WHY can hot milk be whipped into a foam?
⠀
The proteins have curdled in it.
⠀
The albumin and globulin proteins in milk coagulate at 60 °C and their molecules stick together. If you saturate hot milk with air bubbles, films of coagulated protein form around these bubbles. That’s is the milk foam.
⠀
Still commonly cold milk is also poured into the cappuccinator so that the device can control the heating process.