Why your milk foam collapses

The air went in late, or went in too long

Foam collapses because of bubble size. Small bubbles hold each other up and behave like a liquid; large ones drain, merge and float, and what floats separates. Everything below is a different way of ending up with large bubbles.

The first way is aerating at the wrong moment. Air introduced into cold milk gives small bubbles, because the proteins are still able to unfold and wrap around them. Once the milk has warmed past roughly 40 °C that window is closing, and any air added after it goes in as coarse bubbles that never get finer. If you are still hearing the tearing sound halfway through the steam, the foam is already lost.

The second is aerating for too long. Every extra second is more air in a jug that already has enough, and the volume you gain comes as dry, stiff foam. It looks impressive and it drinks like a bad hot chocolate.

The fix: introduce air in the first 3 to 4 seconds only, while the milk is still cold, and stop when the jug has gained about a third in volume.

No whirlpool, so the air never folded in

Introducing air and texturing milk are two separate jobs, and most home steaming only does the first. The air you put in at the start is sitting near the surface as a layer of bubbles. What turns it into microfoam is the whirlpool: with the tip fully submerged and off-centre, the steam drives the whole jug into rotation, and that rotation shears the big bubbles down and drags them through the rest of the milk.

Skip it and you have milk with foam on top, which is exactly what you get in the cup, in two layers, with a dry white cap and a thin drink underneath. You can see when it has worked: the surface goes glossy and moves like wet paint rather than showing individual bubbles.

The fix: after the aeration window, drop the tip 1 to 2 cm below the surface, angle it off-centre until the milk turns, and keep it spinning for another 15 to 20 seconds with no further air.

The milk went past 65 °C

Heat does two things at once, and they pull in opposite directions. Up to a point, warming makes the milk taste sweeter and keeps the foam workable. Past about 65 °C the proteins holding the bubble films have taken all the heat they can, the structure gives out, and the foam starts draining while you are still standing at the machine. The flavour goes at the same time: the sweetness flattens into a cooked, slightly boiled note.

This is the easiest of the four to fix because it is a single number, and the hardest to notice because there is no sound to warn you. The jug just keeps sounding right while the milk goes past the point of no return.

The fix: stop at around 60 °C. With no thermometer, cut the steam the second the jug becomes uncomfortable to hold, which is 5 °C earlier than you think.

You waited too long before pouring

Textured milk is not a stable thing you made and can now use. It is a suspension that starts separating the moment it stops moving, and it has roughly twenty seconds of genuinely pourable life before the foam and the liquid begin to part. This is why the order of operations matters: the espresso is pulled first, the milk is steamed second, and the drink is poured immediately.

Steaming the milk while the shot pulls feels efficient and costs you the texture, because the milk is then waiting for a cup that is not ready. Steaming more milk than the drink needs has the same effect on the second cup, which sits in the jug while you pour the first.

The fix: pull the shot first, steam second, and pour within 20 seconds. Groom the milk on the way: one tap of the jug on the counter to burst the surface bubbles, then swirl until it is glossy again. Steam only what you need, about 120 ml for one cappuccino.