tea doesn't contain soap! Tea leaves contain a molecule called saponin. It has soap-like surface-active properties (Rai et al. 2021), so when the tea leaf releases saponin into the water, you can see bubbles forming. I'm no physics expert, but briefly some parts of the saponin molecule either interact with water molecules, or are repelled by water molecules and interact with air dissolved in water. The saponin molecules then exert a tension that compresses a trickle of water, encapsulating the air.
In practice, some teas “foam” less than others. The later the plucking and the larger the leaf, the higher the saponin content! According to some researchers (Yu and He, 2018), we shouldn't expect to see spring Long Jing (Chinese green tea) “frothy” because it's less concentrated in saponin.
In chemistry, saponins are extracted by alkaline hydrolysis (Wang et al. 2015). Some tea drinkers have noticed the formation of bubbles when using calcareous water (i.e. alkaline water). A saponin that diffuses more depending on the water?
Bibliography
https://www.mdpi.com/2413-4155/3/4/44
https://lesbullesdesavon2015.pagesperso-orange.fr/BullesSavon.html)
