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Why is milk foaming and how does it affect your coffee?

Cappuccino, latte or flute white – any of these drinks is impossible without textured milk. Without foamed milk there would be no such soft tactile perception and expressive body that we like. But do you know what happens when you use a steamer?

During foaming, milk undergoes structural changes at the chemical level. Read this full article to find out why milk foams and what happens to them.

Milk in an espresso cup.

The chemical composition of milk

Milk is a key ingredient in any coffee shop and an essential ingredient in many espresso based drinks. If we understand better what happens when we foam milk, we will improve our skills in making quality cappuccino.

Cow’s milk is a nutrient-rich complex fluid that is mostly made up of water, but also contains several hundred chemical compounds. They can be divided into four groups: proteins or proteins (1-20%), lipids or fats (2-55%), carbohydrates or sugars (lactose 0-10%) and minerals.

Bottles of milk in Tehran. Photo: Mehrshad Rajabi.

It is clear about milk proteins

Proteins are most prone to heat and have the greatest effect on the successful or unsuccessful foamed milk. Let’s take a closer look at them.

Generally, proteins can be defined as molecules made from one or more long chains of amino acid precipitates that are bound together by polypeptides.

It may have sounded too scientific, but it doesn’t really matter if you understand it. What is important is that milk contains proteins with different structures and sizes that are scattered throughout the liquid.

There are two groups of milk proteins: caseins and whey proteins. They have a different structure and behave differently under stressful conditions. Therefore, they respond differently to heating and foaming.

Add milk to the coffee.

Protein structure is simply a way of arranging its atoms. Casein proteins are present in milk in the form of clusters (aggregates) called micelles. These micelles consist of ?-, ?- and ?-caseins, which are proteins with primary structure.

Whey proteins (mainly β-lactoglobulin and α-lactalbumin) are globular proteins with a distinct tertiary and secondary structure.

In short, casein has a simpler structure than whey proteins. And this difference directly affects how two groups of proteins behave when a steamer is taken to work.

Late Art Coffee Photo: Becca Taper.

Caseins are much more thermally stable than whey proteins. In other words, caseins better retain their structure when heated.

Whey proteins have more complex 3D structures that unfold when heated. This occurs at 40 o C (about 104 ° F).

During this process, known as denaturation , whey proteins lose their structure irreversibly. After that, they will always function differently.

Barista spіnyu milk. Photo: Jordan Madrid.

How heating affects milk

Any heat affects the chemical structure of milk proteins. But the degree of exposure depends on the temperature and duration.

Suppose you use pasteurized milk. The pasteurization process means that the milk was heated at 72-80 ° C (about 162-176 ° F) for 15-30 seconds before it reaches you.

Pasteurization denatures serum proteins, but as this process does not last long, some remain intact.

And the reason that ultra-heat-treated or baked-milk differ in taste is that it produces a sulfuric taste during heat treatment.

Infusion of latex art with foamed milk Photo by Tyler Knicks.

But let’s get back to the proteins, because they are the basis of successful or unsuccessful foaming.

In the natural state, the reactive chemical groups in milk are hidden by the complex structures of whey proteins. These groups are triggered when whey proteins unfold during heating.

Because these chemical groups are reactive, they form new bonds within the unfolded structure and combine with other components of milk. And that affects how the milk foams.

The tulip infusion. Photo:  Kat Stokes .

How does this affect milk foaming?

So, what value does the scientific data listed have for your cappuccino?

When we foam milk, we heat and saturate it with steam and air. Proteins create spheres around the air and stabilize at the expense of bubbles.

The protein chains in milk are polar: one end is hydrophilic (attracted to water) and the other end is hydrophobic (repels water). When proteins unfold during denaturation, they trigger the ends and are hydrophobic trying to move away from the water in the milk.

This means that inside each bubble, the hydrophobic ends point inwards because there is no water there. The hydrophilic ends, in contrast, reach for the water component of the milk, which contains the bubbles. Thus, the bubbles remain intact.

Barista pouring late-art.

When milk is foamed between 30 o C and 40 o C (86-104 o F), it is unstable. Within minutes, large bubbles are formed. Raising the temperature to 60 o C (140 o F) stabilizes the foam and improves texture and density. At higher temperatures, smaller bubbles are formed, which are better dispersed.

Bubbles are responsible for stabilizing the bubbles. At temperatures above 40 o C (104 o F) all lipids in the milk melt. These liquid fats prevent the bubbles (merging into one large air pocket) from merging, creating a film on their surface.

Foamed milk is poured into the coffee. Photo by  Tim Wright .

But be careful not to overheat the milk. Burnt milk not only has a sulfuric taste but also stops foaming.

Proteins in their natural state cover air bubbles and protect against fusion. If the milk is overheated, the proteins are denatured and not enough to stabilize the bubbles.

That is why you cannot re-foam the milk – after heating, proteins with an organized structure are not enough to create a stabilizing layer.

Latte art tulip. Photo: Drew Coffman.

It may seem that high-fat milk is better suited for stable foam. But butter, the main type of fat contained in milk, is a large and heavy ball (globule).

More than 95% of milk lipids are contained in globules with a diameter of 0.1 to 15 microns. The fat content can be so large and heavy that it presses on the bubbles, destroying the foam. Fat can mask other flavors, so if you combine your coffee with cream, it will lose the flavor profile.

But before you reach for skim milk, remember that it is the fat that leaves a pleasant tactile impression that many like cappuccino or latte.

Add cold milk to a glass of coffee. Photo:  Alberto Bogo .

What does this mean for you?

When choosing espresso based milk, pay attention to the protein content. Without protein, your milk will not foam. Coffee for coffee is a specific product which for this reason has a high protein content. But you can also use regular milk if you are careful with the temperature.

The ideal temperature for foaming milk is 60-63 o C (140-145 o F). Below, you will get unstable foam with big bubbles. Above, and too much protein is denatured. They will not be enough to stabilize the bubbles.

Skim milk will give you the most stable foam, but it won’t have the creamy taste that everyone likes. The compromise will be semi-fat milk, which will have a reliable foam and rich taste.

Infusion of foamed milk into an espresso based drink. Photo by Trent Erwin.

Understanding the chemical composition of milk can help you create the best espresso drink. By understanding the effect of proteins on milk, you will avoid flat or loose foam.

So, why not test how you learned the new knowledge by making cappuccino?

Author: Yulia Klimanov with the support of Dmitry Koryukin. Photo: Frank Luke.


Translation: Anna Polstiankina.