Tea foam is intriguing because it seems immediately visible and therefore easy to interpret. A cup covered with fine bubbles sometimes appears richer, fresher, or more intense. Conversely, a perfectly smooth surface can give the impression of a less expressive tea.

Yet, foam is not a simple indicator of quality. It results from the interaction between the chemistry of the tea, the water, and the way air is incorporated into the beverage.

At the heart of this phenomenon are natural substances called saponins. Found in several plants of the Camellia genus, they possess a particular property: they can modify the interface between water and air and facilitate bubble formation.

1. Foam cannot exist without air

The first thing to understand is elementary: foam is made of gas bubbles dispersed in a liquid.

Without incorporating air, foam cannot actually form.

When water is poured forcefully into a teapot, when a tea liquor is transferred from one vessel to another, or when matcha is whisked with a chasen, air is mechanically introduced into the liquid.

Bubbles appear.

But pure water generally forms bubbles that disappear quickly. For foam to become more persistent, certain substances present in the liquid must stabilize the interface between air and water.

2. Saponins: the main players in this story

Saponins are a family of molecules naturally produced by many plants.

Their structure has an important particularity: part of the molecule has an affinity for water, while another part interacts more with less polar media.

This architecture gives them so-called surfactant properties. They can position themselves at the interface between air and water and modify the surface tension of the liquid.

Under certain conditions, this allows bubbles created by agitation to last longer instead of disappearing almost immediately.

3. Does tea really contain saponins?

Yes.

Saponins are present in several parts of plants of the Camellia genus, notably in the seeds, but also in the plant tissues associated with tea.

The term "tea saponin" encountered in scientific literature also often covers extracts obtained from different species or parts of the Camellia genus.

This is an important point when consulting studies: research on purified saponins from Camellia oleifera seeds does not necessarily measure exactly what happens in your cup of Sencha.

Nevertheless, it allows us to understand the general physical mechanism: Camellia saponins can act as surfactants and promote the stability of foams.

4. Why do some teas foam more?

Although all teas come from Camellia sinensis, they do not necessarily have exactly the same chemical composition.

Cultivar, leaf age, harvest season, terroir, processing, and storage conditions can change the relative concentrations of many constituents.

The amount of material actually extracted in your cup then depends on the brewing recipe.

Two teas can therefore produce very different foam even when they are prepared in apparently similar ways.

But this difference should not automatically be turned into a quality judgment.

5. Agitation may count for more than the tea itself

Take exactly the same infusion.

Pour the first half gently into a cup.

Then pour the second half from higher up, with more turbulence.

The second may show many more bubbles, even though the composition of the tea is identical.

The difference simply comes from the amount of air incorporated.

6. Matcha: the case where foam becomes intentional

Matcha is the most spectacular example.

In a traditional preparation, the tea powder is suspended in water and then worked quickly with a bamboo whisk.

The movement of the chasen introduces a large amount of air and fragments this air into a multitude of small bubbles.

The desired result is a fine layer of foam, generally composed of relatively small and homogeneous bubbles.

In this case, the foam is therefore not an accident.

It is part of the desired texture and contributes to the perception of matcha.

But here again, two things must be distinguished: a liquid's ability to stabilize foam and the technique used to create it.

An excellent matcha that is poorly whisked may show little foam. Conversely, an energetic technique can create impressive foam without demonstrating, on its own, the superior quality of the powder.

7. Large bubbles and fine foam do not tell the same story

In tasting, not all foams look alike.

Large bubbles dispersed on the surface after pouring a teapot are often simply the result of turbulence.

They can disappear very quickly.

A very fine and more stable foam means that the interfaces between air and water resist their destruction better.

This can be influenced by the presence of natural surfactants, but also by other constituents present in the beverage.

8. Why does foam eventually disappear?

Foam is an unstable system.

The water located between the bubbles gradually tends to flow down under the effect of gravity.

Some bubbles merge.

Others see their gas migrate to larger bubbles.

The liquid film that separates two air pockets sometimes ends up becoming too thin and breaks.

Surfactant substances can slow these phenomena, but not eliminate them entirely.

This is why a freshly created foam generally changes appearance over the minutes.

9. Can temperature change the foam?

Yes, but the phenomenon is complex.

Temperature notably modifies the viscosity of the liquid, the diffusion of molecules, surface tension, and the behavior of the constituents present at the interface.

Research on Camellia saponin extracts has shown that the stability of certain foams can persist over a wide range of temperatures.

But it would be incorrect to convert these results into a simple rule such as: "the hotter the water, the more the tea foams".

In a real cup, several parameters change simultaneously.

10. And the composition of the water?

Tea water is never chemically neutral.

It contains ions and minerals whose concentrations vary greatly depending on its origin.

These constituents can modify the extraction of the tea and the interactions between the molecules present in the liquor.

They can also influence the surface properties of a foaming system.

However, it would be excessive to diagnose the hardness of water simply by looking at the foam in the cup.

Foam is only a final result in which several variables are superimposed.

11. Does foam mean the tea is fresh?

Not necessarily.

It happens that enthusiasts associate abundant foam with a very fresh or very rich tea.

There is a certain logic behind this intuition: the chemical composition of a leaf evolves over time and storage.

But this is not enough to make foam a reliable freshness test.

The amount of bubbles depends too strongly on the preparation, the type of tea, the size of the leaves, the extraction, and the pouring gesture.

To assess freshness, aromas, the condition of the dry leaves, the quality of the liquor, and sensory persistence provide much more interesting information.

12. Does foam indicate a tea rich in saponins?

It can be a clue, but certainly not a measure.

If an infusion contains more surfactant substances, all things being equal, it may be more apt to stabilize the bubbles.

But "all things being equal" is precisely the difficulty.

In a cup, conditions are almost never perfectly controlled.

The height of the pour, the shape of the teapot, the amount of leaves, the temperature, the concentration of the liquor, and the agitation change easily.

Without a laboratory measurement, looking at the height of a foam does not allow one to precisely deduce the saponin content.

13. Why do we speak of a surfactant?

On the surface of a glass of water, the molecules are not surrounded by neighbors in the same way as those located in the heart of the liquid.

This situation creates an interface energy that we observe notably in the form of surface tension.

A surfactant substance comes to accumulate at this interface and modifies its behavior.

Saponins are interesting because they possess precisely this ability.

This is the reason why researchers today study the saponins of Camellia not only for tea but also as natural surfactants in different food systems.

14. Foam can also modify our perception

The subject becomes particularly interesting when we leave chemistry to return to tasting.

Foam slightly changes the way the liquid arrives in the mouth.

It modifies the surface texture and can influence the release of aromatic compounds.

In matcha, a fine foam contributes notably to a more homogeneous and creamier sensation.

Conversely, a coarse foam, composed of large bubbles, can produce a less elegant sensation.

We therefore gradually move from a physicochemical phenomenon to a sensory dimension.

15. Why should not all teas be whisked?

Matcha is a suspension of leaf powder.

It is therefore prepared in a very different way from a leaf tea.

Whisking a Sencha or a Darjeeling vigorously generally brings no particular interest to the tasting.

The bubbles created can even hinder the observation of the color and clarity of the liquor.

We must therefore avoid turning foam into a universal objective.

In some preparation styles, it is part of the desired result. In others, it is only a consequence of the pour.

16. What foam can really teach you

Foam becomes interesting when we stop asking it to say "good" or "bad".

It can simply draw your attention to the physical behavior of the liquor.

A tea that easily produces a persistent foam has a composition and concentration that allow the air-water interfaces to resist sufficiently long.

This can become an additional observation in the tasting, alongside color, body, or length on the palate.

But it must remain one observation among others.

17. A cup is always a system

This Tsuru joins an idea we have already encountered several times: we can almost never explain a cup by a single molecule.

Foam involves saponins and other surface compounds, but also the air you introduce.

Bitterness depends on several molecular families.

Astringency results from interactions between polyphenols and salivary proteins.

Body comes from the sum of all substances dissolved and dispersed in the liquor.

And balance is born precisely from the way all these dimensions coexist.