Molecular Gastronomy: Exploring the Science of Flavor (44 page)

depending on what assumptions you make about the size of the bubbles and

the type of protein coating.

Why, then, does one generally wind up with only a small cubic decimeter

of foam? There’s no shortage of air, so the problem must once again be a lack

of water. If you add some water to an egg white and whisk you will find that

the volume of the foam increases, and if you keep on beating the mixture

vigorously you will eventually end up with several liters of meringue. This

version is less stable than the classic preparation, however. The stability of a

foam depends on the viscosity of the liquid (which is reduced by the addition

of water) and the size of the bubbles (which determines the action of capillary

forces between the bubbles).

Extreme Flan

The fact that emulsions and foams are both dispersed systems suggests

that other such systems found in cooking may display the same behavior. Con-

sider what happens in the case of a quiche. First one lines a baking pan with

dough and fills it with cubes of bacon and a mixture of eggs and milk. Yet the

cook who skimps on eggs (perhaps because they cost more than the milk)

sometimes ends up adding so much milk that the quiche remains liquid after

cooking. Because milk is mostly water, the question now becomes: How much

water can we add to an egg and still obtain the equivalent of a flan?

Orders of Magnitude
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Let’s suppose that the proteins responsible for coagulation are spherical.

In the course of coagulating they can be assembled either in a compact fash-

ion, so that no water is trapped between them; or next to one another along

the edges of a very large cube that encloses a maximum amount of water; or

along the edges of a network that, for the sake of simplicity, may be assumed

to be cubic.

In the first case, the volume of water enclosed is zero. In the second case,

a simple calculation shows that the volume would be impossibly large, on the

order of several cubic meters. Let us therefore consider an intermediate case,

based on observation of a fried egg using an atomic resolution microscope.

This time, an order-of-magnitude calculation leads to a volume of flan—what

physical chemists call a gel—equal to a liter.

To check this calculation experimentally, take an egg and add to it an equal

volume of water, and then heat the mixture. Having successfully obtained

a flan by this method, repeat the experiment while progressively increasing

the quantity of water: twice the original volume, three times, and so on. You

will discover that almost three-quarters of a liter of water can be made to

“gelatinize” with a single egg—the same order of magnitude predicted by the

calculation.

The number of different types of dispersed systems is huge. So far we have

explored a liquid dispersed in a liquid, a gas dispersed in a liquid, and a solid

dispersed in a liquid. But other combinations remain to be investigated.

302 | a c uisine f or t omor r ow

91

Hundred-Year-Old Eggs

Experiments with acids and bases.

e g g s u s e d t o b e p u t i n s a n d , s a w d u s t , o r w a x in order to be

preserved. Asian peoples devised recipes that took advantage of aging, instead

of compensating for its effects, in order to create what were variously known

as hundred-year-old eggs, centenary eggs, and even thousand-year-old eggs—

names that symbolized links with the past as well as longevity. How credible

are these recipes from the chemical point of view? A few experiments reveal

the unexpected behavior of eggs in acid and basic environments.

The origins of the Chinese art of preserving eggs are lost in the mists of

time. Initially, eggs seem to have been immersed in juices extracted from a

local tree. Then it was discovered that by putting them in a mixture of ashes

and earth and keeping them in a dark, cool place one could obtain the same

culinary result but in only ten to twelve weeks. What inspired these practices?

Can others be imagined?

Recipes for hundred-year-old eggs vary from region to region in China.

Some call instead for placing duck eggs in a plaster that contains various ingre-

dients: lime, saltpeter, bicarbonate of soda, mud, fragrant herbs, tea, rice straw,

and so on. The eggs are left to rest for at least three months, and their flavor

is said progressively to improve. It is surprising to note that some of these

ingredients are also used in parts of France; even more recent civilizations

such as our own make use of lime and ashes, which contain potash (potassium

hydroxide). Thus recipes for preserving eggs fall into two classes: ones that

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contain only eggs and others that put them in contact with an alkaline com-

pound. What is the effect of these bases? The effect of acids? After all, at least

one modern French recipe also advises placing quail eggs in vinegar.

An Egg in Vinegar

Let’s experiment by placing a whole egg, in its shell, in a large transparent

container. When it is covered with white vinegar, bubbles soon escape from

the shell. Why? Because the acetic acid of the vinegar is attacking the calcium

carbonate? A lighted candle, placed in the container, eventually goes out, a sign

that the acid gives off carbon dioxide, which, being denser than air, accumu-

lates in the container, driving out the air (the same thing can be demonstrated

more technically by collecting the gas in limewater, which becomes cloudy).

Then, after half a day or so, a thin, red surface layer detaches itself from the

shell. This is why the eggs have a pink shell: the white of the carbonate and

the red of this layer combine to produce the final color (at least in the case of

French eggs; I have heard that eggs in England remain white).

Let’s continue observing. After one or two days of slow gaseous emis-

sion, the egg seems to have gotten bigger. Is this merely an illusion? The

sequence of events shows that the enlargement is real: The final volume

can be more than twice the initial volume. The shell has been completely

dissolved, but the contents of the egg have not spread into the vinegar, for

the acidity causes the white to coagulate. Experiments with several eggs al-

lowed to sit for different periods of time show that this coagulation, limited

at first to a thin outer layer, extends to an increasing proportion of the albu-

men, reaching even as far as the yolk. On reflection, this effect is not entirely

surprising, for one finds the same thing when one pours vinegar on an egg

white in a bowl: The superficial layer of the white coagulates, among other

reasons because the h+ ions contributed by the acid prevent the acid groups

of the proteins from being ionized while triggering the ionization of the base

groups, which thus become positively charged (bases have the opposite ef-

fect). Electrical repulsions between the charged groups of proteins thus un-

fold the proteins, which are then bound by forces called disulfide bridges

that link sulfur atoms. Coagulation occurs because water is trapped by the

resulting network of proteins.

304 | a c uisine f or t omor r ow

Osmotic Expansion

If the dissolution of the shell and the coagulation of the albumen are simply

explained, it may seem less clear why dilation occurs. Could it be that osmosis

is responsible for the increase in size? Water molecules tend to go from areas

where they are most concentrated to areas of least concentration. Whereas the

water concentration is about 95% in the vinegar, it reaches only 90% in the

egg white. Moreover, whereas the acetic acid migrates toward the interior of

the white (this can be verified by measuring the acidity of an egg white that has

sat for several weeks in vinegar), the protein molecules dissolved in the water

of the whites are too large to pass through the coagulated membrane. In other

words, the water of the vinegar enters into the albumen, increasing the water

concentration inside the egg.

To show that this is what happens, one has only to leave the eggs in an

acetic acid solution whose acid concentration is greater than 10%. Once again

the shell is dissolved, but this time the egg ends up being smaller because the

osmosis is reduced. How would an egg placed in a concentrated solution of

chlorhydric acid turn out? One would obtain the same smaller egg produced by

the acetic acid solution, but the coagulation would be more rapid and clearer.

The Floating Yolk

I invite you to conduct your own experiments; many other surprises await

those who are patient enough to observe carefully. For example, when the shell

is dissolved and the egg white is still translucent, you can actually see the yolk

floating in the white.

As for bases, adding caustic soda (sodium hydroxide) to an egg white causes

it initially to coagulate. A chemical reaction produces a nauseating sulfur gas,

and the egg then turns clear again. Obviously the soda dissociates the proteins

after having first precipitated them. If we put eggs in ashes or in lime, which

have lower pH levels, we can wait—for a hundred years.

Hundred-Year-Old Eggs
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92

Smoking Salmon

Sugar and an electrical eld can be used to accelerate smoking.

s m o k e d s a l m o n i s a n e x p e n s i v e d e l i c a c y that France has long

specialized in producing. Manufacturers buy imported salmon and resell their

smoked filets the world over. A team of researchers from the Institut Fran-

çais de Recherche pour l’Exploration de la Mer and the Centre de Coopération

Internationale en Recherche Agronomique pour le Développement perfected

the process currently used to accelerate processing without sacrificing flavor.

The two principal ingredients of the new method are osmosis and electrostatic

smoking.

Smoking, like salting and drying, was originally used to preserve foods.

In all three cases the idea was to eliminate water from foods in order to kill

microorganisms that were already present and to prevent the development of

new pathogenic microorganisms. Yet the old methods often gave an excessively

salty or smoky flavor.

With the appearance of modern refrigeration systems, the technique of

smoking was retained, in a modified form, because it gave filets of fish (and

other foods) the delicious taste we know and love. Present-day products are less

salty and less smoky, but they must be kept at low temperatures, between 0°c

(32°f) and 2°c (36°f).

Fish filets prepared by traditional methods are either immersed in a brine,

sprinkled with salt, or injected with brine. The first treatment, which takes

about four hours, eliminates only about 2% of the water; health regulations

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make it a complicated affair requiring costly treatment facilities. The elimina-

tion of water is ensured only by drying at a temperature of about 22°c (72°f),

with a humidity of 65%, for three to four hours, before the actual smoking

begins. Here again the procedure must be carefully monitored, for the process-

ing temperatures favor the development of microorganisms.

In the procedure patented by Antoine Collignan, Camille Knockaert,

Anne-Lucie Wack, and Jean-Luc Vallet, the salting and drying are done si-

multaneously at a temperature of 2°c (36°f). The filets are immersed in a

concentrated salt and sugar solution (10–30% sugar and 70–90% salt) so that

some of the salt penetrates the flesh by osmosis and dries it out. In fact, the

various molecules are distributed in such a way that their concentration is

everywhere the same: When the filets are immersed in a salt-saturated solu-

tion, the salt migrates toward the flesh and the water comes out. At the same

time the sugar—composed of large molecules that cannot enter the cells of

the fish—promotes the outflow of water as well. All told, filets immersed in

a solution containing some 350 grams of salt per liter and about 1900 grams

of sugar per liter lose roughly 10% of their water. Moreover, the sugar reacts

with the amino acids of the fish and produces agreeable flavors through a

series of Maillard reactions. In certain classic recipes for smoked salmon the

fish is rubbed with sugar until it has a tanned appearance. With the new pro-

cedure, the result is comparable but easier to achieve because it takes place

in solution.

Smoke Without Fire

After rapid rinsing and draining, the smoking takes place in a chamber tra-

versed by a metal conveyor belt that passes under a grate. Smoke is produced

by subjecting sawdust to pyrolysis (dry heat). The smoke is injected into the

chamber after having been cooled to a temperature of 40°c (104°f), condens-

ing the aromatic polycyclic hydrocarbon molecules, which are carcinogenic, so