The Food Lab: Better Home Cooking Through Science

ahead and turned on the blender anyway. The top
popped off in a violent explosion, and my dog leaped
behind the couch in fear as hot tomatoes splattered
across the apartment with all the fury of a VEI-7‡
volcanic eruption. This kind of stuff happens because
there are some mistakes I never learn from and,
more important, because of thermodynamics and the
physics of vapor formation.
See, there are a number of factors involved in the
conversion of hot water to steam. Pressure is a big
one. As we all know, steam takes up much more
space than water. Because of this, if you apply
enough pressure to a body of water, steam will not
escape. When a big batch of tomato soup is sitting in
a blender, there is significant pressure on all of the
soup except the stuff at the surface. Steam escapes
from the top alone, while the liquid below the surface
sits there patiently waiting its turn. Switch on the
blender, though, and suddenly you create tons of
turbulence. A vortex is formed, the surface area of
that body of liquid suddenly becomes much bigger,
and steam is rapidly and violently produced.
Moreover, the added exposure will also heat up the
air in the headspace of the blender, causing it to
expand. This rapid expansion causes the top of your
blender to pop off and the hot tomatoes to go flying.
BOOM!
So, how do you prevent it? There are two ways:
First off, make sure that there’s room for expansion
in the blender. The hot steam needs an escape valve.