Modernist-Cuisine-Vol.-1-Small

6
THE PHYSICS O F
The Stages of Boiling
Boiling is evaporation that happens at the hot bottom of the
fluid rather than at the cooler surface. Boiling begins at
nucleation sites: small, rough surfaces where tiny pockets of
air become trapped by the surface tension of the liquid.
Steam inflates these pockets into bubbles that eventually
break free.
Throw a handful of salt into a pot of simmering water, and
the boiling will accelerate, not because the salt changes the
boiling point but because it adds more nucleation sites.
(Sand works equally well.)
Boiling is not a single, uniform phenomenon. Simmering,
for example, is not actually boiling, and two qualitatively
different stages of boiling exist beyond the familiar rolling
boil. These advanced stages only occur in superheated water
that is beyond the capacity of professional gas burners, so in
the kitchen they occur only rarely and out of view.
nucleation sites. The earliest of these bubbles
collapse almost as soon as they are formed, as
they encounter the still-colder water above them.
This phenomenon is known as cavitation collapse;
the tiny implosions sound like sizzling or
rapid ticking.
As the temperature of the water continues to
rise, the rumble is muted because air bubbles
dotting the bottom and walls of the pot have
grown large enough and buoyant enough to be
swept away by the convection currents to higher
and cooler levels within the pot at which the
bubbles collapse. If you have already put some
grains of pepper in the water as suggested above,
this is the best time to watch the rising and
falling currents. And if you look even more
closely, you’ll see that the surface is actually
quivering (the French say frémissant) as one
plume after another of hot water brushes gently
against the surface before cooling and falling
back down.
Finally, the rumbling and sizzling noises
diminish as streams of bubbles form on the
bottom of the pot and grow big enough to rise all
the way to the surface, where they pop, releasing
steam into the air. This is the beginning of the
actual boiling process (see The Stages of Boiling,
previous page).
As the bottom of the pot gets hotter, even
bigger bubbles make it to the surface in large
numbers, creating a full, rolling boil accompanied
by a gentle burbling sound. Scientists call this
stage nucleate boiling, because the bubbles have
originated at nucleation sites. Water won’t get
much beyond this stage with the limited heating
power of the average stove.
But at higher heating rates (more watts or BTUs
per hour), the bubbles stream from the nucleation
What is a simmer? Some cookbooks
attempt to define a simmer
by the water’s temperature:
a certain number of degrees below
100 °C / 212 °F, although few seem
to agree on just how many degrees.
But the temperature of a simmering
pot of food varies, depending on
the characteristics of the pot, the
burner, and the food (whose
temperature is not uniform
throughout).
So it makes more sense to
define a simmer in terms of what
you can see going on in the pot.
Call it a simmer when only the
occasional small bubble makes it
all the way to the top.
For more on convection in cooking, see Heat in
Motion, page 277.
T HE PHYSICS OF
Skating on Gas
Flick a little water on a medium-hot griddle, and the water
hisses, bubbles, and boils quickly away. That’s called flash
boiling. But when the griddle gets much hotter than the
boiling point of water, the droplets form small balls that
skitter around without vaporizing for as long as a minute, as
if they were on skates of shooting steam. You are watching
the Leidenfrost effect, named after Johann Gottlob Leidenfrost,
a German doctor who described it in 1756.
When a drop of water hits a metal plate at or above about
200 °C / 390 °F, called the Leidenfrost point, the part that first
touches the plate bursts into steam, creating a paper-thin
vapor layer that lifts the rest of the drop. The steam layer
insulates the drop from the plate, so the drop can last for
a long time and roll around the plate like a crazed ball bearing
before evaporating. The same thing happens to drops of
liquid nitrogen spilled on a plate or kitchen counter.
The next time you have some liquid nitrogen in your kitchen,
throw a drop of water on the surface, and you’ll see an
upside-down Leidenfrost effect in which the vapor barrier
comes from the nitrogen, not the water.
Simmering is not boiling, although it does occur when the
1 temperature is near the boiling point. Bubbles of steam form on the
hot bottom, but most collapse quickly as surrounding water cools and
condenses the vapor inside of them. As the temperature rises to
approach the boiling point, some of the bubbles float to the surface.
Nucleate boiling produces the familiar, everyday rolling boil. All of
2 the heat that moves from pot to fluid goes into vaporizing molecules of
liquid near the bottom, sending them upward inside innumerable
steam-filled bubbles. Turning up the burner power doesn’t increase
the temperature of the water; it simply generates more bubbles.
Slug-and-column. Slug-and-column boiling happens when steam
3 bubbles stream off the bottom of the pot so quickly that they blur into
continuous “columns” of steam, often with several columns feeding
into one larger “slug” of steam. This stage of boiling happens only in
liquid that has been superheated above its boiling point; in the
kitchen, this usually only happens when boiling thick sauces (see page
2·68).
Film boiling. Film boiling is the rarest of the stages because it only
4 occurs in fluid so superheated that a continuous blanket of steam
covers the entire heating surface. Because enormous amounts of heat
must be marshaled to produce film boiling, it never occurs in the
kitchen, with one rare exception: Leidenfrost droplets (see next page).
Photos courtesy of: John H. Lienhard IV and John H. Lienhard V "A Heat Transfer Textbook"
4th edition, 2011, http://web.mit.edu/lienhard/www/ahtt.html
316 VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 317