Cappuccinos, lattes, and flat whites all require textured milk. Without foamed milk we wouldn’t get that smooth mouthfeel and the rich body we enjoy. But do you know what is happening when you use the steam wand?
Milk undergoes structural changes at the chemical level when we foam it. Read on to find out more about why milk foams and what is happening when it does.
Milk in an espresso cup.
Milk is a key ingredient in any coffee shop and a significant part of many espresso-based drinks. By better understanding what happens when we foam it, we can improve our chances of preparing a quality cappuccino.
Cow milk is a nutrient-rich and complex liquid that is mainly water but also contains several hundred chemical compounds. These components can be divided into four groups: proteins (1–20%), lipids (2–55%), carbohydrates or sugars (lactose 0–10%), and minerals.
Bottles of milk in Tehran. Credit: Mehrshad Rajabi.
Proteins are the components most affected by heating and that have the most impact on the success or failure of foamed milk. Let’s take a closer look at them.
Generally, proteins can be defined as molecules made of one or more long chains of amino acid residues that are bonded together by polypeptide bonds.
Maybe that’s too much science for you, but it doesn’t really matter whether you understand this. The important part is that milk contains proteins with different structures and sizes that are dispersed throughout the liquid.
There are two groups of milk proteins: caseins and whey proteins. They have different structures and behave in different ways under stress conditions. So they do different things when you heat and foam them.
Adding milk to a coffee.
A protein’s structure is simply the way that its atoms are arranged. Casein proteins present in milk in form aggregates called micelles. Those micelles consist of α-, β-, and κ-caseins, which are proteins with primary structure.
Whey proteins (mainly β-lactoglobulin and α-lactalbumin) are globular proteins with well-defined tertiary and secondary structures.
In short, casein has a simpler structure than whey proteins. And this difference has a direct impact on the way the two groups of protein behave when you introduce them to a steam wand.
A glass of coffee with latte art. Credit: Becca Tapert.
Caseins are much more thermally stable than whey proteins. In other words, caseins keep their structure better when heated.
Whey proteins have more complex 3D structures, which unfold when heated. They start to do this at 40oC (around 104°F).
During this process, which is known as denaturation, whey proteins irreversibly lose their structure. They will always function differently after.
A barista steams milk. Credit: Jordan Madrid.
Any heating affects the chemical structure of milk proteins. But how much it impacts the milk depends on the temperature and the duration of heating.
Let’s assume that you’re using pasteurised milk in your coffee shop. The pasteurisation process means that the milk was heated at 72–80 oC (around 162–176°F) for 15–30 seconds before it gets to you.
Pasteurisation denatures some whey proteins, but because the heating process is kept short, it doesn’t affect all of them.
And the reason that ultra-heat treated or scalded milk tastes different is because a sulphurous flavour develops during heat treatment.
Creating latte art with steamed milk. Credit: Tyler Nix.
But back to those proteins, because they are what will make your foam a success or a failure.
In milk’s natural state, reactive chemical groups are buried within the complex structures of whey proteins. Those groups become exposed when the whey protein unfolds during heating.
Because those chemical groups are reactive, they form new bonds within the unfolded structure and with other milk components. And this has an effect on how milk foams.
Pouring a tulip with steamed milk. Credit: Kat Stokes.
So what does all this science mean for your cappuccino?
When we foam milk, we are forcing water vapour and air into milk while heating it. The proteins create spheres around the air and stabilise into bubbles.
The protein chains in milk are polar: one end is hydrophilic (attracted to water), and the other is hydrophobic (repelled by water). When the proteins unfold during denaturing, they expose their ends and the hydrophobic ones try to get away from the water in the milk.
This means that within each air bubble, the hydrophobic ends are all pointed inwards, towards the water-free interior. The hydrophilic ends are exposed to the watery milk environment that the bubbles are suspended in. This structure helps keep the air bubbles intact.
A barista creates latte art.
When milk is foamed between 30oC and 40oC (86–104oF), it is unstable. Large air bubbles forming within a few minutes. Raising the temperature to 60oC (140oF) results in more stable foam and improvement of texture and density. Smaller and better-dispersed air bubbles are formed at higher temperatures.
Fat plays a role in stabilising these bubbles. At temperatures above 40oC (104oF) all of the lipids in milk melt. This liquid fat helps prevent air bubbles from coalescing (joining together to create a large air pocket) by creating a film on the surface of the air bubbles.
Pouring steamed milk into a coffee. Credit: Tim Wright.
But be careful of heating the milk to too high of a temperature. Not only does scalded milk taste sulphuric, but you will reach a point where the foam will fail.
Proteins in their natural state cover the air bubbles and help to protect them from coalescence. If you continue to heat the milk, more of the proteins will denature and there will not be enough left in their organic state to stabilise the air bubbles.
This is why you can’t refoam milk – after reheating, there won’t be enough proteins with organised structures to create the stabilising layer.
A latte art tulip. Credit: Drew Coffman.
It may seem like milk with more fat content would be better for consistently stable foam. But butterfat, the main type of fat found in milk, is a large and heavy globule.
More than 95% of total milk lipids is in globules ranging from 0.1 to 15 µm in diameter. The fat content can be so large and heavy that it weighs down air bubbles, making foam collapse. Fat can also mask other flavours, meaning you could lose some of the flavour notes of your specialty coffee if you pair it with cream.
But before you reach for the skimmed milk, remember that fat is what gives you the smooth mouthfeel that is so appealing in a cappuccino or latte.
Adding cold milk to a glass of coffee. Credit: Alberto Bogo.
When selecting milk for espresso-based drinks, the key thing to consider is the protein content. Without proteins, your milk won’t foam. Barista milk is a specific product that has high protein content for this reason. But you can use regular milk if you’re careful about the temperature.
The ideal temperature to foam milk is between 60–63oC (140–145oF). Below this, you will get unstable foam with large bubbles. Above this, too many of the proteins will denature. There will not be enough left in their organic state to stabilise the bubbles.
And skimmed milk will give you the most stable foam, but may not have the creamy mouthfeel you desire. Compromise with a semi-skimmed or half-fat milk for reliable foam with rich mouthfeel.
Pouring steamed milk into an espresso-based drink. Credit: Trent Erwin.
Understanding the chemical composition of milk can help create a better espresso-based drink. Through recognising how milk proteins work, you can be sure to avoid your foam falling flat.