Constantly scraping the pan bottom isn't practical. So what is?
When heating milk on the stovetop for custard, bread, homemade ricotta, or even hot chocolate, we're often left with a stubborn film of cooked—or scorched—milk on the bottom and sides of the pan. Constantly scraping the pan bottom as the milk heats to prevent the film from forming isn't practical, but we'd heard that wetting the pan with water before adding the milk would prevent the milk from sticking. We decided to put this bit of kitchen lore to the test.
We pretreated three identical metal saucepans. In the first, we swirled 2 tablespoons of water around the saucepan's bottom and sides. We lightly sprayed the second saucepan's surface with vegetable oil spray. We rubbed the bottom and sides of a third saucepan with butter and left a fourth saucepan untreated. We added 2 cups of whole milk to each saucepan and then cooked each over medium-low heat for 10 minutes before pouring out the milk and examining them.
The saucepan misted with vegetable oil spray had just a tiny bit of milk residue (tasters did not notice any off-flavors from the oil), while the water-treated saucepan was almost as clean. The saucepan treated with butter was no cleaner than the untreated one, and stirring every 2 minutes produced a similar result.
When you add milk to a dry pan, it flows into microscopic imperfections in the pan bottom. As the milk heats, its proteins coagulate and stick to the pan and each other. Misting the pan with vegetable oil spray prior to adding the milk creates a thin film on the pan's surface, which acts as a barrier and makes milk proteins less likely to adhere.
The next time we're heating milk, we'll save ourselves some cleanup and first mist the pan with vegetable oil spray (or swirl in some water if we don't want to add oil).