Fats are liquid when they heat up but solid at room temperature. As they heat up they liquify and begin to coalesce. This can create a greasy mouthfeel, particularly in unhomogenised milk. When the globules are in a liquid phase, they tend to spread out across the films of each bubble. Dairy technologist Pieter Walstra explains,
‘The spreading of liquid fat can also destabilize foam bubbles by causing liquid in the film to flow away from the spreading liquid fat, leading to thinning, and ultimately rupture of the film’ (Walstra 1996).
At higher temperatures this phenomenon is less of a problem.
‘Above 40°C, the milk fat globules contain fully liquid fat and are far less susceptible to disruption or deformation by the foam lamella’ (Mulder and Walstra 1974).
If, as you steam milk, you add a lot of air to the cold milk at the outset, then once the milk temperature climbs to room temperature and you arrest the flow of steam, you will notice that the foam collapses almost entirely. At room temperature, milk is at its least foamable. Borcherding et al (2008) suggest this is due to the existence of solid fats in the milk, which, after they exceed 37°C in temperature, become entirely liquid. The solid particles cause the bubbles to burst.
‘At 20°C, fat globules consisted of about 20% of crystallised and about 80% liquid fat which was obviously detrimental to the foam formation. Above 37°C, milk fat existed solely in a liquid state and the fat globules were more resistant against mechanical treatment’ (Borcherding et al 2008).
As whole milk foams cool down, the milk fat globules reform and the problem comes back. Foams begin to break up again as temperatures drop below room temperature. This suggests over 37°C is the high temperature required for stability to fulfill Foam Factor 3 (stability of your foam at a high temperature).