Blowing On Your Food Really Does Cool It Down

From the archives ~2015

Some time ago, a friend assured me that blowing on hot food didn’t actually cool it down, in fact, the act of blowing on food was more about preparing one’s mouth for taking in something that might burn. I disagreed, and it and it bothered me enough to go researching and find out for sure. I’ve done the (not-so) hard yards. Here’s the answer.

TL;DR — Yes, blowing on food really does cool it down… quite slowly.

There are a couple of things it does: heat transfer from conduction and convection, and evaporative cooling.

Your mouth (and therefore, breath) is much cooler than the hot food you’re about to eat, and this affects the rate of heat transfer. Thermal energy causes molecules to move—the transference of energy from the hot molecules (high energy, the food) to the cold (low energy, the air) cools them until they have the same energy (a constant temperature).

Think about ice cream melting on a hot day vs. how long it takes your dinner to cool on a heated plate—both do eventually reach the same temperature but much faster when there’s a bigger energy difference. Without blowing on the hot food, these molecules would stay hotter for longer, since blowing increases the hot-cold difference between the air and food, and so the process is sped up.

Evaporative cooling does the lion’s share in cooling though, especially when the hot food contains a lot of moisture. When your food comes out “piping hot” (steaming), it is literally giving off energy, ergo cooling down.

It takes energy for the water molecules in your food to change from liquid to gas and that energy transfers from heat in your food to heat in the vapour/air. Without blowing on your food, that cloud of water vapour over the dish actually then effectively blocks more water from vaporising — the water vapour exerts pressure back on to the dish — and so blowing that away allows more water molecules to evaporate.

Pretty cool.