Modernist-Cuisine-Vol.-1-Small

5
THE PHYSICS OF
Why We Blow on Hot Food
THE PHYSICS OF
When to Add the Cream to Your Coffee
Why does blowing on hot food cool it? Your breath is
warmer than the air in the room, after all. Shouldn’t that
warm-blooded puff make the food cool more slowly?
The answer, we all learn as children, is no: blowing on
a bowl of hot soup or a piping cup of tea does actually work.
The reason it works is that the motion of the air passing over
the food matters more than the temperature of the blown air.
The motion accelerates evaporation—and evaporation, much
more than the simple transfer of heat from food to air, is the
main phenomenon that sucks energy out of a hot liquid or any
steaming food.
So the question is really: why does blowing on a hot liquid
make it evaporate faster? The answer is the wispy layer of
“steam” (fog, actually) that covers the top of the cup. Like the
smothering humidity of a sultry summer day, it blankets the
liquid and makes it harder for water molecules to escape into
the air. With the help of this so-called boundary layer, some of
the steam actually condenses back into the tea, redepositing
part of the energy it initially carried away.
Your breath, like a cooling breeze, removes this saturated
blanket of air and allows drier air to take its place. With less
dampening, the energetic molecules on the surface of the tea
break free more readily, and the liquid cools more rapidly.
Most solid foods contain lots of water, so blowing on them
works as well. The effect is not as pronounced as it is with
liquids because convection currents naturally stir a liquid and
bring the hottest parts to the surface; that doesn’t happen in
solid food. Blowing on a potato thus cools the surface but not
the interior. And blowing on a hot object that contains no
water at all, such as a strip of bacon, has no appreciable effect.
Say you’re waiting for a friend to join you for lunch, and the
waiter has poured two cups of coffee. You remember that
your friend likes her coffee white and consider adding the
cream before she arrives to impress her with your thoughtfulness.
But you stop yourself when you consider that the added
cream could make her coffee go cold faster. Will it?
The cream-in-the-coffee conundrum is a classic physics
problem, if not a classic dining problem. The answer hinges
on whether the addition of cream will make the coffee cool
more quickly or more slowly while you wait for your friend.
Several factors come into play. First, the rate of heat loss
due to radiation emitted by the coffee varies with temperature.
According to the Stefan-Boltzmann law, hotter coffee
should radiate energy faster than coffee cooled slightly by
the addition of cream. So that’s one reason to add it early.
Second, black coffee, being darker, should emit more
thermal radiation than cafe au lait. That reinforces the notion
that waiting to add the cream is the wrong approach.
The third factor may be the clincher: coffee with cream in it
is likely to evaporate less quickly than black coffee does.
Evaporation can carry off a lot of heat quickly, so this is a big
win for advocates of adding the cream right away.
The factors all point in the same direction, and experiments
confirm that white coffee cools about 20% less quickly than
black coffee does. Interestingly, the experimenters who came
up with this measurement were unable to determine which of
the three mechanisms just mentioned is the most important.
If you want to impress your friend, go ahead and put cream
in her coffee before she shows up. Just hope she doesn’t ask
you why it’s still warm.
Steaming cool soup? Evaporation cools hot liquid, but a humid layer
quickly forms over the surface of hot soup, slowing evaporation.
Blowing across the soup moves that humid layer aside, allowing more
dry air to come into contact with the liquid. That speeds the cooling.
To keep your coffee warm as long as
possible, should you add cream right
away or just before you drink it?
288 VOLUME 1 · HISTORY AND FUNDAMENTALS HEAT AND E NERGY 289