When it comes to frothing milk, temperature makes all the difference.
In our May/June 2011 issue, we described a way to froth milk by microwaving it in the glass beaker of a French press and then pumping the hot milk into a thick foam with the press’s plunger. Since then, we’ve found that reversing the process and frothing while the milk is still cold produces a creamier, longer-lasting, and more billowy foam.
Here’s why: Agitating milk when it’s cold lets you pump more air bubbles into it. As you pack in more air, the bubbles become smaller and the foam becomes denser. Frothing hot milk produces bigger, weaker bubbles because the proteins have already bound to one another and are less able to coat and stabilize the air bubbles. When milk is heated after frothing, however, the proteins coat the air bubbles before cross-linking and are able to reinforce the structure of the foam that’s already been created.
Here’s the updated technique: