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 100 ml milk at 4 degrees up to a temperature of 60 degrees, with 10 ml of water, the water would have to be at a temperature of 620 degrees.

So, clearly, there’s something else happening.

Steam boilers are usually set around 130 degrees 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 degree 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 30 amps. 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.