An eggy, sulphurous, cooked flavour in steamed milk means it went too hot.
The chemistry
Whey proteins — beta-lactoglobulin in particular — contain sulphur-bearing amino acids.
Above roughly 70 °C, these denature and release hydrogen sulphide and related compounds. That is the smell of cooked egg, and it is the same reaction.
The threshold
60 to 65 °C — sweet, textured, correct. 70 °C — sweetness fading, cooked notes appearing. Above 75 °C — clearly eggy, flat, and no longer foamable.
Why it happens
Steaming for too long, usually because people are watching the foam rather than feeling the jug.
Trying to make the drink hotter so it survives a commute or a slow drinker.
Not stopping when the jug becomes too hot to hold. That is the signal, and it arrives sooner than most people expect.
It cannot be fixed
The reaction is irreversible. Cooling the milk does not recover the sweetness.
Getting a hotter drink properly
Preheat the cup. A cold ceramic cup steals several degrees immediately, and warming it costs nothing.
That is the correct way to serve a hot drink without cooking the milk.
Judging it by hand
Wrap your hand round the jug and stop when you cannot hold it comfortably. Calibrate once with a thermometer and you will not need one again.
Last reviewed 8 August 2026