Molecular Gastronomy: Exploring the Science of Flavor (33 page)

grams per liter, and so on (roughly 1/3 ounce per quart, 2/3 ounce, etc.), until

the saturation point is reached, and put fruit in each of them (plums, for ex-

ample, pierced as the recipe indicates). One observes first of all that the fruit

floats in the syrups containing the most sugar but not in the others. This is

not surprising, for the density of a syrup increases with the amount of sugar

dissolved in it. If we wait to see the effects of the sugar concentration we will

find that our patience is rewarded, for it becomes plain that in heavy syrups,

in which the fruit floats, it has shriveled up, whereas in light syrups, in which

the fruit has sunk to the bottom, it has broken apart.

Of course, the correspondence is not exact, for many kinds of fruit exhibit

varying degrees of ripeness. Some pieces float; others fall to the bottom of the

pan because their composition and, in particular, their sugar concentration has

changed in the course of ripening. Moreover, the pit of the fruit may not have

the same density as the surrounding flesh. We can complicate the experiment

somewhat by making a series of increasingly concentrated syrups and putting

into them whole fruits, quartered fruits with the pit still attached, quartered

fruits separated from the pit, and pits by themselves. The density of the pit

turns out to be irrelevant, for the variability in ripeness of the individual pieces

of fruit is greater than the difference in density between the flesh and the pit.

An efficient way to determine the right strength of the syrup is this: Make

the syrup a little too strong, so that the fruit floats on top, and then add some

water. When the fruit ceases to float, the desired concentration has been

achieved. Let winter come!

220 | investigations a nd mod el s

65

Fibers and Jams

Pectins can be recovered from the brous matter of fruits and vegetables

by extrusion cooking.

d u r i ng t h e w i n t e r , t h e s p e c t e r o f a j a m that fails to set haunts

cooks who make orange marmalade. Sometimes, instead of forming the ex-

pected semisolid mass, the juice remains juice. How can this culinary debacle

be avoided? Jean-François Thibault, Catherine Renard, Monique Axelos, and

Marie-Christine Ralet at the Institut National de la Recherche Agronomique

station in Nantes have found an answer to this question, although the purpose

of their research was mainly commercial. They showed that the technique of

extrusion cooking, used especially in fabricating cocktail crackers, makes it

possible to extract large quantities of pectin, which is to say gelatinizing mol-

ecules, from the pulp of lemons, oranges, and beets.

The occasional failure of jams to set seems paradoxical. Pectins are mol-

ecules that, together with cellulose and other polysaccharides (long molecules

formed by the linking of simple sugars), form the cell walls of most plants;

only cereals do not contain them. But because the method of moderate heat-

ing used to prevent volatile aromas from evaporating does not efficiently break

down the cell walls, the pectins are unable to come together to create a net-

work, or gel, that fills up the entire volume of the solution.

In the case of commercial production a different problem arises: The resi-

due from pressing apples to make apple juice and cider, the pulp of lemons

and oranges left over after juicing, and the pulp of beets that remains after their

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sugar has been extracted all contain large quantities of plant fiber that produc-

ers would like to find a use for. Traditionally pectins are recovered from these

fibers through heating in an acid solution. In this way the pectins are detached

and then, in a further step, purified. The resulting material is used as a gela-

tinizing or thickening agent or to coat potato chips. In the future it may find

a new role as a detergent for cleansing polluted water (because pectins bond

strongly with the metallic ions that are the source of water impurities).

Nonetheless, the acid extraction process has several disadvantages. In par-

ticular, it threatens to degrade pectin molecules by breaking the bonds between

their constituent sugars and altering the chemical groups carried by these sug-

ars that are responsible for gelatinization.

From Polymers to Dietary Polymers

The Nantes chemists had studied another method known as extrusion cook-

ing for several years in connection with the processing of meats and starches,

which gave them the idea of using it to extract pectins from fruit pulp. The

apparatus for this technique (adapted from devices used to extract polymers)

has one or more Archimedean screws that rotate at variable speeds, shearing

off the fibrous material and injecting it into a tube, where it undergoes rapid

expansion. The extracted material then passes through a series of chambers in

which it is heated at controlled temperatures.

Not all pectins promote gelatinization. Their properties depend on their

chemical composition, which varies according to the fruit or vegetable from

which it is derived: Whereas pectins from limes, lemons, oranges, and apples

are efficient agents, the ones extracted from beet pulp do not gelatinize (al-

though beet pectins, because they chelate heavy ions, produce superabsorbent

gels after chemical modification). The differences in the properties of pectins

are a consequence mainly of esterification. The sugar chains that form their

molecular skeleton carry carboxylic acid (–cooh) groups, which are esterified

to varying degrees according to the fruit. As a result, these groups now display

the form –cooch . Depending on the extent of esterification, the pectins as-

3

sociate more or less easily with one another. It is this ease of association that

ensures gelatinization. Extraction for the purpose of producing gelatinizing

or thickening agents therefore must respect the integrity of the pectin lateral

chains.

222 | investigations a nd mod el s

The extrusion cooking method has several advantages. Not only is the ex-

tractive apparatus much simpler and cheaper than that of other techniques

(the screws used at Nantes are only a meter long), but the processing is rapid

and can be automated. Additionally, it yields as many pectins as the traditional

acid treatment and preserves their molecular structure. The Nantes research-

ers found that the shearing off of plant matter during extrusion cooking is the

critical step. Indeed, if this material is not heated, the molecular integrity of the

pectins is preserved despite compression.

Culinary Moral

How can you take advantage of these results cooking at home? At room

temperature, squeeze the juice from the fruit and set it aside, finely grind up

the remaining fibrous matter in a food processor, then add the juice back to the

ground-up material and cook over low heat. But because the pectin molecules

are apt to bond with the copper atoms of your pan instead of with one another,

don’t leave your jam to cool in the pan. Let it set in glass containers instead.

Fibers and Jams
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66

The Whitening of Chocolate

To keep chocolate from turning white, keep it chilled.

w h y d o e s c h o c o l a t e b e c o m e c o v e r e d with an ugly white film after

a few days? Michel Ollivon and Gérard Keller of the Centre National de la Re-

cherche Scientifique (crns), in collaboration with Christophe Loisel and Guy

Lecq of the Danone Group, have recently explained how certain constituents

of this partially liquid mixture migrate and crystallize on its surface, causing

it to change color.

Chocolate is a dispersion of sugar crystals and cocoa powder in cocoa but-

ter. Once crystallized, the cocoa butter serves as a binding agent for the solid

particles, just as cement binds the sand and gravel in concrete. Nonetheless,

chocolate does not easily cohere: The sugar crystals are hydrophilic, whereas

the cocoa butter is hydrophobic. Master chocolate makers add lecithin mol-

ecules as part of a process known as conching in order to promote the coating

of the sugar by the cocoa butter.

Since the 1960s it had been known that the whitening of chocolate resulted

from the thermal behavior of cocoa butter. The mixture of semisolid and semi-

liquid fats that make up cocoa butter can crystallize in six different ways (called

forms 1–6), and only a precise sequence of reheatings and recoolings, collec-

tively known as tempering, yields crystals that are sufficiently stable to prevent

chocolate from whitening. The recooling phases are crucial; failure in any one

of them will ensure a disappointing result.

224 |

Chocolate X-Rayed

The crns and Danone researchers investigated the crystallization process

by measuring variations in viscosity during tempering. X-rays showed that

well-tempered chocolate is crystalized in form 5, and further observation by

means of polarizing microscopy disclosed that the surface of the chocolate

immediately after fabrication is free of crystals; only a few holes, no doubt cre-

ated by the bursting of air bubbles on the surface, and a few minute cracks are

visible. By contrast, when the chocolate is subjected to substantial variations in

temperature, the whitening appears after a few days. Both the surface and the

interior crystalize in form 6.

How are we to interpret these observations? It was long thought that the

whitening resulted from a migration of a part of the cocoa butter toward the

surface, where it recrystallized, probably in form 6. Today we know that the

composition of fatty matter in the body of the chocolate differs from that of the

whitened surface. Hervé Adenier and Henri Chaveron at the University of Com-

piègne have confirmed that form 6 crystals are more stable than those of form

5; the difference in their fusion temperatures is 1.5°c (2.7°f). Moreover, form 6

is more compact than form 5. During the transition between the two forms, a

part of the cocoa butter is pushed out toward the surface of the chocolate.

The difference in fusion temperatures corresponds to a difference in com-

position. The molecules of cocoa butter, triglycerides, are like a comb with

three teeth: The base of the comb is a glycerol molecule, and the three teeth are

fatty acid molecules. Carbon atoms in these acids are held together by means

of single or double bonds. The fusion temperature of a triglyceride diminishes

with the number of double bonds.

Pushed by Fusion

The proportion of the liquid phase naturally increases with temperature

while becoming enriched by triglycerides having one double bond, which melt

at a lower temperature. Whitening corresponds to the recrystallization, during

cooling, of these enriched liquid parts.

Whitening threatens mainly chocolates whose interior, or lining, is rich in

fats. When two blocks of fatty matter are combined, their fats become mixed

The Whitening of Chocolate
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together, especially at high temperatures (when the proportion of liquid and

molecular motion both increase). The surface crystals of the whitened choco-

late are quite different from those of the interior mass, being composed for the

most part of triglycerides having a single double bond.

The composition of the whitened crystals is virtually invariable, no matter

what the fatty content of the lining, and similar to those of whitened choco-

late without a lining. The migration of fats in the lining toward the covering

layer of chocolate leads to an enlargement of the liquid phase of the covering

layer. Consistent with the findings of the cnrs and Danone researchers, this

enlargement accelerates whitening without changing the composition of the

outer crystals.

The moral of the tale is that if you want to preserve the aroma and appear-

ance of chocolate, store it at a low temperature, say 14°c (57°f). In this way you

will limit the migrations that accompany storage. Then warm up the chocolate

before eating it.

226 | investigations a nd mod el s

67

Caramel

The molecules of caramelization nally identied.

s e n e c a m e n t i o n e d c a r a m e l as early as 65 bc, but for more than 2000

years the details of the chemical reactions that give heated sugar its inimitable

flavor were unknown. Exploiting recent results in the chemistry of sugars and

using modern analysis techniques, Jacques Defaye and José Manuel Garcia

Fernandez at the Centre National de la Recherche Scientifique laboratory in

Grenoble recently elucidated the structure and mechanisms responsible for

the formation of the odorant and taste molecules that make up caramel.

Along with the Maillard reaction, which generates the aroma of roast beef,

coffee beans, beers, and bread crust, caramelization is one of the principal