What milk is doing

What’s actually in the jug

Cow’s milk is almost nine-tenths water. The rest is where the work happens: about 5% milk sugar (lactose), around 3–4% fat and a little over 3% protein — and the sugar is why steamed milk tastes sweet with nothing added.

Protein makes the foam

Steam blows air into milk, and the proteins wrap each bubble in a thin skin, like a nanny with a scarf. Heat helps: as some proteins loosen, they grip better. That is how a jug becomes microfoam, bubbles too small to see.

Fat makes it rich, and fussy

Fat makes foam feel silky, but it also fights the bubbles, especially while the milk is still cool — below about 45°C in lab tests. Whole milk and skimmed therefore behave differently in the jug.

Which part of the milk does each job?

• Protein
  • Wraps each bubble in a thin skin
  • Grips the bubbles better as it heats
• Fat
  • Makes the foam feel silky and rich
  • Works against the bubbles
• Sugar
  • Why steamed milk tastes sweet
  • Also called lactose

Protein holds the foam, fat makes it rich while quietly fighting the bubbles, and lactose brings the sweetness.

Same wand, same seconds, two different milks. A flat white has a preference.

Skimmed and whole milk, steamed the same. Which gives the taller, drier foam?

Answer: Skimmed, taller and drier. Whole milk’s fat gives a smaller, glossier foam that pours better, which is why latte art starts with whole.

Warm is sweeter, up to a point

Heat adds no sugar; it makes the sugar easier to notice, because sweetness reads stronger as a drink warms. Baristas aim for about 55–65°C, the range the SCA’s barista course uses: sweet, creamy and drinkable at once.

Steamed milk tastes sweeter than cold milk because…

Answer: Warmth makes the sugar easier to notice. Warmth. Heat adds no sugar at all; sweetness simply reads stronger as a drink warms, and the lactose was there all along.

Past the point

Much past 70°C the proteins start to cook and the milk tastes flat and faintly eggy. By about 80°C, a cook would call it scalded.

Plant milks need a hand

Oat, soy and almond drinks foam differently, and can split in hot, acidic coffee, as committees do. Many carry far less protein — one popular oat barista drink about a third as much. Steam them the same way.

What the carton owns up to

Acidity regulator

Most barista versions add one, such as a potassium phosphate, so the drink does not split in hot coffee.

Oil

Many add oil for richness.

Gellan gum

Some add a stabiliser like gellan gum. Recipes vary widely by brand, so read the carton.