Molecular Gastronomy: Exploring the Science of Flavor (7 page)

the result is a perfectly cooked egg.

Hard-Boiled Eggs
| 31

4Quiches, Quenelles, and

Pu‡ Pastries

Their expansion is caused only indirectly by the eggs they contain; cooking

vaporizes the water, which pu‡s up the dough.

a q u i c h e m u s t n o t b e c o o k e d t o o l o n g, or it will lose its smooth

consistency. It will not be perfectly moist (or
chevelotte,
as they say in some

parts of Lorraine), hence the culinary rule: Cooking must stop when the quiche

begins to rise. Rise? Why does it rise? And why is this a sign that the quiche

is done?

To answer the question, let us examine another dish that also expands: the

quenelle, a cousin of the German
Knödel,
or dumpling. Whether the quenelle

is made of finely minced fish or meat, the flesh has in most cases been mixed

with cream, eggs, and panada (in this case a dough obtained by kneading flour

with boiling water). Once poached the quenelles are placed in a sauce and, if

all goes well, they puff up, even when the eggs have not been beaten.

The quenelle and the quiche therefore have cream and eggs in common.

Which of the two ingredients is necessary to make them rise? Heating the

cream does not appear to make it expand. But a glance at recipes for
petits

choux,
composed of moist bread and eggs, reveals that the egg is the common

denominator of dishes that puff up. Some chefs knew this a century ago: The

anonymous author of a cooking manual published in 1905 wrote that “dishes

that contain eggs can puff up.”

Why should eggs have this property of expansion? A moment’s reflec-

tion yields an insight that even long experience making quiches, varying the

32 |

proportions of egg white, yolk, cream, and bacon, may not: Although there

is no reason why air bubbles should spontaneously appear in the heated egg,

eggs do contain water (almost 90% in the case of the white, 50% in the yolk),

and this water evaporates during cooking. Anyone who has ever fried an egg

has seen the white part rise on contact with the hot skillet. Steam becomes

trapped under the coagulated layer and pushes it up.

Perfectly Pu‡ed Up

Now that we know the reason for the egg’s expansion, how can we maxi-

mize it? If one were to broil an egg white, cooking it from above, the water in

the top part would evaporate and escape without puffing it up. By contrast, if

one heats the white from below, as in the case of a fried egg, one observes the

expansion I have just described. Dishes that are supposed to puff up therefore

must be heated from below. Soufflés? Put the ramekin on the floor of the

oven. Macaroons? Quenelles? Use a bottom-heated metal plate instead of a

broiler pan.

How much can a dish that contains egg swell up through the vaporization

of water alone? An egg white, which weighs about 30 grams and therefore con-

tains about 27 grams of water (not quite an ounce), can generate more than 30

liters of steam (almost 32 quarts). Why, then, should such expansion be limited

to soufflés, macaroons,
pains à l’anise
(sometimes called jumbles or knots in

English-speaking countries), cheese-filled puff pastries, and the like? Because

a significant part of the moisture is lost by diffusion through the upper part

of the dough; indeed, careful scrutiny of these items as they cook reveals the

escape of vapor bubbles. Maximum expansion would require an impermeable

upper layer that keeps all the moisture inside.

Return to Quiche

Let’s come back to quiche. Why is its expansion a sign that it is done? The

liquid that runs around the bacon and into the pastry is composed mainly of

fat, water, and proteins. The proteins coagulate as a result of cooking, binding

together to form a gel, a lattice that traps the water and fat. The more water

there is, the softer the gel. (By contrast, meat, fish, and eggs whose water has

Quiches, Quenelles, and Puff Pastries
| 33

entirely evaporated are tough solids.) In other words, although it does puff up

a quiche, the evaporation of water is also a signal that the quiche is beginning

to lose its tenderness. So there is some truth to the old saying.

But is it the whole truth? No, for the puffing up occurs only if one cooks

the quiche by heating it from below. And so the maxim must be revised to say

that a quiche is done when, having been heated from below, it begins to puff

up. Then, and only then, will it be
chevelotte
.

34 | secrets of the kitchen

5Échaudés and Gnocchi

Is it true that when they oat to the surface of the cooking water they

are done?

é c h a u d é s a r e v e r y o l d p r e p a r a t i o n s : As early as 1651 Nicolas de

Bonnefons mentions small pieces of dough that have been “scalded” in boiling

water. There are many recipes, but from the oldest échaudés to potato gnocchi

and
gnocchi à la parisienne
the principle is the same: One begins with a dough

composed of starch, egg, and water. In the case of potato gnocchi, extra starch

is contributed by the granules found in the potatoes (which are cooked, peeled,

and mashed). Other recipes include parmesan cheese and milk. The dough is

kneaded with a spatula, then placed on a floured baking sheet and rolled into

cylinders, which are cut up into small sections and pressed against a fork to

give them their classic shape. Next comes the cooking. The échaudés or gnoc-

chi are put into a saucepan containing salted water or broth, and when they

rise to the surface they are said to be cooked. Is this maxim trustworthy? If so,

why? The question is not trivial, for many preparations call for the same type

of ingredients and the same type of cooking: Alsatian
spätzle,
various central

European dumplings, and so on.

If we put some échaudés or gnocchi in boiling water we discover that they

fall to the bottom of the saucepan. Then they gradually swell and become

lighter. At first they follow the convection currents in the saucepan, continu-

ally coming back to the bottom. But after a while—thirty seconds or so—they

begin to float. It is therefore accurate to say that échaudés and gnocchi rise to

the surface.

| 35

Are they actually cooked at this point, as the old saying has it? If you taste

them, you will find that they are edible. But are they really done?

Denser, on Average

Let us analyze the problem for classic gnocchi, which are made of potato,

flour, and egg. This dough preserves its form in the course of cooking because

the egg coagulates on contact with the boiling water, the temperature being

higher than 68°c (154°f). The potato has been cooked beforehand, with the

result that the starch granules that fill its cells have swelled up. As for the flour,

it is composed of starch granules and gluten; the latter is made of proteins,

which with the kneading of the dough form a network that embeds the starch

granules in the cellular structure of the potato. The starch contained in flour,

like that of the potato, is insoluble when cold, but in hot water the suddenly

porous starch granules absorb the water molecules.

This mechanism explains the swelling of gnocchi during cooking. And be-

cause the density of the starch is greater than that of water (when flour is

mixed with water it falls to the bottom of the container), the total density of

the gnocchi diminishes, approaching—without ever quite reaching—that of

the water. Why, then, do the gnocchi wind up floating on the surface? It must

be that some substance whose density is lower than that of water, either air or

steam, is incorporated with it in the form of bubbles. What sort of experiment

would determine which one it is?

If we cook some gnocchi in water from which the dissolved air has been

expelled by heating the water for a long time (the bubbles that form at the bot-

tom of the pan in the initial stage of heating and then rise to the surface of the

water are air bubbles, not steam bubbles), we find that the gnocchi nonetheless

rise. So it is not air that makes them do this.

The only other possibility is that water vapor forms bubbles that cling to

the gnocchi and cause it to rise. Remove one of the gnocchi that have come to

the surface of the degassed water, gently roll it on a cutting board in order to

puncture any steam bubbles, and then put it back into the water, and it sinks

again. Once its surface crevices have again been filled with microscopic steam

bubbles, it will rise to the top once more.

If you want to see this with your own eyes, cook some cauliflower florets.

Their surface is so irregular that it efficiently traps the steam; a glistening

36 | secrets of the kitchen

gaseous layer can be seen to cover the florets just when they begin to float. Pat

the surface and you will release the steam, causing the florets once again to

descend to the bottom.

A Qualied Maxim

Our investigation of échaudés and gnocchi is not yet finished, for we still

do not know whether the fact that they float means that they are cooked. When

is
an échaudé cooked? When the egg has coagulated? When the starch has

stiffened? To answer this question, let’s form échaudés of various sizes—half a

centimeter and 10 centimeters in diameter, for example. Let’s then cook them

together and measure the internal temperature when they rise to the surface

of the water. We find that the temperature of the small échaudé is much higher

than that of the large échaudé. This proves that buoyancy is not a reliable sign

that échaudés are cooked or, at least, if one judges according to this criterion,

that échaudés of different sizes will be cooked to different degrees.

Moreover, the temperature of the largest échaudé sometimes turns out to

be lower than that at which starch stiffens and egg coagulates. In other words,

the simple fact of buoyancy is insufficient. If one aspires to a standard of pre-

cision worthy of the great chefs, it is necessary to prolong cooking beyond

the point at which they come back up or to draw up a table that gives cooking

temperatures as a function of an échaudé’s size.

Échaudés and Gnocchi
| 37

6The Well-Leavened Soufé

Water evaporates upon contact with the heated sides of the ramekin and

causes the soufé to rise.

h o w c a n o n e m a k e a p e r f e c t s o u f f l é every time? The question is

not only of practical interest to cooks. Measuring the pressure inside a cheese

soufflé and the loss of water during cooking will also help food scientists

understand the dynamics involved.

Contrary to a widely held belief, this dish is within the reach of beginners.

In the case of a cheese soufflé, for example, begin by preparing a bécha-

mel sauce, heating flour and butter, and then adding to this milk and grated

cheese. Next, off heat, fold in egg yolks and (very carefully) beaten egg whites.

The result is cooked in the oven at a temperature of 180–200°c (about 375–

425°f), depending on the size, for twenty to thirty minutes.

It has long been supposed that soufflés rise because they contain air

bubbles that expand upon heating. A simple calculation shows that this

effect can generate an expansion of only about 20%. However, expert pas-

try cooks are perfectly capable of making soufflés that double or triple in

volume.

Soufflés rise because of the vaporization of the water found in the milk

and eggs. The proof is readily seen: When one cuts into a soufflé with a knife

in order to share it with a party of delighted dinner guests, a cloud of steam

bursts forth. At exactly this moment, to everyone’s great dismay, the soufflé

collapses.

38 |

A Model Soufé and a Soufé Model

What is the maximum possible expansion of a soufflé? A simple thermody-

namic model is useful in thinking about this question. The warm air inside the

oven transfers its heat first to the ramekin and to the upper part of the soufflé,

establishing a temperature gradient between the periphery and the center of

the mixture. When the temperature reaches 100°c (212°f) near the sides of the

ramekin and at the soufflé’s upper surface, the water inside evaporates and a

crust is formed.

This account is supported by internal temperature readings. When one

sinks the probe of a thermocouple (a very precise sort of thermometer) into a

soufflé during cooking, one finds that the upper crust has the same tempera-

ture as the oven and that at the center the temperature falls before rising again

near the bottom.

Moreover, this measurement reveals a hidden dynamic at work in soufflés.

If the probe is placed at a fixed position in relation to the ramekin, it registers

first an increase in temperature, then a slight decrease or leveling off, and

finally a jump back up to about 70°c (158°f). What is happening is that as the