Foam depends on protein structure, and protein degrades as milk ages.
What happens
Proteolysis. Enzymes and bacteria break down casein and whey proteins over time, even under refrigeration.
Damaged proteins cannot form the stable film around air bubbles that microfoam requires.
The practical timeline
Fresh, several days from the date — foams well, holds texture, pours cleanly.
Within two days of the date — noticeably harder. Foam is coarser and collapses sooner.
At or past the date — will barely foam, even if it still tastes fine.
Milk can taste perfectly acceptable and be useless for texturing.
Temperature
Keep it under 4 °C right up to steaming.
Cold milk gives you more time in the stretching window before reaching 37 °C, which makes the whole process easier.
Milk that has warmed on the counter is harder to texture, and warming and re-chilling accelerates degradation.
UHT milk
Foams, and differently — the ultra-heat treatment has already denatured much of the protein. It produces a stiffer, drier foam and it does not taste as sweet.
Usable, not preferred.
Homogenised versus not
Homogenised is what you want. Non-homogenised milk has larger fat globules that destabilise foam.
Almost all supermarket milk is homogenised.
The practical rule
Buy milk more often, keep it at the back of the fridge, and use it well before the date.
Last reviewed 8 August 2026