How to Steam Milk
Two seconds of air while the milk is still cold, then spin it smooth and stop at 60°C. Everything else about steaming milk is a consequence of those three numbers.
Milk is half of most coffee drinks by volume and nearly all of what goes wrong with them.
The good news is that it is a short technique with a small number of failure modes, and each failure has a single cause you can name. Once you know which one you are looking at, the fix is obvious.
The method
This is for a flat white or a latte. The only thing that changes between drinks is how long you aerate.
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Start cold, and fill to the spout
Cold milk straight from the fridge, into a jug filled to the bottom of the spout. That is roughly a third to a half full.
Cold buys you time: you only get to add air below about 37°C, so starting at 4°C gives you a working window and starting at room temperature barely gives you one. Underfilling means the wand cannot reach deep enough to spin the milk, and overfilling means it climbs out as it expands.
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Purge the wand first
Open the steam valve for a second into the drip tray before the wand goes in the milk.
Steam wands hold condensed water between drinks, and a tablespoon of water in a small jug of milk is enough to thin it noticeably.
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Set the angle, then open the steam fully
Put the tip just under the surface and off centre, with the jug tilted slightly so the wand points down one side rather than straight into the middle.
Open the steam all the way. A half-open valve gives you neither the air nor the spin, and it is a common habit among people who find the noise alarming.
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Aerate: a two-second hiss
You want a steady sound like tearing paper. Not a shriek, not a gurgle. If the jug is silent, the tip is too deep; if it is screaming and spitting, it is too shallow.
Two seconds for a flat white, four or five for a cappuccino. That is the only difference between those drinks at this stage.
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Submerge and roll it smooth
Drop the wand deeper so the hissing stops, and keep it off centre so the milk turns in a tight whirlpool.
This is where microfoam actually gets made. You have already added all the air you are going to; the vortex now shears those bubbles down until they are too small to see. A jug that is not visibly spinning is not texturing.
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Cut the steam at 60°C
Sixty to 65°C. With a thermometer it is easy; without one, the moment the side of the jug becomes too hot to keep your palm on is about right, and that lands around 60°C for most people.
Turn the steam off before lifting the wand out, or you will spray milk across the machine. Then wipe the wand and purge it immediately, because dried milk is much harder to remove than wet.
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Groom it, and pour straight away
Tap the jug once on the bench to collapse any large bubbles, then swirl it until the surface looks like wet paint, glossy and moving as one mass.
Pour within about thirty seconds. Textured milk separates as it stands, with foam rising and liquid sinking, and a jug left for a minute has to be swirled hard to bring it back.
How much air each drink takes
Aeration time assumes a domestic machine and a 350 ml jug. A commercial machine with more steam power needs less.
| Drink | Aerate for | Foam depth | Served at |
|---|---|---|---|
| Cortado | 0 to 1 second | Almost none | 80 ml |
| Piccolo latte | 1 to 2 seconds | 3 mm | 100 ml |
| Flat white | 2 seconds | 5 mm | 160 ml |
| Caffè latte | 2 to 3 seconds | 5 to 8 mm | 250 ml |
| Cappuccino | 4 to 5 seconds | 1.5 to 2 cm | 150 to 180 ml |
| Latte macchiato | 4 to 5 seconds | 2 cm, settled | 300 ml |
Count from when the hissing starts, not from when you open the valve.
What is actually happening
Three things, in order, and each has a temperature attached to it.
Air gets trapped. Holding the tip at the surface drags air into the milk as bubbles. Milk proteins, mostly casein and whey, unfold at the surface of each bubble and link up into a film that holds it together. This only works well while the milk is cold, which is why aeration has to happen in the first seconds. Past about 37°C the proteins have already begun to change and new bubbles do not stabilise, so air added late stays as large bubbles that pop within a minute.
The bubbles get broken down. Spinning the milk hard shears those bubbles against each other and against the jug. Microfoam is not a different substance from foam; it is foam whose bubbles have been made too small to see individually, which is what gives it the sheen and the weight of wet paint. A whirlpool is not decoration, it is the mechanism.
The sweetness peaks, then goes. Lactose does not increase with heat, but your ability to taste it does, and milk reads as sweetest around 60 to 65°C. Past roughly 70°C the whey proteins denature and release sulphur compounds, which is the scalded, faintly eggy flavour of over-steamed milk. That change is irreversible. There is no technique that recovers milk taken to 75°C, and reheating already-steamed milk makes it worse again.
How different milks behave
Protein builds the foam, fat destabilises it but carries flavour. Every milk is a different point on that trade.
| Milk | Foams | Notes |
|---|---|---|
| Whole dairy | Best overall | The 3.5 percent fat gives silkiness; enough protein to hold it. The default for good reason. |
| Semi-skimmed | Very well | Slightly stiffer, drier foam. Easier to over-aerate. |
| Skim | Stiff and dry | Lots of foam, no body. Good for cold foam, poor for latte art. |
| Barista oat | Nearly as well as dairy | Formulated with added protein and stabilisers. The reliable non-dairy choice. |
| Standard oat | Poorly | Thin foam that collapses. Fine in the drink, bad on top of it. |
| Soy | Reasonably | Good protein. Can curdle against very acidic coffee; warm it gently and pour the milk into the coffee. |
| Almond | Badly | Little protein. Expect large bubbles and fast collapse. |
| Half and half | Fast, then collapses | High fat destabilises the foam. Aerate briefly, stop at 55°C, pour immediately. |
Reading your mistakes
Large visible bubbles on the surface. Too much air, or air added after the milk warmed up. Shorten the aeration and make sure it all happens in the first couple of seconds.
Stiff foam that sits on top like shaving cream. Over-aerated and under-spun. You added plenty of air and never broke it down. Submerge the wand sooner and make sure the milk is actually turning.
No foam at all. The tip never reached the surface, or the valve was not fully open. You should hear the aeration; silence means nothing is happening.
Milk tastes scalded or slightly eggy. Over 70°C. Stop earlier. Nothing fixes it afterwards.
Foam and liquid separate in the jug. Normal, and it happens within thirty seconds. Swirl it back together and pour faster next time.
Latte art blurs and disappears into the crema. The milk was too thin, meaning not enough air, or poured from too high. Drop the spout to the surface once the cup is two thirds full.
Milk splits in the cup. Usually non-dairy milk against hot acidic coffee. Pour the coffee into the milk rather than the other way round, keep the milk below 60°C, and use a barista formulation.
Questions
What temperature should steamed milk be?
Sixty to 65°C. Milk tastes sweetest there, and above about 70°C the whey proteins break down and release the scalded, slightly sulphurous flavour of over-steamed milk. That change cannot be undone.
Why is my milk full of big bubbles?
Too much air, or air added too late. Milk only takes air well while it is still cold, below about 37°C, so all the aeration has to happen in the first couple of seconds. After that, submerge the wand and spin.
Can I re-steam milk?
No. Reheating already-steamed milk pushes it further past the point where the proteins denature, and the foam will not rebuild properly. Steam what you need and start fresh.
What size jug should I use?
A 350 ml jug for one drink, 600 ml for two. The milk should reach the bottom of the spout. Too large a jug and the wand cannot create a vortex in a small volume; too small and the milk climbs out.
Do I need a thermometer?
For the first few weeks, yes. After that the side of the jug becoming too hot to hold comfortably is a reliable marker for about 60°C, and most baristas work by hand rather than by gauge.
Why does my oat milk split?
Acid and heat together. Use a barista formulation, keep it below 60°C, and pour the espresso into the milk rather than the milk onto a hot shot. A very acidic light roast will split some oat milks regardless.
Can I steam milk without an espresso machine?
You can froth it, which is not quite the same. A handheld frother, a French press plunged rapidly, or a sealed jar shaken hard then microwaved all produce foam. None of them produce true microfoam, because that needs the shear of a steam jet, so latte art is out.
Sources
- Specialty Coffee Association brewing standards Background on serving temperatures.