Does Blowing on Hot Food Really Make It Cooler?

The Science of Why Blowing on Hot Food Cools It Down

The hotter soup is, the more effective blowing on it is at cooling it.
The hotter soup is, the more effective blowing on it is at cooling it. Elisabeth Schmitt/Getty Images

Does blowing on hot food really make it cooler? Yes, blowing on that nuclear coffee or molten pizza cheese will make it cooler. Also, blowing on an ice cream cone will melt it more quickly.

How It Works

A couple of different processes help cool hot food when you blow on it.

Heat Transfer from Conduction and Convection

Your breath is near body temperature (98.6 F), while hot food is at a much higher temperature.

Why does this matter? The rate of heat transfer is directly related to the difference in temperature.

Thermal energy causes molecules to move. This energy can be transferred to other molecules, reducing the movement of the first molecule and increasing the movement of the second molecule. The process continues until all the molecules have the same energy (reach a constant temperature). If you didn't blow on your food, the energy would be transferred to the surrounding container and air molecules (conduction), causing your food to lose energy (become cooler), while the air and dishes would gain energy (become warmer).

If there is a big difference between the energy of the molecules (think hot cocoa cold air or ice cream on a hot day), the effect takes place more quickly than if there is a small difference (think hot pizza on a hot plate or a refrigerated salad at room temperature). Either way, the process is relatively slow.

You change the situation when you blow on food. You move your relatively cooler breath where the heated air used to be (convection). This increases the energy difference between the food and its surroundings and allows the food to cool more quickly than it would otherwise.

Evaporative Cooling

When you blow on a hot drink or a food containing a lot of moisture, most of the cooling effect is due to evaporative cooling.

Evaporative cooling is so powerful, it can even lower the surface temperature below room temperature! Here's how it works.

Water molecules in hot foods and drinks have enough energy to escape into the air, changing from liquid water to gaseous water (water vapor). The phase change absorbs energy, so when it occurs, it lowers the energy of the remaining food, cooling it. (If you aren't convinced, you can feel the effect if you blow on rubbing alcohol on your skin.) Eventually, a cloud of vapor surrounds the food, which limits the ability of other water molecules near the surface to vaporize. The limiting effect is mainly due to vapor pressure, which is the pressure the water vapor exerts back on the food, keeping water molecules from changing phase. When you blow on the food, you push away the vapor cloud, lowering the vapor pressure and allowing more water to evaporate.

Summary

Heat transfer and evaporation are increased when you blow on food, so you can use your breath to make hot foods cooler and cold foods warmer. The effect works best when there is a large temperature difference between your breath and the food or drink, so blowing on a spoonful of hot soup will be much more effective than trying to cool a cup of lukewarm water.

Since evaporative cooling works best with liquids or moist foods, you can cool down hot cocoa by blowing on it better than you can cool a molten grilled cheese sandwich.

Bonus Tip

Another effective method of cooling your food is to increase its surface area. Chopping up hot food or spreading it out on the plate will help it lose heat more quickly!

More Answers To Food Science Questions