Steaming Milk: The Temperatures and Science Behind Fine Foam and Sweetness

The texture of a latte or cappuccino is made as the steam wand adds air to milk while heating it. Fine foam (microfoam) depends on milk's proteins and fat, and on temperature.

1. Steaming does two things at once

• Adding air: with the wand tip near the surface, air is drawn in and foam forms. This volume-increasing stage is called stretching.
• Mixing and heating: with the tip a little deeper, the milk whirlpools, large bubbles break into fine ones, and condensing steam delivers heat.

Longer stretching makes more foam, as for a cappuccino; shorter stretching makes a thin layer, as for a latte.

2. Proteins hold foam; fat breaks it

Milk proteins (caseins and whey proteins) coat bubble surfaces, forming films that stabilize the foam, while fat works against it. According to Huppertz's (2010) review, skim-milk foams are especially stable when formed at 40–50°C, and at low temperatures where milk fat is partly solid (around 20°C) the fat hinders foaming. Products of fat breakdown such as free fatty acids also strongly weaken foam — one reason old or poorly stored milk foams badly.

3. Temperature and sweetness

Warm milk tastes sweeter because sweet perception strengthens with temperature: the TRPM5 channel that carries sweet signals responds more strongly as temperature rises toward body temperature (Talavera et al., 2005). That's why many baristas aim for a drinkable 55–65°C or so. Much hotter, and the heat itself masks sweetness.

4. When it's too hot

Above roughly 70°C, whey proteins begin to denature extensively and sulfur-containing compounds give a cooked-milk smell (Walstra et al., 2006). The foam also turns coarse and collapses quickly. Overheated milk won't recover its texture when cooled, so stop steaming once, at the temperature you want.

5. Plant milks

Oat, soy and almond milks differ from dairy in the types and amounts of protein and fat, so they foam differently. Barista versions are often adjusted with proteins or emulsifiers to foam well. Some plant milks curdle with coffee's acidity and heat; pouring the milk first and adding espresso afterward, or steaming slightly cooler, sometimes helps.

Putting it into practice

• Start cold: milk straight from the fridge in a cold pitcher gives more time to add air, for finer foam.
• Judge by hand: when the pitcher bottom becomes too hot to hold, it's around 60°C. A thermometer is more precise.
• Finish: tap the pitcher to pop large bubbles, swirl to blend foam and milk, and pour right away.

References

• Huppertz, T. (2010). Foaming properties of milk: A review of the influence of composition and processing. International Journal of Dairy Technology, 63(4), 477–488.
• Walstra, P., Wouters, J. T. M. & Geurts, T. J. (2006). Dairy Science and Technology (2nd ed.). CRC Press.
• Talavera, K., Yasumatsu, K., Voets, T., Droogmans, G., Shigemura, N., Ninomiya, Y., Margolskee, R. F. & Nilius, B. (2005). Heat activation of TRPM5 underlies thermal sensitivity of sweet taste. Nature, 438, 1022–1025.