I microwave water in order to make tea. After it is microwaved and I drop the tea bag(green tea) into the water, thick white foam builds up on the top.

What is it and should I worry about drinking it?

  • Not having seen this myself, I wonder: is there anything unique about your situation? Possibly something about the tea, the water, or how you've cleaned your cup?
    – Cascabel
    Mar 14, 2012 at 18:52
  • +1, I've wondered about this too, although I don't get foam. However, when I drop in a Stash green tea bag, I'll get a torrent of white bubbles that rush to the surface, which doesn't happen when I use, for example, an English breakfast blend.
    – fire.eagle
    Mar 14, 2012 at 20:52
  • Can you get a picture of this for us?
    – ashes999
    Aug 25, 2013 at 14:55
  • This phenomenon has now also some answers on Physics Stack Exchange, different from those below.
    – tanius
    Nov 22, 2018 at 2:49

5 Answers 5


When you boil water in a cup in a microwave, it will often boil without forming bubbles, because unlike a kettle with a rough heating element or inner surface, a clean ceramic cup has few nucleation points. Nucleation points allow pockets of gas to form, which become bubbles as the water boils.

When you add the teabag to the hot water, you are essentially introducing thousands of nucleation points very quickly, and so lots bubbles form very quickly - your foam. You should exercise caution when heating water this way prior to adding a teabag, as if you heat it for too long it can superheat, and will boil explosively out of the mug when you add the teabag.

  • 2
    But why "thick white foam"? Bubbling/boiling water usually makes rapidly disappearing bubbles that might be described as thin and mostly transparent, not thick and white.
    – Cascabel
    Mar 14, 2012 at 20:33
  • 4
    Foam is in the eye of the beholder Mar 14, 2012 at 21:19
  • 1
    There are thin transparent bubbles when i let the tea bag sit. The thicker foam appears when I bob the tea bag in the water. Perhaps the mixing action creates the thicker foam. Thanks Elendil!
    – jrounsav
    Mar 14, 2012 at 22:54
  • 1
    For the reason Elendil points out it is advised to put a spoon into a cup of water when heating it in the microwave. That way the air has something where bubbles can form, preventing this superheating also called boiling retardation. As long as the cup is nonconductive a metal spoon will not do any harm in the microwave.
    – Umbranus
    Nov 8, 2016 at 13:51

Tannins produce foam in tea, and also streams and rivers.


There's nothing to worry about when you see the foam appear. When hot water comes in contact with tea, it extracts the amino acids and proteins that result in such foam.

The reason that you get more foam on the surface is when you microwave the water is perhaps dip the bag in hot water. When you put the tea bag in the cup first, part of the bubbles that appear will dissolve due to the moving water. Try to see if this makes a difference.

Check this page for more information: https://www.teasenz.com/chinese-tea/foam-surface-tea.html


This may not be direct answer to your question about what the foam is made up of. But when it forms and how to avoid it.

This often happens if the water is not warm enough.

If you like to avoid it, you can try these steps:

  • Most tea leaves should be placed in water near boiling point. That's 95C (or 200F) for black tea and 90C for Green tea. If you're not near sea level altitude just make sure the water reaches boiling point.

  • Pour the boiling water over the tea bag already placed in your cup. Dunking the dry tea-bag into hot water can lead to the issue ElendilTheTall talks about as well as foam.


The "white" foam is caused by denatured proteins in the tea leaves when heated. Same when you boil meat, eggs or fish.

  • 3
    I have never heard of plant leaves containing significant amounts of protein, and I am pretty sure tea leaves have close to 0% protein content. Besides, if this were true, it would happen with tea boiled by any method, not just in microwaves. Any sources for your claim?
    – rumtscho
    Jan 31, 2013 at 17:06
  • Leaves, or at least their chloroplasts, are full of the most abundant protein on earth, Rubisco: en.wikipedia.org/wiki/RuBisCO It convrts carbon dioxide to sugar. Still, I think the foaming is more likely due to the tannin content of the leaves. Mar 4, 2013 at 15:44
  • Tannins interact with proteins though, it doesn't have to be an either/or. Nov 18, 2014 at 21:52

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