Molecular Gastronomy: Exploring the Science of Flavor (29 page)

potatoes that are deep-fried.

Deep-Frying Considered

How should French fries be cooked? On this point cooks are apt to disagree,

for each chef has his or her own method. One needs to ask what one is looking

for in a plate of French fries and then rationally to examine which procedures

allow this expectation to be satisfied.

Few connoisseurs will quarrel with the opinion that good French fries must

be tender at the center, with minimal greasiness, and that they should be crispy

without being overly brown. To achieve this result we must recognize that deep-

frying involves a diffusion of heat from the outside inward, with two principal

consequences: the formation of the crust and the cooking of the interior.

Potatoes are composed of cells that contain mostly water and starch gran-

ules. When the heat reaches the center of the fries by conduction, some cells

are dissociated as the starch granules release their long molecules into the

192 | investigations a nd mod el s

heated cellular water. With the complete evaporation of this water a crust is

produced on the surface of the fries.

If one cooks a potato stick into which a thermocouple has been inserted

(a more rapid and more reliable way of measuring temperature than with a

thermometer), one finds that the interior heats up very slowly: Even when the

temperature of the oil is 180°c (356°f), the temperature in the center reaches

85°c (185°f) only after several minutes, for the potato is thermically inert. In

other words, if the oil is too hot in the first round of frying, the surface will

burn before the inside is cooked.

Conversely, the oil must not be too cold to begin with, for then the crust

will be slow to form and the fries will soak up oil. In practice, seven minutes

of cooking at a temperature of 180°c (356°f) yields good results for fries mea-

suring 12 millimeters (about half an inch) thick. A second round of cooking

in oil heated to a slightly higher temperature, 200°c (392°f), produces perfect

fries; remove them from the oil when they have turned just the right golden

brown color.

French Fries
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56

Mashed Potatoes

Proteins change the behavior of starch in water.

w h y d o m a s h e d p o t a t o e s m a d e w i t h m i l k stick less than ones

made with water? By showing that proteins modify the thickening and gela-

tinization of starch, Jacques Lefebvre and Jean-Louis Doublier at the Institut

National de la Recherche Agronomique station in Nantes have at last provided

an explanation of this venerable piece of culinary knowledge and useful indica-

tions for the use of flour in sauces.

Flour and potatoes have in common the fact that they contain a great deal

of starch, in the form of granules that contain two sorts of molecules: amy-

lose, which is composed of a linear chain of hundreds of glucose groups, and

amylopectin, a polymer that is similar to amylose but has instead a ramified,

or branched, structure. In each granule of starch these two types of molecule

exhibit a crystalline form.

Whereas the starch granules in flour are exposed to the surrounding at-

mosphere by the milling of grains of wheat, the starch in potatoes inhabits a

watery environment enclosed by their cell membranes. Starch does not dis-

solve, for amylopectin is highly insoluble in water, and amylose is soluble only

in fairly hot water, at temperatures higher than 55°c (131°f).

When one cooks potatoes by putting them in a hot fluid (air, water, or oil)

the heat is drawn to the center by conduction, triggering the same sort of ex-

pansion as the one that occurs when, in preparing an espagnole, a white sauce,

or a béchamel, one pours a boiling liquid (water, milk, or broth) into flour: The

194 |

water molecules dissolve the amylose molecules and alter the structure of the

starch granules, disorganizing the amylopectin crystals and causing the gran-

ules to swell. During these transformations the starch granules soften, and dis-

solving the bulky amylose molecules makes the solution flow less easily. Thus

a sauce thickens when flour is added to it, both because the swollen granules

get in the way of one another and because the solution in which these granules

are dispersed becomes viscous.

If cooks do not always understand the molecular details of the thickening

of sauces, they nonetheless know that sauces must be kept hot. For example,

white sauces form a gelatinous mass when they cool because a gel forms when

the neighboring amylose molecules in the water solution combine with one

another, forming a network that traps the water, swollen starch granules, and

the various dissolved compounds.

Doublier and Lefebvre showed that the phenomena of swelling up and ge-

latinization are modified when proteins such as casein are present outside the

swollen granules. Caseins, found in milk, aggregate into structures called mi-

celles, which are dispersed in the cellular water and coat droplets of fatty mat-

ter. As emulsifying agents they are widely used in the food processing industry

to make ice creams, dairy products, custards, and so on.

These proteins reduce the quantities of amylose that leak out of the starch

granules and also limit the extent to which they swell up. Caseins subse-

quently bring about a separation in the water phase: Protein-enriched water

droplets separate from the rest of the sauce, which is then enriched by amy-

lose in a continuous phase. This increase in amylose concentration favors its

gelatinization.

The Proteins of Mashed Potatoes

How do these mechanisms operate in the making of mashed potatoes?

When one cooks potatoes the starch granules are not fully expanded because

there is not enough intracellular water to be absorbed by the granules. Mash-

ing the potatoes with a bit of reserved cooking water hastens the incorporation

of amylose into solution and the swelling up of the starch granules, with the

result that the whole thing forms a sticky mass. On the contrary, mashing

them with milk, which contains caseins, limits the swelling of the starch and

therefore yields a smoother, more pleasing consistency. This phenomenon also

Mashed Potatoes
| 195

must be taken into account when one thickens sauces with flour, for the gelatin

of the sauce stock has the same effect on the flour as milk proteins.

In studying variations in viscosity as a function of the rate of flow, the

Nantes researchers provided another hint that cooks ought to find useful. Al-

though perfectly expanded starch granules are thixotropic, which is to say that

they are deformed when subjected to flow and therefore form a less viscous

solution in the mouth than at rest in the saucepan, sauces thickened with flour,

in which proteins limit the degree of expansion, preserve an almost constant

level of viscosity: The few amylose molecules that leak out of the starch gran-

ules into solution are aligned with the direction of flow, while the unswollen

granules are deformed to a correspondingly minor degree.

196 | investigations a nd mod el s

57

Algal Fibers

Algae contain bers whose nutritional value is comparable to that of veg-

etable bers.

i n p a r t s o f t h e f a r e a s t algae have been used as vegetables in soups

and salads since ancient times. In France they serve mainly as a source of io-

dine and fertilizer and as gelatinizing or texturing additives. Although eleven

species of algae were recently accepted as vegetables by the French health au-

thorities, their chemical composition and metabolism are poorly understood.

Analysis of the fibers they contain nonetheless has illuminated the sources of

their nutritional value.

The modern vogue for fibers began in the early 1970s when the British phy-

sician Denis Burkitt discovered a correlation between certain digestive, meta-

bolic, and cardiovascular diseases and low levels of consumption of foods rich

in fibers. Fibers are macromolecules that resist digestion by human enzymes.

For the most part they make up the cell walls of plants—cellulose, for example,

and various other molecules composed of chains of monosaccharides, or ele-

mentary sugars. Their chemical complexity dampened the enthusiasm initially

aroused by Burkitt’s finding, but researchers have now developed sufficiently

powerful analytical tools to carry on his work.

Fibers and Digestion

At the Institut National de la Recherche Agronomique station in Nantes,

Marc Lahaye and his colleagues have used such tools to study the dietary fibers

| 197

of marine macroalgae. It used to be customary to classify fibers according to

their degree of solubility in response to various enzymatic treatments. Water-

soluble fibers—certain pectins (fruit polysaccharides that cause jellies to gel),

algal polysaccharides, and certain kinds of hemicellulose—were distinguished

from insoluble fibers such as cellulose, other kinds of hemicellulose, and lig-

nin. The soluble fibers, many of which have interesting rheological properties,

were thought to reduce the blood concentration of cholesterol and to act on the

metabolism of glucids and lipids. Insoluble fibers, on the other hand, seemed

to accelerate bowel movements.

Researchers have confirmed the main features of these properties in recent

years, and the Nantes team refined the classification of the dietary fibers in

algae by modifying a technique of molecular separation known as the gravi-

metric method, which was used to precipitate macromolecules in various

environments, such as water and alcohol, after the elimination of starch and

proteins by enzymatic treatment. In 1991 Lahaye and his coworkers used it to

determine the quantity of polysaccharides solubilized in environments that

approximately reproduce those of the digestive tract.

They went on to apply this method to algae and found that the total con-

centration of dietary fibers in wakame, for example, can be as great as 75%,

as opposed to only 60% in Brussels sprouts, the root vegetable that is richest

in fiber. To discover how such fibers behave in the digestive tract, the Nantes

chemists first studied
Laminaria digitata,
several thousand tons of which are

processed every year in order to produce the gelling agents known as alginates

(in 1992 alone some 65,000 tons were recovered and treated by the French

seaweed fertilizer industry). Its polysaccharides—glucose polymers known as

laminarins—are essentially soluble in very acid environments, whereas algi-

nates dissolve in a neutral environment. The insoluble fibers of
Laminaria
spe-

cies are principally constituted by cellulose.

On the other hand, the fibers of dulse (
Palmaria palmata
), a common red

seaweed consumed by Europeans since the eighteenth century, seem to be

continuously solubilized in the successive sections of the digestive system. In

the case of sea lettuce and sea hair—two of the eleven species of algae that

can now be sold as vegetables or condiments in France—the soluble fibers are

xylorhamnoglycuronane sulfates (made from sugars and composed, in particu-

lar, of xylose and rhamnose), and the insoluble fibers are principally glucans

(glucose polymers). Their solubilization more nearly resembles that of dulse.

198 | investigations a nd mod el s

Algal Thickening Agents

Improved understanding of the nature and assimilation of these foods has

stimulated interest in developing markets for underused algal fibers and for

the large quantities of residues that are now extracted from alginates every

year. The extraction procedure begins with repeated washings in an acidified

water solution, which eliminates the laminarins and fucans and transforms

the alginate into alginic acid. Dietary fibers are solubilized in these baths. Then

the alga is tossed in a warm basic environment (often with sodium carbonate),

and the insoluble part is separated out with the aid of a flocculating agent and

air currents. The resulting flocs, or tuftlike masses, constitute a second coprod-

uct consisting mainly of cellulose.

Fibers from vegetable sources—beets, cereals, and fruits—are now incorpo-

rated in breakfast products or used as an ingredient in various prepared foods.

Algal fibers can serve the same purposes. Looking to the future, research in the

chemistry of algal fibers holds out the prospect that processing methods can

be developed similar to those used in the milling of wheat and other grains,

which depend in large part on an understanding of their elaborate structure.

But whereas the chemistry of starch is already well developed, the listing of

the elementary sugars that make up algal macromolecules has only recently

been completed, and the exact order of their sequence is in many places still

uncertain, as is the type of chemical bonds linking the sugars. Now that chem-