Milk and Texture

Phenomenon

In a café latte, the milk foam is glossy, like wet paint. Poured into the cup, it blends with the coffee, and it drinks smooth and slightly sweet.

At home, you try steaming milk yourself. You end up with two layers: on top, a big mass of coarse, dry foam you could pile up with a spoon, its large bubbles popping one by one; underneath, hot, thin milk. Sometimes you keep going until the jug is too hot to hold, and it tastes "cooked", with the sweetness gone.

The same milk — so why can it turn into two completely different things? What is that steam wand actually doing?

Variables

While you steam milk, several things are happening in the jug at once:

The texture of the final cup is decided by all of these together.

Experiment

A thought experiment: steam the same cold milk twice. The first time, the steam tip stays right at the surface of the milk from start to finish; the second time, it stays buried deep in the milk the whole way.

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What will each jug look like? Which has more foam? Will their temperatures differ much?

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A second thought experiment: same milk, same technique — one jug stopped at around sixty degrees Celsius, the other taken very hot, close to eighty.

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Which tastes sweeter? Which has better foam? Which could you drink straight away?

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Observation

In the first experiment, the jug with the tip held at the surface makes a constant hissing sound and swells quickly into a big mass of coarse foam, with nothing mixed in underneath. The jug with the tip buried deep has almost no foam; it just gets hot.

Yet their temperatures barely differ: wherever the tip is, the steam is heating. That is the core of this chapter — steam is doing two jobs:

• Adding air: with the tip near the surface, the jet of steam draws air into the milk.
• Heating: when steam meets cooler milk it condenses into water, releasing a great deal of heat Confidence: [Established Mechanism]. As long as the steam is on, heating is happening.

Air is something you control — whether to add it and how much; heating you control only by when you stop.

In the second experiment, most people find the jug at around sixty degrees sweeter and smoother Confidence: [Rule of Thumb]; the one near eighty is too hot to drink straight away and often tastes cooked. One thing should be clear: a few dozen seconds of heating does not add any sugar to milk Confidence: [Established Mechanism]. Why milk tastes sweeter in a certain temperature range has no reliable answer yet Confidence: [Open Question].

Model

What foam is: bubbles wrapped in protein

Milk foam is a huge number of small bubbles dispersed through milk. Bubbles on their own are very unstable; they quickly burst and merge. What holds them up is the milk's protein: proteins move to the boundary between air and liquid and form a thin film around each bubble Confidence: [Established Mechanism].

How much protein there is changes the foam: one study found that in skim milk, the higher the protein content (within the range it tested), the smaller the bubbles Confidence: [Strong Evidence].

Microfoam means bubbles that are very small, very numerous and evenly dispersed through the milk: the surface is glossy, it flows, and it can be poured into coffee and blend in. Its opposite is dry foam: big bubbles piled on top, separated from the liquid below.

Fat: the source of richness, and a foam breaker

The fat in milk brings a rich mouthfeel and aroma, but it is not friendly to foam: milk fat hinders both the formation and the stability of foam Confidence: [Strong Evidence]. Temperature changes this too: one study measured from fridge temperature to near boiling and found the harmful effect of fat was most pronounced in the cool-to-warm stretch Confidence: [Strong Evidence].

Research done on a café-style steam wand points the same way: the higher the milk's fat content, the less stable the foam, but the less liquid drains out beneath it Confidence: [Strong Evidence]. In other words, skim milk often foams more easily into a large, stiff mass, while whole milk makes less foam but a richer, smoother texture. Which is better depends on what you want Confidence: [Preference].

There is also something about freshness: free fatty acids, produced when milk fat breaks down, strongly impair foaming Confidence: [Strong Evidence]. Milk that has sat open for a long time, or warmed up and been chilled again, is harder to foam well Confidence: [Rule of Thumb].

Steam does two jobs: stretch first, then roll

Since steam adds air and heats at the same time, steaming milk is about separating these two jobs in time. The usual approach has two stages Confidence: [Common Practice]:

1. Stretching (adding air): the tip sits just below the surface, you hear a soft, steady hiss, and the milk slowly rises. For less foam (say, a latte) keep this short; for more (a cappuccino) make it longer.
2. Rolling (spinning and heating): lower the tip a little so the milk spins in the jug. The spinning pulls big bubbles under, breaks them up and mixes them in, while heating continues until the temperature you want.

Air is best added at the start, while the milk is still cool Confidence: [Rule of Thumb]: there is still time before you stop, leaving the rolling stage room to make the bubbles fine. Add air once the milk is already hot, and often all you get is big bubbles floating on top. Using milk straight from the fridge and a cold jug Confidence: [Common Practice] works on the same principle: the colder you start, the more time you have.

So the answer to "how do you control them separately" is: air through the tip's depth and how long you stretch; temperature through when you turn the steam off.

Temperature: why stop at 55–65 °C

Many cafés heat milk to around fifty-five to sixty-five degrees Celsius Confidence: [Common Practice]. There is more than one reason:

• You can drink it right away. The temperature people like to drink hot beverages at is roughly around sixty degrees, and it varies from person to person Confidence: [Strong Evidence]. Hotter drinks take a long time before you can drink them, and scald more easily.
• It tastes sweeter and smoother. As above, this is most people's experience Confidence: [Rule of Thumb]; the reason is not settled.
• Too hot changes the taste. Heated close to boiling, milk often develops a cooked taste, and the foam more easily turns coarse and dry Confidence: [Rule of Thumb].

This range is a convention, not a rule. Some people simply like their coffee very hot, and there is no right or wrong in that Confidence: [Preference]; just know that what you trade for the extra heat may be some of the sweetness and texture.

Plant milks

Oat, soy and other plant milks have different proteins and fats from dairy milk, and foam differently. "Barista" editions of plant milks made for coffee are usually formulated to deal specifically with foaming and stability Confidence: [Common Practice]. The principle of steaming is the same — air first, then roll and heat — but each product's "feel" has to be learned anew.

Back to the cup

Milk dilutes and softens coffee: a stronger espresso holds up better in milk Confidence: [Rule of Thumb]. Names like latte, cappuccino and flat white differ mainly in how much milk and how much foam; the exact proportions vary widely between places and cafés Confidence: [Common Practice], so the names are only a rough direction.

Transfer

The "two jobs" model applies to any way of foaming milk:

Reading a jug of foam: foam too coarse and floating on top usually means air went in too late or too much, or not enough rolling; almost no foam means the stretching stage was too short; a sharp screeching sound is commonly taken as a sign that the tip is in the wrong place Confidence: [Common Practice].

Foam keeps even less well than coffee Confidence: [Rule of Thumb]: once steamed, big bubbles slowly rise and burst and the liquid sinks to the bottom. So pour soon after steaming.

The Atlas "Milk Lab" scene is still being built; once it is ready, you will be able to drag the tip's depth and position and see when adding air, rolling and heating each happen.

Misconceptions

"The hotter the milk, the better it tastes." Hotter milk is not sweeter, and heating does not add sugar to it. Above roughly seventy degrees, most people taste less sweetness, not more, and the cooked taste appears Confidence: [Rule of Thumb] — and you have to wait a long time to drink it. Liking it hot is a preference, but "the hotter the better" does not hold.

"Whole milk always foams best." Research has repeatedly found that fat weakens foaming; skim milk often makes more, stiffer foam. Whole milk's strength is texture: less foam, but richer and smoother. Whether foam is "good" depends on whether you want quantity, stability or mouthfeel.

"The more foam, the better." The amount of foam should follow the drink. Too much foam on a latte covers the coffee and will not pour a pattern; being fine and fluid matters more than being plentiful.

Real Lab

This chapter has no real-lab card. You can do these without an espresso machine — pick either.

French-press frothing (change only one variable: fat content).

1. Prepare one cup of whole milk and one cup of low-fat or skim milk, the same amount of each.
2. Heat each to around sixty degrees Celsius (with a thermometer; without one, until the cup feels hot in your hand but you can still hold it for a few seconds).
3. Pour into the French press, no more than a third full. Pump the plunger the same number of times at the same speed for each.
4. Tap the press on the counter a few times and swirl it so big bubbles burst and the foam turns fine.
5. Pour into two identical cups and compare: which has more foam? Which still has foam after five minutes? Which tastes richer and sweeter?

Temperature side by side (change only one variable: temperature). Heat the same milk to around sixty degrees in one cup and around seventy-five in another, and have someone shuffle them for you. Taste once the hotter one has cooled enough to sip, comparing which is sweeter and whether there is a cooked taste. Careful: milk at these temperatures is very hot, so take care when pouring and handling it.

With an espresso machine. Using cold milk and a cold jug, practice the two stages deliberately: stretch for a few seconds while listening for the hiss, then lower the tip so the milk spins, and stop at around sixty degrees by thermometer. In the next jug change only one thing: double the stretching stage, and compare the amount and texture of the foam.

Write it down: what you changed, and how the amount of foam, its fineness and the taste each changed.

Check Your Model

Predict. A friend steams milk by keeping the tip buried deep at first, and only once the milk is already hot lifts it to the surface to add some air. Before looking at any result, write down: what will this foam be like?

Diagnose. A reader says: "I always use whole milk and steam it very hot, because I think that gives the most foam and the best flavor. But my lattes always have a lump of dry foam on top and taste a bit cooked." What might they have misunderstood? What would you suggest they change first?

Explain. In your own words, explain to a friend: why do we say steam does two jobs? Why do many cafés stop milk at around fifty-five to sixty-five degrees?