Microfoam is milk with air bubbles so small they cannot be seen individually, distributed evenly through the liquid.
The structure
Air bubbles of roughly 10 to 100 micrometres, surrounded by:
Denatured proteins. Heat unfolds casein and whey proteins, and they migrate to the air-water interface and form a stabilising film around each bubble.
Fat globules, which coat and lubricate, giving the characteristic mouthfeel and slowing collapse.
Why it feels the way it does
Bubbles that small are below the threshold of tactile detection. Your tongue registers a continuous liquid with more body, not a foam.
That is the difference between microfoam and foam: foam has bubbles you can feel.
Why steam is required
Two things happen simultaneously at the wand tip:
Air injection at the surface. Violent shear in the vortex, which breaks the injected bubbles down progressively.
Whisking injects air without the shear. That is why frothers make foam and steam makes microfoam.
Why temperature limits it
Proteins denature progressively. Below 37 °C they can still wrap new bubbles. Above 70 °C they are spent and the structure collapses.
That is the whole reason for the stretch-then-texture sequence and the 65 °C ceiling.
Why it separates
Thermodynamically, foam is unstable. Bubbles coalesce and rise. Even perfect microfoam separates within a couple of minutes.
Pour promptly.
Last reviewed 8 August 2026