Milk is a very nutritious food. Milk contains 84% ​​water. The remaining 16%, protein, fat elements, lactose.

It also supplies many important vitamins and minerals. A 1-cup (237-mL) serving of whole milk provides:

Calories: 146
Protein: 8 grams
Carbs: 11.4 grams
Fat: 8 grams
Calcium: 300 mg (23% of the Daily Value (DV))
Riboflavin: 0.337 mg (26% of the DV)
Vitamin D: 2.68 mcg (13% of the DV)
Phosphorus: 246 mg (20% of the DV)
Vitamin B12: 1.32 mcg (55% of the DV)

When milk is boiled at 203°F (95°C), these substances form a thin layer on top of the milk, called whey(also called milk cream).

When the milk boils well and the water starts to evaporate, this top layer prevents the water vapor from escaping outside the pan. This increases the vapor pressure and pushes the whey upwards. The milk overflows.

When this top layer is blown, stirred with a spoon, or sprayed with water, the vapor escapes from the pan. When you break a hot puffy puri, the vapor exits and puri becomes thin. Same concept for milk. 

Water does not have the protein or fat to form this top layer. So the steam escapes easily. But milk overflow.

It’s best to heat your milk slowly over medium heat and stir it while it comes to a boil. Stirring and heating gently help hold the water, carbs, fat, and protein in milk together.

As soon as you see bubbles forming around the edges of the pot and just a few in the middle, turn off the heat.

The higher you heat your milk, the more likely it is that you’ll denature the proteins and cause curdling. When cooking at a higher heat, you’re also more likely to notice taste and color changes from the Maillard reaction.

Continue to stir your milk as it cools. That should prevent skin from forming on top of the milk. If it does form, it’s perfectly safe to eat. However, if you don’t care for its chewy texture, you can skim it off and discard it.