Molecular Gastronomy: Exploring the Science of Flavor (6 page)

it immediately. But is this method less effective in extracting the juices?

Lost Juices

Nothing beats an experiment: Let’s cut up a big piece of meat into two equal

parts and place half of it in cold water and the other half in boiling water; then

let’s heat and periodically weigh the two pieces. We will find that the mass is

very rapidly reduced in the boiling water and more slowly in the cold water.

After about an hour of cooking, however, the two pieces have lost the same

amount (give or take a gram). After this point the mass no longer varies, even

with several additional hours of cooking. Moreover, in a blind tasting the two

broths are indistinguishable: Liebig’s broth theory, doubtful in principle, turns

out to be false in practice.

However, our experiment suggests a new way of treating the boiled meat

once its juices have been released. If it is left to cool sufficiently in the broth,

its mass can increase by more than 10% because the meat absorbs the liquid.

Why not let the meat cool in a juice made from truffles, for example?

Making Stock
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2Clarifying Stock

Does convection sufce to clarify a broth?

c o o k b o o k s m a k e m a n y c u r i o u s c l a i m s. One by the late Bernard

Loiseau asserts that adding ice cubes to a cloudy broth “stuns” the suspended

particles, causing them to fall to the floor of the stock pot. One may quibble

with this way of stating the matter, but does it contain an element of truth?

The Modeling of Broth

Let’s begin by selecting particles close in size to those that actually cloud

beef broth. Ground coffee is a good candidate because it consists of particles of

various sizes. But because, unadulterated, it would excessively tinge the color

of the broth, let’s dilute it by running water through the grounds until the col-

oring agents are rinsed out. The result is a black powder of mixed granularity.

Let’s now divide this powder into two equal parts and put them in identical

glasses containing the same quantity of water. After we heat the contents of the

two glasses in a microwave oven, the particles suspended in both liquids reveal

the presence of energetic currents that cease after a few seconds. Now, very

carefully, put ice cubes in one glass and in the other a mass of hot water equal

to that of the ice cubes. Nothing happens in the latter case, but in the glass with

the ice cubes the suspended powder shows signs of intense agitation.

The observed motion is not surprising: The ice cubes cool the water in

the upper part of the glass while melting and releasing cold—that is, dense—

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water. This dense water falls, and the hot water at the bottom rises and cools

on contact with the ice cubes, which are warmed in turn, and so on until the

ice cubes have melted.

What happens to the particles? Have they all been “floored” by a knockout

blow from the cold water? Not really. A strange segregation appears: Although

both the large and small particles in suspension are carried downward by the

convective current and seem to be deposited at the bottom of the glass, in real-

ity the upward current carries the smallest particles back with it. Why don’t the

large particles rise again as well? Probably because particles that fall in a fluid

have a maximum speed.

If the liquid were immobile, the particles would be subject to two forces: the

weight of the particles themselves, pulling them toward the bottom, and the

upward thrust—or buoyancy—described by Archimedes’ principle (equal to

the weight of the fluid displaced by the particles). Ultimately the particles wind

up forming a deposit because they are denser than the water and because the

resultant of these forces pushes them toward the bottom.

Yet the falling particles are subject to another force that slows them down.

The intensity of this drag depends on the viscosity of the liquid and the radius

and velocity of the particles. To simplify matters, let’s begin by considering this

force in isolation, acting independently of the others. During free fall, the force

is initially zero (because the rate of fall is zero), and the particles are acceler-

ated by the downward force. Gradually, however, the upward drag asserts itself,

offsetting the resultant of the weight and the Archimedean thrust so that the

particles end up falling at a constant speed, which is their maximum velocity.

The Segregation of Particles

When the liquid rises, after having fallen to the bottom of the glass, it tends

to carry both the small and the large particles with it. However, these particles

have different maximum rates of fall and therefore react differently. Because

their radius is small, the small particles fall to the bottom at a speed that is less

than that of the fluid’s upward motion. By contrast, the large particles, with

their greater maximum speed, fall too fast for the ascending fluid to be able to

carry them back up again; they remain at the bottom of the glass.

How can we test this hypothesis? Would it be possible to reproduce the

experiment in a more viscous fluid and in this way modify the maximum veloc-

Clarifying Stock
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ity of the particles? Marc Fermigier of the École Supérieure de Physique et de

Chimie in Paris, citing Darcy’s law for flow through porous media, observes

that the increase in viscosity may slow the speed of convection and thus alter

the phenomena being studied: In pure water the current pushes the particles

back up along the curve of its path, below the convection cell, because it is able

to penetrate the granular medium of these particles; the more viscous the fluid,

however, the harder it is for the current to pass between them.

Fermigier’s colleague Eduardo Weisfred adds that the large particles, being

more inert than the small ones, tend to shift to another line of current below

the convection cell, so that they are carried toward areas where the current is

less swift and where sedimentation is possible; the small particles, on the other

hand, follow the fluid throughout its complete motion.

What is the practical lesson of all this? That only the large particles form a

significant deposit, and the small ones will continue to cloud the bouillon—

just as the lamb, the innocent polluter in La Fontaine’s fable, is said to have

fouled the wolf’s drinking water.

28 | secrets of the kitchen

3Hard-Boiled Eggs

How to center a yolk and get the cooking time just right.

c o o k b o o k s s a y t h a t t o o b t a i n a hard-boiled egg with a centered yolk,

the egg must be cooked in water that has already been brought to a boil. Ex-

perience often demonstrates the soundness of this advice, but sometimes one

follows it and the yolk ends up being off center. Other times the yolk comes out

in the center when the egg has initially been placed in cold water. What good

is advice if it isn’t always good?

First things first. Why is the yolk sometimes decentered? Because it has

changed position inside the egg. Why has it changed position? Because it is

subject to the forces of gravity and buoyancy. Do these forces push the yolk

upward or downward? It is often supposed that the yolk is more dense than the

egg white. Let’s find out whether this is true by doing an experiment. In a tall,

slender glass—a transparent “shell” that will permit us to see the respective

positions of the yolk and the white—place a yolk and, on top of this, four or

five whites. The yolk slowly rises, which is easily enough explained by the fact

that it contains lipids, or fats, which are less dense than the water that makes

up most of the egg white.

Does the fact that the yolk comes to float atop the whites account for its

decentering in the hard-boiled egg? We can test this hypothesis by another

experiment. First, place an egg on its side in a saucepan filled with water. Then

bring the water to a boil and let the egg cook for ten minutes or so, making

sure that it remains motionless throughout. On peeling off the shell of the

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hard-boiled egg we find that the yolk has moved toward the top. Let’s repeat

the experiment with an egg that has been left in a vertical position long enough

for the yolk to rise. If we cook the egg in the same position we discover that

the yolk is decentered once again toward the top. These experiments confirm

that the difference in density between the yolk and the white is responsible for

the position observed.

“Yes, but what about the membranes that center the yellow in the egg?”

those who know of their existence will ask. The experiment with the glass

removes these membranes and so eliminates their effect, it is true. But the

cooking experiment demonstrates that they are not sufficient to hold the yolk

in place.

How, then, can we reliably obtain a cooked egg with the yolk in the cen-

ter? The answer may be deduced from the preceding experiments: We must

prevent the yolk from rising in the shell. How? By closing off the vertical axis

of movement, and with it the yolk’s ability to float. In practice this means ma-

nipulating the egg while it cooks. Put one in boiling water, rolling it around in

the saucepan for about ten minutes, and then remove the shell. You will find

that the yolk is centered.

The same experiment, only starting with cold water, produces a centered

yolk as long as one rolls the egg around for a longer time, which becomes te-

dious after a while. The cookbooks are partially correct, then, but they offer no

insight into the forces actually at work. The key to success, it turns out, lies in

not allowing the yolk to stay still.

Hamine Eggs

What is the recipe for a perfectly cooked egg? The question may seem odd

because tastes vary so greatly. Some people like the dark green ring on the

surface of the yolk, for example, and others detest the sulfurous odor that ac-

companies it. Cookbooks say to cook the egg for only ten minutes in boiling

water, without any further explanation. Let’s look first at the problem of cook-

ing time.

Why ten minutes rather than, say, five or fifteen? Because after five min-

utes the egg has not yet hardened, and after fifteen minutes it is assumed

that the egg white will be rubbery and the yolk sandy. Yet the latter result

is not universally observed throughout the world. Hamine eggs prepared in

30 | secrets of the kitchen

Jewish communities in Greece and elsewhere are famous for their tender-

ness, although they are cooked for several hours. How do these cooks avoid

the sulfurous smell of overcooked eggs? And why does the yolk in their eggs

not take on the greenish color found in eggs cooked longer than ten minutes

in France?

These questions give rise to others. How do eggs cook in the first place? The

white consists of about 10% proteins (amino acid chains folded upon them-

selves in the shape of a ball) and 90% water. During cooking the proteins

partially unfold (they are said to be “denatured”) and bind with each other,

forming a lattice that traps water—in other words, a gel.

The tenderness of the cooked egg white depends on the quantity of water

trapped (the loss of a part of this water is what makes overcooked fried eggs

rubbery and overcooked yolks sandy) and on the number of proteins making

up its lattices (more lattices mean that more water is trapped, rigidifying the

entire system).

Experiments in which eggs are cooked at different temperatures provide a

clue to the mystery of hamine eggs. When an egg is cooked in boiling water,

at a temperature of 100°c (212°f), not only does its mass progressively dimin-

ish as water is eliminated from the gel that forms, but many kinds of protein

coagulate as well. By contrast, when an egg is cooked at a temperature just a bit

higher than the temperature at which its proteins have all coagulated—about

68°c (154°f)—it thickens (thanks to the coagulation of only a few proteins)

while retaining its water, a guarantee of tenderness and smoothness.

Hamine eggs traditionally are cooked in embers, with a temperature range

of only 50–90°c (122–194°f). There is therefore no paradox, only a good em-

pirical understanding of the coagulation of the egg’s proteins during cooking.

Cooking eggs in boiling water nonetheless has an advantage: Because the tem-

perature is constant, one obtains a constant result by fixing the cooking time.

However, this temperature does not take into account the nature of the egg.

Isn’t it time that we avail ourselves of the benefits of modern technology?

High-quality thermometers now make it possible to cook eggs at temperatures

closer to those at which their proteins are denatured: at 62°c (144°f) one of the

proteins in the white (ovotransferrin) is cooked, but the yolk remains liquid

because the proteins that coagulate first in this part of the egg require a tem-

perature of 68°c (154°f). Obviously this would mean longer cooking times, but