Molecular Gastronomy: Exploring the Science of Flavor (21 page)

these levels have little to do with the aroma of the fermented dough, unlike the

aldehydes and two compounds that so far have not been identified.

Such studies are being published at a time when bean and soy flours, cus-

tomarily added to bread as whitening agents, are used less and less often.

When the dough is subjected to longer and more rapid kneading by mechani-

cal means, these flours produce hexanol, which brings a stale, oily smell. In re-

viving neglected techniques of fermentation such as the sponge method, a new

generation of bakers seeks to avoid the disadvantages of standardized produc-

tion and to reinvigorate the market for a food that is as old as humanity itself.

136 | investigations a nd mod el s

37

Curious Yellow

The unsuspected nature of an egg yolk.

b e hol d t he e g g, w h o l e a nd r a w, in its shell. Where is the yolk? No

doubt, the physicists will say, on a vertical axis—for reasons of symmetry. Yet

there are other possibilities. The yolk could be in the upper part, in the center,

or in the lower part. How can we determine its location? Put a yolk in a tall,

narrow glass and cover it with several whites: The yolk rises to the top, which

suggests that the same thing occurs in a whole egg.

But could it be that the membranes that surround the yolk and bind it to

the rest of the egg prevent the yolk from rising in its shell? There are several

ways to show that this is not the case. If you boil an egg standing on its end

and examine the yolk you will see that it is lodged in the upper part of the shell.

Coagulation may have disturbed the internal arrangement of the egg, you say?

Place a whole fresh egg in its shell in vinegar. After two days or so, the shell

will be dissolved by the vinegar, and you will see the yolk floating on top of the

white (the egg retains its shape because it is held in place by membranes and

because the external layers have been coagulated by the action of the vinegar).

Or try an even simpler experiment: Remove the top of the shell of a fresh egg

and look to see where the yolk is.

More elaborate procedures can be followed as well. Although radiography

gives poor results (because the shell is opaque to X-rays), ultrasound yields sur-

prising results. In images obtained by immersing an ultrasound probe in an

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egg through a hole in the top of the shell, the yolk can be seen to be composed

of concentric layers similar to those of a tree.

Why is it that no one who has eaten a soft-boiled egg has ever suspected

the existence of this structure? The yolk is an alternation of layers called deep

yellow, 2 millimeters thick, and clear yellow, 0.25–0.4 millimeters thick. These

layers are produced by the hen during the day and during the night, respective-

ly. The difference between the two types results from the rhythm of feeding,

which produces a weaker concentration of yellow pigments during the night

than during the day. Of course, these layers become mixed together when you

pierce a yolk, so you cannot see them.

Granules and Plasma

If we continue our investigation under the microscope, we see that the two

layers, light and deep yellow, are not homogeneous. Instead they are composed

of granules dispersed in a continuous phrase called plasma. Marc Anton and

his colleagues at the Institut National de la Recherche Agronomique station in

Nantes separated the granules from the plasma by centrifugation and observed

that roughly half of the yolk is made up by water, a third by lipids, and about

15% by proteins. Proteins and lipids often are associated in particles that are

distinguished according to their density: low-density lipoproteins (ldls) in the

plasma and high-density lipoproteins (hdls) in the granules. Isolating them

makes it possible to test their properties. For example, it can be shown that the

ldls combine to form a gel when they are heated to a temperature of about

70°c (158°f). It is these structures—composed of proteins and lipids (notably

cholesterol)—that are responsible for the setting of the yolk during cooking.

It has long been claimed that mayonnaise, which consists of droplets of oil

dispersed in water (from either egg yolks or vinegar), is stabilized by lecithins

and other phospholipids in the yolk. Anton and his colleagues sought to an-

swer this question by determining whether the emulsifying properties of the

yolk come from the plasma or the granules. Because the solubility of proteins

depends on acidity, the Nantes biochemists began by studying their solubility

in terms of pH (a measure of acidity) and salt concentration. They found that

plasma proteins are completely soluble at all levels of pH and all degrees of salt

concentration, whereas the solubility of the granular proteins varies: They have

low solubility at a pH of 3—that of mayonnaise—but become more soluble

138 | investigations a nd mod el s

at neutral pH in a low-salt environment (sodium ions replace calcium ions,

which establish bridges between the granular proteins inside the granules,

with the result that these proteins are released).

Protein solubility is not the only thing that must be taken into account in

order to make a successful emulsion, however. The less upward movement

there is by the oil droplets in the water phase, the more stable the emulsion.

In the plasma this movement is minimal for a pH of 3, and salt concentration

has no effect in an acid environment; emulsions obtained from granules, on

the other hand, are sensitive to both acidity and salt concentration. Emulsions

made with whole egg yolks behave like those obtained with plasma.

In sum, the component elements of plasma are responsible for the egg

yolk’s emulsifying effect, and proteins do a better job than phospholipids of

preventing the oil droplets from moving upward, thereby stabilizing the emul-

sion. Is this because the proteins in the ldls of plasma act by electrostatic

repulsion at the surface of the oil droplets, causing the droplets to repel one

another? Or because they protrude from the surface of the droplets and act

instead by steric repulsion? At a pH of 3, proteins are electrically charged and

repel one another; at a pH of 7, however, it is the proteins’ steric properties that

stabilize the droplets by blocking their tendency to fuse with one another. The

exact mechanisms of this behavior have yet to be understood.

All this for an ordinary egg yolk.

Curious Yellow
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38

Gustatory Paradoxes

The environment of aromas a‡ects our perception of them.

a g l a s s o f v i n e g a r i s u n d r i n k a b l e, but it becomes palatable if one

adds a large amount of sugar to it. Yet the pH—the acidity of the vinegar mea-

sured in terms of the concentration of hydrogen atoms—is unchanged. Why

is the sensation of acidity weakened? Because the perception of tastes depends

on the environment in which taste receptors operate. The same interactions

take place in the vicinity of the olfactory receptors. Chantal Castelain and her

colleagues at the Institut National de la Recherche Agronomique station in

Nantes sought to identify these synergies in order to lay the basis for a rational

theory of the aromatization of foods.

Most foods consist of water and fats, which are insoluble in water. When

foods are put in the mouth, the taste molecules reach the taste receptors after

having first diffused through the saliva (a watery solution). Simultaneously

the odorant molecules pass into a vapor phase, in the air contained in the

mouth, and from there migrate toward the olfactory receptors in the nose.

This simple description is complicated by the fact that taste and odorant mol-

ecules are never completely soluble or insoluble in water and that, depending

on their chemical composition, they are variably distributed in liquids and air.

It is clear, then, that the aromatization of foods can be mastered only if the

diffusion of molecules between liquid phases (either water or oil) and gaseous

phases is understood.

140 |

To analyze these various migrations, the Nantes biochemists constructed a

device in which a neutral oil can be placed in contact with water and one can

measure the distribution of molecules among four compartments: water, oil,

the air above the water, and the air above the oil. They began by dissolving the

molecules, in either the water or the oil, and then measured their concentra-

tion in the three other compartments. The aromatic molecules were found

to follow different trajectories depending on experimental conditions. When

dissolved initially in oil, for example, they pass into the air above the oil while

diffusing in the water and passing from there into the air above.

Several molecular mixtures were tested: esters, aldehydes, alcohols, and ke-

tones. Paradoxically, transfer was observed to be more rapid from oil to water

than from water to oil in the case of the esters and ketones but more rapid from

water to oil than from oil to water in the case of the alcohols and aldehydes.

Because many alcohols are soluble in water, for example, one would have

expected them to have trouble migrating toward oil, in which case the transfer

ought to be slower from water to oil than from oil to water.

In the Nose

To better understand how foods are aromatized, these studies must be

supplemented by an analysis of the effects of odorant molecules on the recep-

tors in the nose once they have penetrated the mucus layer covering these re-

ceptors and dissolved in the hydrophobic phases of the cellular membranes.

The Nantes researchers are investigating the perception of molecules above

the water and oil phases. Having dissolved a single molecule in two liquids—

oil and water—in concentrations such that a sensor registers the same partial

pressure of the molecule above each liquid, they analyzed the perceptions of

people who breathed the air above the two.

Paradoxically, again, the human nose sometimes perceives a difference.

For certain molecules, the nose registers a very similar result to that of the

detectors (in the case of linalol, for example, which has an odor of lavender

and bergamot, depending on the concentration), but no general law has been

deduced because other odorants such as 1-octane 3-ol (which has a smell of

mossy decomposing undergrowth and mushroom), benzaldehyde (a smell of

almond), and acetophenone (a smell of beeswax) give rise to different percep-

Gustatory Paradoxes
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tions. Obviously the presence of water vapor can affect the perception of the

aroma.

Such phenomena are likely to be found in connection with taste as well.

The texture of foods determines flavors by affecting the length of time dur-

ing which taste and odorant molecules remain in the mouth and the rates of

diffusion of these molecules from foods toward the olfactory and gustatory

receptors. Thus a mayonnaise that is too acid becomes less so when it is beaten

because its firmer texture slows down these transfers.

142 | investigations a nd mod el s

39

The Taste of Food

The texture of vinaigrettes determines their odor.

c o ok s w e l l k n o w t h a t a d d i n g too much flour to a sauce makes it

tasteless. The reason is that the flavor of foods does not depend solely on the

odorant and taste molecules they contain but also on interactions between

these molecules. Molecules with no odor, such as proteins and starch mol-

ecules, bind with certain odorant molecules and prevent them from acting

on our senses. One would like to know exactly which aromas are masked in

this way.

Identifying all the chemical bonds between the various molecules in a food

is not enough, for its physical structure plays a role as well. Even in homoge-

neous phases such as solutions, the release of odorant molecules depends on

the viscosity of the system. However, foods are not solutions but dispersed sys-

tems. In foams, air bubbles are trapped in liquids or solids; in emulsions, oil

droplets are dispersed in water; in suspensions, solid particles are distributed

in liquids; and so on. The odorant molecules that are present within the dis-

persed or continuous phases of such physicochemical systems are not released

in the same fashion as if they are simply dissolved.

Complicating matters still further is the fact that in emulsions, for example,

dispersed droplets of oil are held in place by layers of tensioactive molecules.

Because of their solubility in both oil and water, these molecules almost nec-

essarily bind with odorants, which then become inaccessible to the olfacto-

ry system. To identify the phenomena that determine the release of odorant

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molecules, Marielle Charles, Élisabeth Guichard, and their colleagues at the

Institut National de la Recherche Agronomique and École Nationale Supéri-