The skim that forms on top of milk

Hey, Mattie Alicia: Why does a skin form on top of milk when you heat it up? — Insomniac in U.C.

Milk is a pretty unstable liquid. Treat it badly, heat it too high too fast, and all hell breaks loose. The sticky skin is casein and calcium clumping together as water evaporates from the surface. Skim it off, and you’re whisking away nutrients. That scorched stuff on the bottom of the pan is casein and whey that sinks when it’s heated. More lost nutrients. Nighty-night.