Molecular Gastronomy: Exploring the Science of Flavor (11 page)

as a function of time, for each temperature, display an elbow shape, which

indicates that the softening consists of two phenomena. The first is rapid and

probably is associated with the diffusion of the water toward the interior of the

lentils; the second is slower and seems to be associated with the gelling of the

starch in the hot water, which causes a starchy paste to form.

The researchers also observed that the percentage of lentils that burst open

during cooking increases exponentially as a function of time as the tempera-

ture rises above 80°c (176°f). Temperature thus affects both the integrity of

the lentils and their firmness: At temperatures above 86°c (187°f), the propor-

tion of lentils that fall apart exceeds the proportion of lentils that become soft

while retaining their form—hence the culinary rule suggested by these studies,

namely that lentils should be cooked at a temperature lower than 80°c (187°f).

Of course, this takes time.

Softening Lentils
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14

Souféed Potatoes

Analysis of a classic dish shows how to avoid the greasiness of deep frying.

s o u f f l é e d p o t a t o e s l o o k l i k e s m a l l, crispy golden balloons. They

are said to have been discovered on August 25, 1837, during the dedication of

the railroad line linking Paris and Saint Germain-en-Laye. The menu for the

official luncheon was to include fried slices of potato, but when the train had

trouble climbing the last hill the chef was forced to interrupt the frying; once

the guests were finally seated, he immersed the slices once again in very hot

oil in order to make them crispy. They puffed up.

Since then cooks have differed over the proper way to make this difficult

masterpiece of classic French cuisine. Physicochemical analysis has recently

illuminated the mechanisms that cause the potatoes to puff up and revealed

how to limit the absorption of oil by the potatoes during frying.

Cookbooks do not say why the recipes they give for souffléed potatoes

should work. It has long been claimed that this dish and the ideal thickness

of the sliced potatoes were studied by the French chemist Michel-Eugène

Chevreul (1786–1889), a pioneer in the chemistry of fats. The story is plau-

sible, given the importance of heated fat in this dish, but I have found no

trace of any such investigation in the works of Chevreul. Four years after

Chevreul’s death, however, chef Auguste Colombié noted in his
Éléments

culinaires à l’usage des demoiselles
(1893), “Thanks to the good offices of M.

Decaux, the gracious and learned laboratory assistant of the late Chevreul,

who kindly furnished me with the necessary thermometers, I was able to

62 |

make three scientific experiments on the puffing up of potatoes, Wednesday

14 April 1884, at the warehouse showroom of the Compagnie Parisienne du

Gaz.” There follow several pages in which Colombié presents the results of

his experiments, with no reference to Chevreul. It therefore seems probable

that historians of cookery have identified Colombié with Decaux and Decaux

in turn with Chevreul.

The Technolo³ of

How should souffléed potatoes be prepared? Most traditional recipes rec-

ommend cutting the potatoes lengthwise into slices between 3 and 6 millime-

ters (1/8 and 1/4 inches) thick. The slices are washed, dried, and then cooked

in oil that has been heated to a temperature of 180°c (176°f). Once the slices

have risen to the surface, after six or seven minutes, they are removed from the

oil and allowed to cool before being put back and cooked a second time, only

now at a higher temperature. The authors of these recipes attribute success to

the thickness of the slices, the length of time between the two immersions, or

the temperature of the oil in each case.

Which is the relevant parameter? Why do the potatoes puff up? How can

this puffing up be optimized? In testing the classic recipes one needs to keep

two things in mind: that potato cells contain granules of starch, which swell

when the cellular water is heated, forming a purée, and that because a potato

is a thermally isolating material, its center is slow to cook. If the oil in the first

round of frying is too hot, an excessively thick and rigid crust forms before the

center is cooked, and the potato will not puff up.

Water Vapor Repels Oil

Next, if we weigh the fried slices, we find that the oil does not replace the

water eliminated by heating, as was long assumed. Given a surface of 100

square centimeters (or roughly 15 square inches), about 80 cubic centimeters

(almost 5 cubic inches) of steam manages to escape per second. In other words,

the pressure of the steam keeps the oil from seeping in. Besides, if the slices

quickly rise to the surface, this is because the water has been replaced by steam

and not by oil (a potato is composed of 78% water and 17% starch, which is

denser than both water and oil).

Souºéed Potatoes
| 63

The behavior of steam bubbles provides the key to the phenomenon of

soufflage
. In order for the slices to puff up, steam must suddenly be generated,

deforming the crust, whose dried-out cells create a steam-resistant compart-

ment within each slice. When vaporization is slow, small trains of bubbles

trickle out through openings in the crust, and the pressure of the steam is

insufficient to cause the slices to expand, hence the need for hotter oil during

the second round of frying.

Puffing up also requires that the compartments formed during the first

round of frying be impermeable. The centers of the slices continue to cook

during the interval between the first and second rounds, and water is redis-

tributed through the dried-out areas. As the temperature falls the crust prob-

ably becomes detached from the center as well. The second round of frying

then causes the residual water in each slice to evaporate, triggering expansion

because the steam has a hard time escaping through openings in the compart-

ment walls.

This explains why the thickness must be carefully controlled: If the slices

are too thin, one does not obtain a crust with an intermediate layer of puréed

starch granules, and the quantity of steam generated therefore is insufficient;

if the slices are too thick, more time is needed for the center to cook and an

overly thick crust forms on the outside, hindering the expansion. It also be-

comes clear why the greatest care must be taken in handling the potato slices.

For if the thin crust is pierced, large vapor bubbles are suddenly able to escape

through the openings, and the pressure is no longer sufficient to cause the

slices to puff up.

Finally, how can the amount of oil absorbed by the puffed potatoes be mini-

mized? Sam Saguy at the University of Jerusalem has shown that the oil is

present mainly on the surface of the sliced potato, in quantities that increase

with the rugosity of the surface and repeated use of the same oil: The more

uneven the surface, the more oil that adheres to it (because of an increase

in tensioactive molecules that results from repeated heating, hence the foam

produced by old oil). It is a good idea, then, to fry potatoes in clean oil, to use

as sharp a knife as possible, and to wipe off any oil coating the surface of the

cooked potato slices so that when the water inside cools and condenses it is

not absorbed.

64 | secrets of the kitchen

15

Preserves and

Preserving Pans

Why are unplated copper pans recommended for cooking fruit preserves?

l e t ’ s e x a m i n e a f e w m o r e d i c t u m s. L.-E. Audot, author of
La cui-

sinière de la campagne et de la ville
(1847), says that in order to make fruit

preserves “it is indispensable to use an unplated copper pan (earthenware

or terracotta ones being liable to burn [the preserves] or impart a bad taste).”

Sixty years later, geologist Henri Babinski, in his
Gastronomie pratique
(1907),

advised, “For preserves made from red fruits, it is preferable to use an enam-

eled pan, which does not transmit any sharp taste, as often happens with

unplated copper pans.” During the same period, professors at the École du

Cordon Bleu recommended that cooks “avoid using any iron or tin-plated

utensil.”

What is one to make of these conflicting opinions? Should copper or enam-

eled cast iron be used? If copper, tin-plated or unplated? Although copper

preserving pans may retain a certain luster that encourages culinary nostalgia

and adds to the aesthetic quality of the kitchens in which they are displayed,

they are also a bother because they have to be thoroughly cleaned (which can-

not be done with ammonia, by the way, because this would give the preserves

a disagreeable taste) before being used. Why not use stainless steel pans or

enameled containers instead? Does copper give better results because of its

superior thermal conductivity? Or does it possess other unsuspected proper-

ties that make it preferable to these alternatives?

| 65

The Role of Copper in Preserves

Nothing beats an experiment. Let’s begin by putting red currants or rasp-

berries in an unplated copper pan. To be rigorous about it, let’s first measure

the pH of the pan’s contents (pH is a measure of acidity running from 0, for

very strong acids, to 14, for very strong bases). The acidity of such fruits some-

times is surprisingly high. Indeed, a pH of about 3—which is to say about as

much as certain vinegars—is not unheard of. Next, tilt the pan and you will

see that the copper has been stripped away by the fruit and its juice. In other

words, the copper ions covering the metal have dissolved.

Do these ions have an effect on the preserves? Let’s conduct another ex-

periment, dividing a previously cooked batch of preserves in a chemically inert

container (glass, for example) and then adding a copper salt to one of the two

halves. When the two portions have cooled, one observes that the one con-

taining copper ions is firmer than the other. Why? Because the solidity of the

preserves depends on the presence of pectin molecules, extracted from the

fruits, which form a network that traps the water, sugar, and fruits. Adding

lemon juice generally promotes gellification because pectin molecules contain

carboxylic acid –cooh groups that, depending on the degree of acidity, may

or may not combine. If the environment is insufficiently acidic, the carboxylic

acid groups are ionized in –coo– form so that the electrical charges they carry

have a mutually repulsive effect; in an acidic environment, by contrast, these

groups are neutralized and the pectin molecules no longer repel one another.

What is the role of copper in all of this? In preserves copper is found in the

form of ions and possesses two positive electric charges that interact with the

two negatively charged groups, causing the pectins to bond with one another.

In other words, copper reinforces the pectin gels, hardening preserves, as ex-

perience shows.

And Tin?

Given that copper is a suitable material, why should the tin that covers

the inner surface of old preserving pans be harmful? Could it be that the old

dictums are nothing more than the worthless residue of empirical advances in

culinary practice? As it turns out, putting red fruits such as raspberries or cur-

rants in tin-plated containers produces no unwelcome consequences. Because

66 | secrets of the kitchen

tin does not act on red fruits, one might suppose that copper is the culprit, but

fruits placed in copper are not altered either.

It is nonetheless generally the case that metals act through their salts. Try

sprinkling a pinch of various metallic salts—silver, aluminum, copper, tin,

iron, and so on—over red fruits. The tin salts immediately cause a disagreeable