Milk foam is built by protein, not by fat. Casein and whey wrap around the air bubbles and hold them, while fat gives the foam its creamy texture and mouthfeel. That is why whole milk at 3.5 per cent fat produces the rich, glossy foam baristas want, and skimmed milk gives a bigger but drier, stiffer foam that collapses faster. Temperature matters just as much: steam to 60 to 65 °C and stop, because above 70 °C the proteins denature, the foam breaks down and the milk tastes cooked.
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:
+ 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:
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.
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.
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.
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.
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.
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!
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.
Yes.
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.
Don't.
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.
Whole milk, with its higher fat content, is commonly used for creating a creamy and rich foam. Its 3.5 per cent fat gives body while the protein does the stabilising. Skimmed milk foams to a greater volume but the result is dry and stiff, better suited to a traditional cappuccino cap than to latte art.
Yes, and it is the easiest milk to work with. Start from fridge temperature, around 4 °C, which gives you longer to texture before the milk reaches 65 °C. Automatic frothers handle full cream milk best of all, since their fixed programme assumes standard protein and fat levels.
Because air was introduced for too long or too deep in the jug. Aerate only for the first two or three seconds, with the steam tip just below the surface, then submerge it to spin the milk and break the bubbles into microfoam.
60 to 65 °C. Lactose sweetness peaks in that range, and the foam stays stable. Past 70 °C the whey proteins denature, the structure collapses and a boiled note appears that no amount of coffee will hide.
Barista versions do, because they have added protein and stabilisers. Oat is the most reliable, soy foams well thanks to its natural protein, and almond is the hardest since it has little protein and tends to split in acidic coffee.
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