🔄 Frothing

Frothing means working fine air bubbles into hot milk to make a stable foam for coffee. In coffee bars a barista usually does it with the steam wand of an espresso machine: the steam heats the milk and blows air into it at once. The foam is held by milk proteins, casein and whey proteins.

The proteins gather on the surface of the bubbles and form a film that stops them from bursting. A 2024 study found that caseins settle quickly at the boundary between air and liquid, while whey proteins can build a stiffer film; scientists still disagree about which of them gives the more stable foam. The foam itself contains more casein than whey protein.

In coffee bars the steam heats the milk to about 65–70 °C. Overheating is risky: the proteins denature heavily and the milk gets a slightly bitter taste. The steam also condenses and adds water, 4–6% by volume in lab tests. Whole milk gives a thicker foam, skimmed milk a lighter and thinner one.

In the 1950s Italian espresso machines with steam wands reached the markets of the Mediterranean and Britain, where cappuccino became popular. At home people use a hand frother with a mesh plunger, which doubles the volume of the milk in 10–20 seconds, a battery-powered electric whisk, or an automatic jug with induction heating that switches itself off. Under the World Barista Championship rules, cappuccino foam has to be at least 1 cm thick.

Cappuccino foam or latte foam? In a classic cappuccino, espresso, hot milk and foam are in equal parts and the foam is dense; a latte has more milk and thin, velvety foam, known as microfoam. Under the World Barista Championship rules, cappuccino foam must be at least 1 cm thick. A steam wand heats the milk and blows air into it at once, while a hand frother only adds air, so the milk is heated beforehand.

For foam, the milk label matters. Whole milk (at least 3.50% fat in the EU) gives a thicker foam; skimmed (no more than 0.50%) a lighter one. Look at the protein line in the nutrition table: more protein means a steadier foam. Fresh chilled cartons and long-life cartons both appear in supermarkets and corner shops; the carton for coffee is simply milk, and the foam depends on the method and temperature.

Why it matters to you. In a classic cappuccino, espresso, milk and foam are in equal parts and the foam is dense; a latte has more milk and thin foam. Do not overheat the milk: coffee bars stop at 65–70 °C.