You have peeled, sliced and washed potatoes.
You put them to cook in milk, at first everything is normal.
Then, when the milk starts to boil, a fairly abundant white foam forms on the surface, which can easily boil over if you don't do anything.
This white foam comes mainly from the starch that escapes from the potatoes during cooking.
This starch is released into the hot milk, gelatinizes and forms a sort of "cloud" or foam on the surface as steam bubbles rise to the surface.
As this happens in milk, the phenomenon is reinforced by its proteins and fats, which stabilize the bubbles, making the foam finer and more persistent.
This foam is not dangerous, and has virtually no taste: it's essentially starch, with a little milk protein, and can be left or skimmed (with a skimmer or a simple tablespoon) to avoid spillage, which can happen very quickly.
Once most of the foam has been removed, the potatoes continue to cook normally.
Note that 2 things can increase the amount of foam:
- The variety and age of the potatoes: flourier varieties and older tubers yield more starch, and therefore more foam.
- the intensity of the boiling and the size of the pan: a high boil with little free surface produces more bubbles and therefore more surface foam.
To sum up: the foam produced when cooking potatoes in milk is simply a reaction between the starch in the potatoes and the milk. It's not at all a problem for the cooking process or your recipe, but you do need to be careful, or even skim it off, to avoid overflowing, which can happen very quickly.
