About milk in coffee and the formation of milk foam

Coffee with milk: composition

Regardless of the specific type of coffee drink with milk, i.e. whether it is a cappuccino or a Flat White, any of the drinks will contain two liquids: coffee and milk. The composition of that drink then depends on what the coffee and milk are made up of.

What makes up coffee:

2-3% caffeine

3-5% tannins

13% protein

10-15% oils

carbohydrates, oligosaccharides, polysaccharides, cellulose, acids, phenols, lipids, diterpenes, amino acids, proteins, minerals,...

+ 80-90% water, which extractsthe ingredients from coffee

What makes up milk:

13% of the dry matter that makes up:

  • 3-4% fat
  • 3.4% protein (casein)
  • 4.7% carbohydrates (lactose)
  • vitamins, minerals (calcium, protein, B2, B12, potassium, phosphorus, etc.)

+ 87% water in milk composition

Now you know what you're drinking in your favourite cappuccino. So it's time to explain the preparation. I won't go into the basics of milk coffee drinks - espresso - any further, I've already written about its preparation and the coffee extraction process (see link in the text).

However, the preparation of milk froth is no less interesting! To use the exact scientific term for milk, it is: "an emulsified colloid of fat globules dispersed in a water-based solution". To better understand what it looks like in the milk, check out the picture of a drop of milk under a microscope:

A microscopic view of the composition of milk with visible fat globules. Image source: www.sciencedirect.com

Principle of milk churning and milk foam formation

Mechanically learning the correct technique to whip milk is one thing. But understanding what practically happens in milk when it is whipped is the key to understanding all the laws of milk frothing. You'll know how to work better with milk and why milk behaves one way or another in a jug when whipped.

So the theory of the chemical and physical changes when milk is whipped is:

You let steam from the nozzle of a coffee maker into the milk - simplified as water with fat globules and proteins -. Depending on where you place the tip of the steam nozzle in the milk, you decide whether to heat with stirring or aerate the milk.

Simply put, if you have the tip of the nozzle well below the surface, you use the power of the steam to swirl the milk (swirl it in the pot) and its temperature to heat the milk. When you start with the steam nozzle with its tip close enough to the surface of the milk, you take advantage of theability of the steam nozzle to draw air into the milk in addition to the heat and flow of the steam. And that's exactly when the milk starts to froth.

Frothing milk with the steam nozzle of the coffee machine

The force of the steam flow from the nozzle can force air bubbles into the milk - a watery solution. With each additional bubble, the amount of foam increases. How do the air bubbles in the milk not escape to the surface and burst? Themilk proteins are responsible for this air-holding magic.

Proteins or proteins (like casein) form micelles in milk that are one part hydrophobic (water insoluble) and one part hydrophilic (water soluble). It is this dual property of milk proteins that means that their hydrophobic part will stick to the air bubble, while the other hydrophilic part will hold water in the milk.

The more air bubbles trapped by the proteins in the milk, the thicker the foam will be.

What temperature to heat the milk to when whipping?

How do the proteins react to the increase in temperature of the milk? When heated, their structure changes - denaturation of proteins. If you watch the temperature of the milk correctly, denaturation will proceed as planned. That is, just enough to release enough of the protein to form a protective layer around the air bubble.

Heating the milk above the ideal temperature causes the protein structure to break down and the air bubbles will escape from the milk. Which means no milk foam, just boiling milk. Another great thing happens when milk is heated. Milkgets sweeter with heat. Complex carbohydrates are transformed by heating into simple sugars whose sweetness is more easily perceived.

ⓘ The optimum temperature for cappuccino milk is 55-65°C.

Why whisk? Is it wrong to put cold milk in your coffee?

Yes and no. It depends on what kind of coffee you add cold milk to. Better said, what temperature the coffee is. A few lines above, I wrote about how increasing temperature affects the proteins in milk. And the effect is also apparent with a sudden change in temperature. Such as cold milk rushing into hot coffee. This has the effect of precipitating the milk protein.

More sensitive coffee drinkers mighthave digestive problems with such coffee at the very least, feelings of heaviness from the stomach. Cold milk also cools the coffeedramatically, which takes away from the quality of the flavor and our experience of an afternoon with a cup of coffee.

If we're talking about iced coffee with milk, like a milky frappé, then cold milk is obviously fine.

Why milk is whipped at all is just the answer to warming up milk to combine with hot coffee. The fastest way to achieve this is with a steam nozzle. It also gives the covetedcreamy - whipped texture for delicious milky coffee drinks and latte art creation.

How does milk foam become microfoam? How to make a velvety cappuccino

To turn aerated milk - fluffy foam into creamy smooth microfoam, the second step follows: stirring the milk foam. So at this point, we have the proteins trapped in the milk with enough air bubbles. It's time toevenly distribute these bubbles throughout the volume of milk in the jug.

The steam nozzle of the coffee maker will give off enough force to move the milk around in the teapot. Take advantage of this and adjust the position of the nozzle so that the steam stream will swirl the milk. In this way, the trapped air will be distributed comprehensively in the milk. Instead of big bubbles of air in one - the top part of the milk, of course - there will be lots of microbubbles throughout the volume of milk in the jug.

The even texture of the milk from the small air bubbles creates just the right amount of dense yet smooth microfoam. This is the secret to a velvety smooth cappuccino.

Whisking the milk into a cappuccino succeeds, but the foam doesn't last.

Gravity is omnipresent and so it affects the milk froth. If you want a more detailed explanation, think back to summers by the water and how the air floated you to the surface. It works the same way with air in milk.

The protein tries its best to hold it in. The moment the air is stronger - typically when multiple air bubbles conspire to form one big one, the protein loses and the bubble escapes from the milk and the foam breaks up. Which eventually happens with absolutely all milk foam.

Let's recap. Smaller air bubbles = more chances for the proteins to hold the foam longer. That's another point for microfoam, which is prettier to look at, nicer to taste, and more stable to boot!

Fat percentage and its effect on milk foaming

I'll go back to the composition of milk now. As I wrote, milk is basically water with, among other things, proteins that can hold air bubbles. In addition to proteins, fat globules are quite prominent and significant in milk. In other words: globules of butterfat (microscopically small globules).

It is true that: where there is a globule of fat there is no place for an air bubble. So the fatter the milk, the less air it can hold. And you'll definitely think of whipping from low fat milk. Practically, you'd have a really rich froth, but would you like that coffee from nonfat milk?

Fat is indispensable in the kitchen. Fatty food just tastes better, just the bread buttered is more delicious and not dry. Well, milk works the same way. Fatis a carrier of flavour. The dry foam (scooped out with a spoon in stodgy "ala Italian" cafes) makes asmooth and glossy milk cream.

With whole milk, a perfect emulsion of water, air and fatis created. It looks great in combination with espresso, tastes even better, and you simply can't create Latte Art withanything else.

Already know what milk to use in your latte machine?

  • The proteins in milk are the most important for whipping - so choose animal milk
  • increasing temperature causes denaturation of proteins - so start with chilled milk
  • the fat in milk is the carrier of the flavour and smooth texture of milk foam - so use whole milk

Can you whip the foam from vege milk substitute into coffee?


If you choose the right plant-based milk alternative. In particular, I recommend specially formulated plant-based drinks for baristas. Then also soy and pea milk with higher protein content than other non-animal milks.


If you choose your homemade plant "milk" or a store bought plant drink that is not designed for baristas and does not contain a lot of protein. Such as nut milks.