The difference arises from the components in water and milk.
If you forget to turn off the stove, you might have to clean up a mess of spilled milk. It’s hard to imagine the same happening when boiling water, so why does milk boil over while water does not?
First, let’s take a look at what water and milk are made of.
- Water is primarily composed of water molecules, or H2O, that cluster together to form a liquid. There are some minerals present, but at too low a concentration to affect how it boils.
- On the other hand, milk is a complex chemical mixture, the main components being proteins, fats, and water.
What Happens to Milk When Heated?
The components of milk must interact with each other to create the stable, drinkable liquid we see, but the way they interact will change when heated.
Let’s explore what happens when milk is heated, focusing on one of the proteins found in milk.
When we boil milk, the internal structure of the proteins changes, and they become much firmer than in their usual liquid state. This presents a new opportunity for nearby fat molecules, which clump together to form a gel-like substance.
Being lighter than water and energized by heat, this gel-like substance begins to rise to the surface.
Bubbles formed on the surface of milk due to steam being trapped by the layer of fat and protein. (Source: Alesia.Bierliezova/Shutterstock)
At the bottom of the pot, things are very hot. At such high temperatures, water begins to turn into steam. The steam here is also lighter than water, and like the proteins and fats, it starts to rise to the surface. When it nears the top, it encounters the gel-like substance that prevents it from passing through.
As the steam continues to push upward, bubbles form, similar to how our breath pushes into a soap film to create bubbles. That’s how bubbles are created on boiling milk.
Why Can’t the Bubbles Stay at the Top?
From this point onward, the process continues until the fat, protein, or water is depleted. However, long before the milk runs dry, the bubbles will grow large enough to be pushed over the edge of the pot by newly formed bubbles rising from below.
In summary, the process consists of three steps: Fats and proteins coagulate into a gel-like compound – Boiling water turns into steam rising from the bottom of the pot, forming milk bubbles – finally, the milk overflows.
When bubbles form in water, they can easily escape from the surface. (Source: youranedopekin/Freepik)
In the three steps mentioned above, boiling water only has one step, which is steam rising from the bottom of the kettle/pot.
Since water lacks any substance to trap the escaping steam (like the gel-like layer in milk), it can freely rise to the surface and escape into the air, therefore not creating bubbles and never overflowing.
The only time water might spill over while boiling is likely due to us overfilling the pot.
Is There a Way to Boil Milk Without Spilling?
Now that we know why milk boils over, we can devise ways to prevent it from spilling.
The simplest way to manage the overflow is to address the layer of bubbles formed on top. Since milk spills over due to excessive bubbling, we can prevent it from spilling right there.
Stirring allows steam to escape while mixing the top layer with the rest, thus preventing milk from spilling over. (Source: BlackBoxGuild/Envato Elements)
This can be done by stirring the milk, allowing not only the steam to escape but also redistributing the layer forming on top throughout the pot, preventing steam from being trapped for a time. However, right after you stop stirring, this gel-like layer will form again.
A more passive method to achieve a similar result is to place a large wooden spoon across the pot. This way, there will be a continuous escape route for steam rising to the surface.
Placing a wooden spoon over the pot of milk is a temporary way to prevent spilling. (Source: Ahanov Michael/Shutterstock)
This method works for a while, but as heating continues, the rate of steam generated will exceed the rate at which steam escapes. Consequently, the trapped steam begins to accumulate on both sides, and milk bubbles can still overflow. Overall, to avoid spilling milk while boiling, you need to closely monitor the cooking process. As soon as it begins to boil, you can lower the heat so that the milk doesn’t overheat and spill over, nor does it dry out.
