There’s an increase in perceived sweetness, primarily a biological illusion driven by our tongue, combined with a structural change in milk proteins.
Taste receptors on our tongue contain a microscopic ion channel called TRPM5, which is responsible for signalling sweet, bitter, and umami tastes to your brain. The TRPM5 channel is highly sensitive to temperature. It opens significantly wider when food or drink is warm (specifically between 15 - 35C, sending a much stronger electrical signal to your brain for the exact same amount of sugar.
When you sip milk at 65C, it cools slightly in your mouth to that perfect 35C window. If you heat the milk past 70C, it scalds your taste buds, desensitising the TRPM5 channels and flattening the flavour profile entirely. Also past 70-75C there’s irreversible protein degradation and sulphur compounds are released, masking the sweetness of the lactose and ruins the flavour profile of speciality coffees.
Chemically, the only major transformation happening at 65C for milk involves whey proteins (beta-lactoglobulin). As temperature rises toward 60-65C, these globular proteins begin to denature. Their hydrophobic ends expose themselves and trap air bubbles introduced by the steam wand, creating a stable, velvety microfoam. This physical texture coats your tongue, slowing down the transit of the milk across your taste buds. This prolonged contact time gives your TRPM5 receptors more opportunity to register the lactose that was already there.