Cream Down and Foam on Tea: The Science Explained

You brew a strong pot of pu'er, get caught up in conversation, and when you look back, the once-clear liquor has turned cloudy, as if a little milk had been added; reheat it and it slowly clears again. When you pour water, a layer of fine white bubbles often rises on the surface too. Neither means the tea has gone bad; they're normal behaviors of several constituents in the liquor as temperature and water quality change.

How Cream Down Happens

"Cream down" (lenghouhun, also called "rurong") is known in English as tea cream. While hot, the caffeine, tea polyphenols, theaflavins, thearubigins, and other constituents in the liquor are each dissolved in the water; as the temperature drops, they bind to one another through hydrogen bonds and similar interactions, forming larger complexes big enough to scatter light, and the liquor looks cloudy.

Key points:

• The main players are caffeine and polyphenols: pigments such as theaflavins and thearubigins in particular bind strongly with caffeine, which is why black tea and strong ripe or raw pu'er readily show cream down.
• Most of it is reversible: reheat the tea and the complexes dissolve again, and the liquor clears.
• Part of it is irreversible: research divides black tea's cream down into "redissolvable" and "non-redissolvable" fractions. The non-redissolvable fraction contains more metals such as aluminum, calcium, iron, and copper and is hard to redissolve even with boiling water.

Does Cream Down Mean Good Tea?

You can't generalize, but there's some logic to it. For cream down to appear, the liquor needs a high enough concentration of caffeine and polyphenols, so tea rich in constituents and brewed strong enough is more likely to show it. Some longtime tea drinkers take "pronounced cream down" as a clue that the liquor is full-bodied.

But it's also influenced by many factors:

• Leaf amount and steeping time: the same tea shows it when brewed strong and not when brewed light.
• Water quality: calcium, magnesium, and other ions in hard water make the cloudiness more pronounced and are more likely to produce non-redissolvable precipitates. See Water Chemistry.
• Type of tea: most catechins in ripe pu'er have already been transformed, so its cream down is usually lighter than that of strong young raw tea or black tea.

So cream down is a sign that "constituents are concentrated enough," but not a standard for judging quality. To judge a tea, go back to aroma, taste, and spent leaf. See Tasting Pu'er.

Cold Brewing and Cream Down

When cold-brewing pu'er, because the water is cold from the start, less caffeine and fewer ester catechins are extracted, so there's usually little noticeable cream down and the liquor is clearer. Conversely, tea brewed hot and then iced often shows pronounced cream down, a problem bubble tea shops commonly have to deal with when making iced black tea.

What Is the Foam on Tea?

The fine white bubbles that rise on the liquor when you pour come mainly from tea saponins in the leaves. Tea saponins are a type of saponin whose molecules are hydrophilic at one end and lipophilic at the other, making them natural surfactants that lower the surface tension of water, so the surface traps air easily, just as soapy water foams easily. Proteins and polysaccharides in the liquor also help stabilize the foam.

What affects the amount of foam:

• Height and force of the pour: a high, fast pour brings in more air and more foam. See Pouring Technique.
• The first infusion: during the rinse, the leaves are just unfurling and surface constituents dissolve quickly, so foam is often most noticeable.
• Material: teas from different materials and processes differ in saponin and protein content, and so in how much they foam.

You can skim the foam off gently with the lid, or leave it alone. It has nothing directly to do with whether the tea is clean.

Sheen and Tea Film

Sheen: good tea liquor often has a glossy layer on its surface, which tea drinkers call "oil sheen" (youguang). This sheen comes from colloidal substances in the liquor (polysaccharides, pectin, and protein–pigment complexes) making the surface more reflective, and it also depends on the liquor's temperature and the color of the cup. A clear liquor with a sheen is often considered a sign of a full-bodied tea.

Tea film: sometimes a thin film forms on the surface of cooled tea that breaks into flakes when touched with a spoon. It's mostly formed by calcium and magnesium ions in the water binding with oxidized polyphenols in the liquor, and it's especially common with hard water; switching to softer water or filtered water usually reduces it greatly. It's different from sheen: sheen is an overall gloss, while tea film is a visible thin layer that breaks when touched.

FAQ

Why does tea turn cloudy when it cools? This is cream down. As the temperature drops, caffeine binds with tea polyphenols, theaflavins, and thearubigins into larger complexes that scatter light, making the liquor cloudy; reheating usually clears it.

Does cream down mean good tea? It means the caffeine and polyphenols in the liquor are concentrated enough, a sign of rich constituents, but it's also affected by leaf amount and water quality, so you can't judge quality by it alone.

What is the foam on tea? Mainly natural surfactants such as tea saponins, with proteins and polysaccharides stabilizing the foam; the higher and harder the pour, the more foam.

What is the thin film on the surface that breaks apart? Mostly a tea film formed by calcium and magnesium in hard water binding with oxidized tea polyphenols. Switching to softer water usually reduces it.