Molecular Gastronomy: Exploring the Science of Flavor (46 page)

structuring of meat. The gels are firmer when the myosin is extracted just after

slaughter, before the onset of rigor mortis (a consequence of irreversible bonds

being established between the actin and myosin). Sodium pyrophosphate, a

molecule belonging to the same family as atp, dissociates the actin–myosin

complexes and so makes it possible to obtain firmer gels. The incorporation of

these myosin extracts in meats that are then sliced into thin sheets has made

it possible to increase their cohesion while limiting the loss of juices, which

are trapped in the gel.

Pure Beef
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95

Fortied Cheeses

The right bacteria can strengthen the avor of cheeses.

a g o u d a o r a c h e d d a r t a k e s o n its full gustatory quality only after

several months, and many cheeses, even ones that have been aged for a long

time, do not have the powerful flavor that one might want. The aging of cheese

has been a lively topic of debate in the gastronomic world for centuries. The

milk that is curdled and seeded with lactic bacteria acidifies in the course of

maturing: The transformation of the milk sugar lactose into lactic acid pre-

vents contamination by pathogenic microorganisms, and the lactic bacteria

release aromatic compounds that contribute to the taste of the cheese. Mireille

Yvon and her colleagues at the Institut National de la Recherche Agronomique

(i nra) station in Jouy-en-Josas have studied which strains of lactic bacteria

produce the greatest quantity of these aromatic compounds.

The i nra biochemists knew that the flavor patiently acquired by cheeses

results from the microorganisms responsible for maturing. Fats and sugars are

progressively transformed, and proteins are dissociated into their constituent

elements, amino acids, which are then transformed into aromatic molecules.

For example, the amino acids leucine and valine produce compounds having a

cheese note, whereas phenylalanine, tyrosine, and tryptophan are the precur-

sors of floral and phenolic notes (as they are described by trained tasters).

Does the slow dissociation of proteins into amino acids limit the formation

of aromatic compounds? No, for the direct addition of free amino acids does

not improve the taste of cheeses any more than does the seeding of milk by

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lactic bacteria (whose capacity for dissociating proteins has been increased by

genetic engineering). In the latter case, the amino acids are released in greater

quantities, but the taste is not changed. In the early 1990s, Yvon and her team

concluded instead that what limits the development of flavor is the transforma-

tion of amino acids into aromatic compounds.

Stimulating Additions

Two biochemists in the Netherlands, W. Engels and S. Visser at Wagenin-

gen University, noticed that the flavors typical of Gouda were obtained when

methionine was added to lactic bacteria (without milk) and went on to identify

two enzymes that seemed to be responsible for the phenomenon. Yvon and

her colleagues, for their part, had observed in vitro that lactic bacteria degrade

certain amino acids to form aromatic compounds such as aldehydes and car-

boxylic acids. The first step in this transformation, known as transamination,

involves a reaction between an amino acid and a molecule named ketogluta-

rate, which produces a keto acid in addition to glutamate (a molecule that,

as we have seen, is used as a flavor enhancer in Asian cuisine and in many

commercial products because it communicates the taste that we know today as

umami). During transamination, an amine group (–nh ) is converted from an

2

amino acid to a keto acid, which is then chemically modified and transformed

into aromatic compounds.

In 1997, the biochemists at Jouy succeeded in purifying and characterizing

aminotransferase, the enzyme in the lactic bacterium
Lactococcus lactis,
which

is responsible for the transamination of leucine and methionine. Nonetheless,

under actual aging conditions, the presence of this enzyme does not signifi-

cantly affect the aromatic quality of cheeses. Why? Was the diffusion of reactive

agents in the lactic bacteria too slow? Did the lactic bacteria lack the molecules

necessary for them to receive the amine groups?

The i nra researchers tested the second hypothesis by seeding warm (pas-

teurized) milk with lactic bacteria and then adding rennet (which curdles the

milk and transforms it into cheese). In this way they obtained a curd that

they then molded and pressed and finally immersed in a brine enriched with

ketoglutarate. They followed the transformation of the amino acids during

the aging process, and a panel of tasters analyzed the development of the

cheese’s odor.

Fortified Cheeses
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In the control cheeses, which had not been enriched by ketoglutarate, very

few amino acids were dissociated, and the odor was weak. By contrast, the ad-

dition of ketoglutarate augmented the transformation of several amino acids.

The transformation of ketoglutarate led to the formation of powerfully aro-

matic compounds, such as isovalerate in the case of leucine and benzaldehyde

in the case of phenylalanine, demonstrating that the odor of the cheeses was

increased by the addition of ketoglutarate. Similar results were obtained for

Cheddars, tested for comparative purposes.

In Search of E‡icient Microorganisms

While they were studying the effects of adding ketoglutarate, the i nra

biochemists observed that the glutamate produced during transamination is

transformed by an enzyme produced by other bacteria in ketoglutarate. Be-

cause glutamate is abundant in milk, even before aging, they had the idea

of introducing the gene for dehydrogenase glutamate, the enzyme they had

discovered in a lactic bacterium, which they suspected would produce ketoglu-

tarate from glutamate.

The effects of introducing this gene were followed in vitro and in a control

cheese. The modified lactic bacteria were found to trigger the transamination

of the amino acids no less completely than lactic bacteria to which ketogluta-

rate had been added. What is more, lactic bacteria containing the dehydroge-

nase glutamate gene produced more highly aromatic carboxylic acids.

It is clear, then, that fortified bacteria can be used to make better cheeses.

The problem is that genetically modified organisms are not universally ac-

cepted by consumers. Therefore biochemists are searching for strains of lactic

bacteria that naturally produce dehydrogenase glutamate. In this case, at least,

genetically modified organisms will have served as a research tool. Is this not

one of their chief advantages?

318 | a c uisine f or t omor r ow

96

Chantilly Chocolate

How to make a chocolate mousse without eggs.

t h e w o r d s
c h a n t i l l y c r e a m
conjure up images of fresh strawberries,

ice cream, and airy desserts. Chantilly is a kind of foam, or mousse, made

by whipping cream in a chilled bowl. When the whisk is guided in a circular

motion, through a vertical plane, its wire loops steadily introduce air bubbles

in the cream that are stabilized by the molecules of the casein (a protein) and

by the crystallization of the fatty droplets. This crystallization takes place at a

low temperature, which is why the cream and the bowl must be chilled before-

hand. This cooling process also prevents the cream from turning into butter.

To obtain the best results, stop whipping the cream once strands begin to form

inside the loops of the whisk.

Can the fundamental principle of Chantilly cream be applied to fatty matter

other than milk? Because chocolate contains cocoa butter, for example, it ought

to be possible to make Chantilly chocolate.

A Chocolate Emulsion

Our chances of obtaining such a foam will increase if we begin by creating

a physicochemical system similar to cream but with a chocolate base. Physi-

cal chemists know that cream is an emulsion, a dispersion of fatty droplets in

water (in this case the water contained in milk, which also dissolves sugars,

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such as lactose, and mineral salts, but these ingredients, although they contrib-

ute to the taste of Chantilly cream, are unimportant for our purposes here).

The fatty droplets in an oil-in-water emulsion such as cream do not com-

bine with one another, for they are stabilized by casein micelles and calcium

phosphate. The casein molecules are bound together by the calcium phosphate

into tensioactive structures, or structures with a hydrophobic tail immersed in

the fatty droplet phase and a hydrophilic head immersed in the water phase.

The emulsion we need to make Chantilly chocolate can be formed in an

analogous manner by mixing together water, tensioactive molecules, and cocoa

butter. One simply pours a little water into a pan (which will be improved from

the gastronomic point of view if it is flavored with orange juice, for example,

or cassis purée) and adds some tensioactive molecules, either proteins from

the yolk or white of an egg or gelatin (often used to thicken butter and cream

sauces, which are also emulsions). One could rely simply on the lecithin al-

ready present in chocolate, but let’s use gelatin instead and dissolve it in the

water by heating. Then whisk in the chocolate. The result is a homogeneous

sauce—precisely the chocolate emulsion we were looking to create.

From Emulsion to Foam

With this emulsion we can make a foam. Put the pan in a bowl partly filled

with ice cubes to crystallize the chocolate around the air bubbles that we will

next introduce by whisking the chilled sauce, either manually or with an elec-

tric mixer. The procedure is then exactly the same one followed in the case of

Chantilly cream. Whisking creates large air bubbles in the sauce, which steadi-

ly thickens. Once the crystallization temperature is reached, the volume of the

sauce suddenly expands, and its color changes from dark to blond chestnut.

This lighter color results from the air bubbles, which can be seen under a

microscope. They also gradually change the texture of the sauce: After a while

strands of chocolate form inside the loops of the whisk, just as in the case of

Chantilly cream. In this way one obtains a foam that, unlike classic versions,

is unadulterated by crème fraîche or stiffly whipped egg white. It is a purely

chocolate mousse!

Want to try? Melt a half pound of chocolate with about 6 ounces of water.

Three things can go wrong. If your chocolate doesn’t contain enough fat, melt

320 | a c uisine f or t omor r ow

the mixture again, add some more chocolate, and then whisk it again. If the

mousse is not light enough, melt the mixture again, add some water, and

whisk it once more. If you whisk it too much, so that it becomes grainy, this

means that the foam has turned into an emulsion. In that case simply melt the

mixture and whisk it again, adding nothing.

Chantilly Chocolate
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97

Everything Chocolate

How to introduce chocolate into all kinds of pastry.

a t c h r i s t m a s a n d o n n e w y e a r’ s e v e, chocolate is obligatory. But in

what form? Chocolate puff pastry, perhaps? Chocoholics know that chocolate

contains cocoa butter, and they would like nothing better than to be able to

substitute it for ordinary butter in puff pastry. But they also know that the hard-

ness of chocolate stands in their way. A few simple observations about state

transitions will make it possible to solve this problem and to adapt the majority

of classic recipes for pastry to new uses.

To make puff pastry one first makes a paste by kneading flour with a little

water, sometimes butter. Next one rolls out the dough and places a layer of soft-

ened butter over it. The edges of the dough are then folded back over the butter

so that it is completely covered. This envelope is then folded and rolled out six

times, with the result that the dough that finally goes in the oven is composed

of hundreds of alternating layers of dough and butter.

How can we incorporate chocolate in the dough? Dark chocolate cannot

be used in place of butter, despite its cocoa butter content, because it is too

hard. Eighty percent of cocoa butter—the only fatty matter permitted by law in

France to be used in making chocolate, although other kinds have been pro-

posed—is composed of three triglycerides, or molecules made up of glycerol

(commonly called glycerine). These molecules are associated with three fatty

acids: palmitic acid, stearic acid, and oleic acid.

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Controlling Fusion

This composition explains the remarkable physical properties of cocoa but-

ter. If cocoa butter were composed of only one sort of molecule it would melt

at a fixed temperature, just as frozen water melts at 0°c (32°f) under normal