Molecular Gastronomy: Exploring the Science of Flavor (22 page)

eure de Biologie Appliquée à la Nutrition et l’Alimentation stations in Dijon

studied salad dressings in collaboration with researchers at Amora-Maille, the

well-known producer of mustards and condiments.

Fragrant Salad Dressings

In the vinegar-based sauces studied, the watery phase of these emulsions

consisted of a mixture of wine vinegar, lemon juice, and salt. Drops of sun-

flower oil were then emulsified in it (that is, kept in a dispersed state) thanks to

the action of whey proteins. Finally, a mixture of xanthan (a polymer obtained

by microbial fermentation of glucose) and starch was incorporated to stabilize

the sauce. To this basic salad dressing the physical chemists added fixed quan-

tities of odorant molecules: allyl isothiocyanate, in the oil phase, which yielded

a hint of mustard; and, in the water phase, phenyl-2-ethanol and ethyl hexano-

ate, which gave rose and fruit notes, respectively. When several emulsions were

compared each one was found to contain oil droplets of a particular size. A

jury of trained tasters evaluated the sauces, noting the intensity of the various

accents—lemon, vinegar, mustard, and so on.

The results of these taste tests were difficult to analyze: Although the acid

taste was preponderant, the tasters struggled to describe the other sensations.

Their training nonetheless enabled them to perceive that the intensity of the

overall odor, as well as that of the taste and odor of egg, the mustard odor, and

the butter taste, increased with the size of the oil droplets and, conversely, that

the intensity of the citrus fruit odors diminished as the oil droplets got larger.

Migrations of Odorant Molecules

To interpret these observations more precisely, the researchers analyzed the

concentrations of volatile molecules in the air above the sauces. They found

that the water-soluble components were present in lower concentrations as

the oil droplets became smaller and, conversely, that in this case the oil-soluble

molecules were more abundant.

What accounts for these differences? Consider first the oil-soluble odor-

ant molecules. When emulsions containing fixed proportions of water and oil

144 | investigations a nd mod el s

are vigorously whisked, the oil droplets become smaller and more numerous.

The lipophilic molecules therefore have a shorter distance to cover in order to

reach the surface of the droplets. Moreover, because the total surface area of

the oil–water interface has increased, the tensioactive molecules form a thin-

ner layer above the surface of the oil droplets, with the result that the lipophilic

odorant molecules diffuse more easily outside the droplets. Finally, the extent

of the water layer (in which these odorant molecules bind with the thickening

agents) that must be traversed before they reach the air above the emulsion

is smaller.

For the odorant molecules that are more soluble in water, the effect is oppo-

site: The smaller the oil droplets, the harder it is for these molecules to diffuse

in a thinner layer of water, and so they are less readily perceived.

Are these general laws? Because foods contain various kinds of molecules,

many studies will be necessary to characterize all the relevant mechanisms

and then to elucidate their relative importance. “Life is short,” said Hip-

pocrates, “the art is long, opportunity fleeting, experience misleading, judg-

ment difficult.”

The Taste of Food
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40

Lumps and Strings

The enemies of successful sauces form because water di‡uses slowly in

starch paste and gelatin.

l u m p s — t h e s h a m e a n d d e s p a i r of the cook. Cookbooks suggest vari-

ous ways to avoid them in making béchamel and other sauces that are thick-

ened with flour. Some authorities say to make a roux first, cooking butter and

flour and then adding milk (cold according to some, hot according to others);

others insist on the opposite method, namely pouring the roux into the milk,

which, again depending on the author, should be either cold or hot.

Which method is better? If we are willing to waste a bit of flour, butter, and

milk in order to test the four possibilities we will find that the formation of

lumps is at bottom a question of speed: When the roux is added gradually to

the milk or the milk to the roux, lumps do not appear; however, they do form

when the two ingredients are mixed together at once, especially when the roux

is poured into boiling milk.

This experiment gives us a method, but it does not explain the phenom-

enon. Let’s study the matter further by simplifying it. The butter serves

chiefly to cook the flour, eliminating its bland taste by creating odorant and

taste molecules (the browning is associated with the chemical reactions that

form these molecules), but it is not the cause of the lumpiness, which also

results from combining flour and water. Why? Flour is composed principally

of starch granules, themselves made up of amylose and amylopectin. These

two types of molecule are both polymers, which is to say chains of glucose

molecules (linear in the case of amylose, ramified in the case of amylopectin).

146 |

Amylopectin is insoluble in water, but amylose is soluble in hot water. When

the starch granules are put into hot water, they lose their amylose molecules,

and the water fills the space between the amylopectin molecules, causing the

granules to swell up and form a gel known as starch paste.

This description gives us a handle on the problem of lumps, for it suggests

that flour deposited in hot water is quickly enveloped by a gelatinized layer

that limits the diffusion of water toward the dry central core of the lump. But

it is inadequate as an explanation, however, because it fails to explain why,

if the water diffuses into the periphery of the lumps, it does not ultimately

reach the center.

One-Dimensional Lumps

As we have seen, everything is a question of speed. To measure the rate of

gelatinization, let’s simplify the matter further by making a one-dimensional

lump whose center we can observe. Put some flour in a test tube and then

pour water over it (if the water is colored one can follow its diffusion into the

flour by observing the movement of a distinct boundary). At room temperature

the water infiltrates the upper layer of the flour fairly rapidly at first but then

penetrates further very slowly, by less than a millimeter an hour. Heating the

water causes the granules to swell, so contact with this gelatinized region lim-

its the advance of the colored boundary. In this case the rate of diffusion rises

to several millimeters per hour.

It follows, then, that the center of the lumps remains dry because of the

slowness of the water’s diffusion through the gelatinized periphery. When the

water has diffused and swollen the starch granules, binding them together,

they form a layer that retards further diffusion toward the center to such a

degree that it remains dry (a phenomenon characterized more precisely by

specifying the many molecular interactions that take place between the starch

granules and the water). In other words, placing a quantity of flour measuring

a centimeter in diameter in hot water causes it to become moistened to a depth

of one or two millimeters from the periphery, with the center remaining dry

longer than the time needed for most culinary purposes.

How can we get rid of these lumps? No fundamental physics is needed

to solve the problem. It suffices to break up the lumps—with a whisk, for ex-

ample—into particles smaller than the thickness of the starchy layer.

Lumps and Strings
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Soaking Gelatin

Does this theory of the formation of flour lumps apply to other types of

lumps? Cooks know that leaves of gelatin must soak in cold water before being

used in a hot liquid. Failure to observe this rule creates strings that are as

difficult to eliminate as lumps of flour in hot water. Are these strings like-

wise composed of a dry center and a sheath in which the water and gelatin

molecules are mixed? To find out, first measure the water’s rate of diffusion

in cold gelatin by placing a small amount of coffee grounds on its surface. A

hemispherical colored zone extends outward from this point at a rate of only

about a centimeter a day.

Next, let’s repeat the experiment we conducted with the flour, only this time

replacing the flour with gelatin. One then observes that the boundary of col-

ored water sinks into the gelatin only very slowly. But a new phenomenon now

appears: Under the layer of gelatinous solution, the gelatin that is untouched

by the water has melted like butter.

Similarly, in a hot broth, a sheet of gelatin that has not been soaked before-

hand has trouble dissolving and conserves a solid central part that melts from

the heat. Its molecules stick to one another, forming the dreaded strings. In a

sheet that has been soaked long enough to allow the water to gradually pen-

etrate into the center, on the other hand, heating causes the gelatin to dissolve

without strings.

148 | investigations a nd mod el s

41

Foams

The stability of foams depends on the arrangement of the proteins at the

interface between the water and air.

f o a m s — l o w i n f a t b e c a u s e they are essentially composed of air—first

came to prominence with the rise of
Nouvelle Cuisine
in France in the 1960s

and then gained broader popularity as a consequence of the growing interest in

lighter foods on both sides of the Atlantic. Today, with the advent of molecular

gastronomy and, in particular, the fame of the Spanish chef Ferran Adrià, they

are very fashionable among gourmets. In the early days foams were obtained

by vigorously beating egg whites, but the variety of eggs combined with igno-

rance of the optimal conditions for making foams led to irregular results, a

fatal handicap from the point of view of the food processing industry. Physico-

chemical analysis of protein foams has yielded a better understanding of the re-

lationship between the composition of proteins and their foaming properties.

Composed of air bubbles separated by liquid films, foams retain their form

only if the liquid forming the walls of the bubbles does not subside or if these

walls are able to support themselves despite the draining away of the liquid.

Beating an egg white reveals one of the conditions of stability, namely, that the

air bubbles must be sufficiently small that the surface forces are stronger than

the forces of gravity, which cause the water to fall and the air to rise.

By comparing the layers formed by the various proteins along the boundary

where water and air meet, Roger Douillard and Jacques Lefebvre at the Institut

National de la Recherche Agronomique station in Nantes and Justin Tessié in

Toulouse showed that the stability results both from the interactions of the

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proteins present in the walls of the liquid films that separate the bubbles and

from the viscosity of these films.

A key parameter in the study of foams is the interfacial tension of the pro-

tein films at the air–water boundary. The physical chemists measured this

tension in terms of the force needed to extract a very clean platinum blade

immersed in a solution covered with a protein film: The greater the amount of

liquid that coats the blade, the greater the force needed to extract it. The pro-

teins modify interfacial tension because they consist of chains of amino acids,

with hydrophilic (water-soluble) parts and hydrophobic (insoluble) parts. Ar-

ranged at the water–air interface in such a way that their hydrophilic parts are

in contact with the water and their hydrophobic parts with the air, they favor

an increase in the surface area common to both air and water and facilitate the

formation of foams.

A foam is stabilized by increasing the viscosity of its liquid phase (for ex-

ample, by adding sugar and glycerol) and, above all, by modifying the drainage

properties of the absorbent films. In protein foams these films are rigidified

by intramolecular and intermolecular bonds, such as disulfide bridges between

the cysteine groups of proteins, and weak bonds (van der Waals forces and

hydrogen bonds).

The physical chemists from Nantes and Toulouse were particularly inter-

ested in the role of proteins in the formation of foams and sought to analyze

the scale of interfacial tension as a function of protein concentration. They

knew that very soluble proteins, which are adsorbed to only a small degree on

the air–water interface, do little to reduce interfacial tension when their con-

centration rises. But the behavior of almost all other proteins was difficult to

analyze because of their molecular complexity: Not only are proteins polymers,