Friday, April 1, 2011

Tea Foam

This post is quite off topic for what I normally post, but it caught my interest recently.

Foam that collects on the surface of tea, sometimes.

The reason it interests me is because it only happens with some teas, and only some of the time; hot, iced, black, green...etc.

I was brewing a Kenyan CTC tea the other day, and when I poured it, foam collected.
When I brewed a full leaf Assam, there was no foam.
I poured a green tea into two different cups; one had foam, and one did not.

Now, the picture (disclaimer) I did not take. The picture is also of instant (powdered) iced tea. I already know the reason that instant tea foams; the amount of solubles, like sweeteners, as well as the mixing process that introduced oxygen into the mix.

But why does loose leaf tea occasionally collect foam?

Search Google and a variety of answers (sometimes incredibly comical) will appear.
Some of the reasons I found:

-Soap residue from a dishwasher that couldn't perform its function properly.

-The introduction of oxygen into the water, when poured, stirs up tannic acid and causes it to foam.

-The mixing of hot and cold water makes foam.

-The dissolved solids in the water cause the foam.

-Hot water scalds the tea and creates the foam.

-A chemical reaction between sugar and caffeine.

-Extremely fine dust and foreign particles on the leaves float to the surface and collect as foam.

-Microwaved water makes tea foam.

-The foam is denatured proteins that detach from the leaves when hot water is poured onto them.

I do not know if any of these are the correct answer, but it was entertaining reading and looking some of this information up! If anyone else digs up anything, please feel free to share!



  1. I think proteins and/or amino acids definitely play a role in the "good foam". The more torn type of black tea, more heavily-rolled green tea and some early spring green tea seem to have more foam, probably because inner contents escape out of the cell more easily. Pectins (a source of fruity flavor in oolong) can also cause foam.

  2. Gingko,

    Always so knowledgeable!
    Thank you for your input, as always!

  3. Hi Sir William! I agree that this topic is fascinating to ponder. (and in case you missed it, I also touched on this subject in my "original bubble tea" post from summer 2009. sorry about the shameless plug ;)

    As you and Gingko mention above, there are so many possible causes for bubbles and foam... I generally think bubbles are my friends... but foam seems like a warning sign of actual soap residue! I think tea drinkers should not (or at least very very rarely) use soap on their tea wares.

  4. Brett,

    I am quite glad to hear that someone else had a similar post! This means that the topic is relevant!
    Your mention of "saponin" is quite interesting! I could see how that could be an indicator of a brothy, thick texture.
    I too agree with you on the point of not using soap on teaware. It can be harmful to the taste and texture of tea! All of my teaware is rinsed with just water for that very reason!
    It is a pleasure to have you add to discussion!

  5. It is a joy and a comfort to learn that there are others out there who have had these musings. Alas, it seems there is no consensus on the cause of tea foam, but I thought that you might be interested in a related phenomenon that seems to have been explained, "tea scum". Harold McGee's *On Food and Cooking* (p. 434) explains that the use of very hard water that produces a surface scum on tea consisting of calcium carbonate and phenolic aggregates, which conclusion is expatiated upon by this blogger ( Let us hope that Mr. McGee someday turns his culinary curiosity to the question of tea foam.

  6. Anon,

    "Tea scum is a complex organic material derived from the oxidation of tea soluble (chemicals, and the reaction is pushed along with the help of) calcium salts and (is) accompanied by (the formation of) calcium carbonate."

    This line in that article definitely sums up the results found!
    Quite an interesting study! I can see why hard water would create foam/scum on top of the liquid!
    Very interesting link! Thank you for adding your valuable input to this discussion!

  7. I notice that when I make tea from our well water, which is very hard and high in iron, I get a foam verging on "scum" that is not pleasant. I always buy bottled water for tea.

  8. Deb,

    Yes, well water is high in minerals and will definitely cause scum/foam.
    From my experience, the more solubles in the water, the more foam.
    Water that has a good ph balance will make clean tea.
    Thank you for voicing your input!

  9. Hi, everybody!

    I also noticed that the foam forms when the water is not very hot (like 70 degrees) and more evidently in microwave and when the water has more water stone.
    My explanation is like this: tea changes surface tension of water (already mentioned in posts). If you heat water a litle bit, also water stone is less soluble as well as oxygen. So when you add tea bag plenty of little oxygen bubles are formed (becouse of higher temperature) and are cought as foam on the surface of the tea. The thin layer of insoluble stone helps to catch them. In my experience, when the water is boiled, the foam is not so evident, becouse the oxygen escapes more easy.
    I think that the foam is obviously not from the soap becouse I clean my cup just with hot water.

    Anyway I like the foam :)