As much as we love great espresso, there is something pretty amazing about a latte or cappuccino. While we know a lot about the science of coffee, there doesn’t seem to be nearly as much information available about milk and milk steaming.
So what really happens when you steam milk? Why does microfoam break down after a few minutes? And why does skim milk steam differently than whole?
Using a little science, we want to help answer those questions and more. Before we get deep into the physics and chemistry of steaming, let’s start with the basics—the makeup of milk.
The Makeup of Milk
With a cookie or a shot of espresso, milk is darn delicious. But what is it about milk that makes it so desirable? Think of it this way: cow’s milk is essentially a dietary supplement for young calves. It’s full of all the essential nutrients that a baby cow needs to turn into a healthy, happy heffer–namely water, sugar, protein, fat, and minerals.
By the numbers, water makes up the bulk of milk, roughly 87%. The second most prevalent compound in milk is sugar, making up around 4.8%. Cow’s milk has a specific kind of sugar called lactose. Lactose is why milk has a sweet flavor when fresh, and a sour flavor when old (bacteria in milk gradually breaks down lactose into lactic acid as milk ages).
Percent Concentration in Cow's Milk
|Water||85.5 - 89.5||87.5|
|Total solids||10.5 - 14.5||13.0|
|Fat||2.5 - 6.0||3.9|
|Proteins||2.9 - 5.0||3.4|
|Lactose||3.6 - 5.5||4.8|
|Minerals||0.6 - 9.0||0.8|
While it may sound like whole milk has substantially more fat than 2% or skim, it only has 3.9% fat content on average. The fat in milk gives it a creamy texture. In non-homogenized milk, fats float around in various sized globules, often combining into a fat cap at the very top of the milk jug. With homogenized milk, the milk is forced through a small nozzle; this breaks up the fats globules into uniform droplets and dispenses them evenly through the milk. Milk proteins make up around 3.4% of the total volume of whole milk. The protein in milk is responsible for creating and stabilizing the foam. We’ll circle back to the effect of protein and fat on milk foam in a moment.
The remaining .8% is mostly different minerals, such as calcium and magnesium. These minerals have a fairly limited effect on the flavor and steaming qualities of the milk.
How Steaming Works
When you steam milk, you’re essentially doing two things simultaneously: making foam by introducing air and heating the milk. For a quick refresher on milk steaming technique, check out our guide to steaming here: How to Steam Milk. Here’s how and why that technique works:
When you turn on your steam wand, jets of steam shoot out through the holes on the steam tip. When the steam tip is at the surface, the jets of steam act like tiny, fierce whisks–they rapidly inject air by breaking the surface of the milk and folding air into the milk. If the steam tip is overly exposed or is above the surface, large bursts of air are forced into the milk creating big, unwieldy bubbles. If the steam tip isn’t exposed at all, the surface won’t break; the milk will still warm up from the hot steam but there won’t be any air introduced, meaning no foam.
While air is getting introduced, the steam is also heating the milk. Remember those proteins from earlier? In cold milk, they exist in tightly coiled bundles. Then, as milk warms, the proteins unwind and start to wrap around the air bubbles, forming a kind of protective jacket. This is because one side of the protein is hydrophobic, meaning water repellent, and the other is hydrophilic, or attracted to water. The hydrophobic side will cling to the air bubbles, while the hydrophilic side holds close the water in the milk. It’s this action that captures the air in the liquid and gives milk its unique foaming properties.
Unfortunately, if the milk gets too hot, the proteins will completely break down, or denature, releasing the air and ruining your foam. This is why it's important to try to introduce air before the milk hits room temperature. After milk hits 100°F or room temperature, it is significantly harder to get those air bubbles to turn into a nice, velvety microfoam.
Heating milk has another added benefit–heat helps enhance the perceived sweetness of milk. The longer chain carbohydrates (sugars) naturally present in milk break down into simpler sugars with heat. Just like the difference between refined sugar and wheat, the simpler the carbohydrate is, the easier it is to taste.
Steam doesn’t just add air and heat the milk; the force of the steam jets moves the milk in the pitcher. By carefully positioning the steam wand, we can use the force to stir in all the great foam. Why is this important? Air is lighter than milk. If left to its own devices, it will float to the surface and form a thick head, almost like a freshly poured lager. While that can be nice to drink, it’s nigh impossible to pour into great latte art. To get the perfect steam wand position, all you have to do is set your steam wand so it is just off center. You’ll know you’ve hit the sweet spot when the milk spins in the pitcher like a tornado or vortex.
Now a bit of bad news–even the best microfoam will eventually collapse. For that, we have gravity to thank. Over time, the light air bubbles rise and heavier liquid milk drains from the foam. As the liquid drains, the small bubbles combine into larger and larger bubbles. This continues until the pressure of the air inside the bubble exceeds the strength of proteins holding it in and the bubble pops.
The Effect of Milk Fat on Foam
You might have heard skim milk is significantly better for foaming. Or your coffee friend swears the whole milk is the best at making luscious cappuccinos. It turns out both things are kind of accurate.
Fat has a destabilizing effect on foam. Remember those unwinding proteins from earlier? The hydrophobic part of milk protein is as likely to attach to fat as it is to air. It just wants to get as far away from water as possible. The more fat there is in milk, the less air it can hold.
With that in mind, skim milk will yield the most stable and stiff foam. Unfortunately, it's extremely difficult to use this kind of foam to pour latte art; instead of flowing smoothly out of the pitcher, skim milk has a tendency to sort of plop into the cup.
Whole milk tends to produce a creamier, more flavorful foam. Let’s be honest–fat tastes great (see the myriad uses of butter in French cuisine). It also helps produce a milk foam that is more velvety than dry, as the foam in whole milk is really an emulsification of air, water and fat.