In an attempt to prepare custard or, more rarely these days, to sterilize raw milk bought directly from the farm, you've put some milk on the stove to heat. A few minutes of inattention later, disaster strikes: half of the liquid has escaped the pot and spilled onto the stovetop, accompanied by a loud "psssshhhhh" that surely caught your ears' attention, along with the unmistakable smell of burning, foreshadowing the long minutes you'll now spend scrubbing to clean it all up.
Yet, you knew this would happen—milk is notoriously tricky, after all: there's a reason behind the expression "watch it like milk on the stove." But here we are: once again, you've been caught unaware, and this likely won't be the last time.
Illustration image from Pixabay
But why is milk prone to boiling over, while a pot of water causes no such trouble (beyond rattling the lid or creating the occasional splash in a vigorous boil)? It's all due to the unique composition of milk. While over 90% of it is water, the remaining 10% consists of various other substances—some of which give it foaming properties.
You see, for a liquid to boil over, you need not only bubbles, as boiling can create even in water alone, but also for these bubbles to be stable, forming what is known as foam.
For that to happen, a "surfactant" is necessary—this is a molecule that loves to position itself at the boundary between air and water, helping to stabilize bubbles in the liquid. So, what fulfills this role in milk? Primarily proteins, which, thanks to their structure of amino acids—some of which are hydrophilic (love water) and others hydrophobic (repelling water)—gather around the steam bubbles formed by boiling.
But it gets worse. These proteins don't just create foam (which is great for a cappuccino); they can also coagulate, forming a solid layer on the surface of the milk—known as that infamous "skin" (thickened further by fat globules in the milk, which also rise to the surface, thanks to buoyancy). This skin prevents the bubbles from bursting and releasing their gas, and it gradually gets pushed upward in the pot... and psshhhhhhhh...
Experienced cooks might point out that a similar thing happens when cooking pasta; the water, which wouldn't normally foam on its own, also boils over noisily onto the stovetop here. The culprit? The starch in the pasta, which, though a carbohydrate rather than a protein, creates a similar effect by forming a thin gelatinous layer on the liquid's surface, which is pushed up by steam bubbles until... psssshhhhhh...
How can you avoid these mishaps? First and foremost, by keeping an eye on your cooking (milk, pasta, etc.) so you can take the pot off the heat in time. After that, there are a few tricks to buy (some) peace of mind: for one, there's no point in cranking the heat to the maximum for quicker results. A moderate boil will be more than enough to heat the milk and will also give the bubbles more time to pop as they're formed—especially if you've wisely chosen a pot with plenty of room (relative to the amount of liquid).
Some recommend placing a wooden spoon across the pot to break up the foam and/or skin before it reaches the top. While this method can help sometimes, it's by no means a surefire way to go and read a book while your milk heats up.
Lastly, there's an aptly named tool, the "milk watcher" (anti-monte-lait). It's simply a slightly heavy disk (made of metal or porcelain) that sits at the bottom of the pot. When small bubbles form, they get trapped beneath the disk, gathering into a larger bubble until it's big enough to lift the disk and escape to the surface.
The advantage? Twofold: not only do larger bubbles, which are more unstable than small ones, pop easily and fail to create foam, but they also pack enough "force" to break through the milk's skin. Additionally, since the disk keeps rising and falling, it makes noise... alerting you that you should probably come and check what's going on!