Have you ever wondered why we use steam to heat milk for lattes and cappuccinos? Surely direct contact with a heating element would be simpler.

So, why steam?

Actually, there’s some interesting science involved.

Obviously steam is extremely hot and provides an abundance of energy. We also know that steaming a jug of milk to 60 degrees C will increase its weight by approximately 10%. This is due to the steam condensing and adding water to the milk.

However, if we do a thermal equilibrium equation, (high school chemistry), it becomes apparent that the heat of the water is not solely responsible.

With a little math, we can calculate that to heat 100ml milk at 4℃ up to a temperature of 60℃, with 10ml of water, the water would have to be at a temperature of 620℃.

So, clearly, there’s something else happening.

Steam boilers are usually set around 130℃ which is nowhere near the 620 degree temperature stated above; but, they still manage to heat the milk.

How?

“Latent Heat of Condensation.”

When a material changes phase it either releases or absorbs energy.

As gas condenses to liquid it releases an enormous amount of energy (exothermic). On the contrary as a liquid evaporates it absorbs a similar amount of energy (endothermic).

So it turns out that the phase change is doing most of the work. The 130℃ water is adding 16.8 kilojoules of energy and the latent heat of condensation is providing another 25 kilojoules.

In electrical terms that approximates 7 kilowatts of energy delivered in around 6 seconds. On a 240 volt supply that works out to 30amps. An entire 3 group espresso machine, at peak-service, may draw a comparable amount.

Steam engines, refrigerators, and nuclear power plants, are all examples of latent heat being put to similar use.

In essence, the steam boiler acts as a giant capacitor, sipping on electricity throughout the day and storing it as latent heat within the system. This allows us to heat milk in only a few seconds without drawing massive amperage.