Molecular Gastronomy: Exploring the Science of Flavor (31 page)

trolled conditions therefore has only a limited effect on characteristics other

than color.

From Grass to Cheese
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60

The Tastes of Cheese

Lactic acid and mineral salts ˆve goat cheese its distinctive taste.

a r o m a s a r e t h e s t a r s o f t h e f o o d i n d u s t r y : Many firms produce

and sell them to the large food processing conglomerates that make yogurts,

soups, sauces, and so on. Nonetheless, foods that are aromatic and little else

please only the nose, for they are lacking in taste—hence the interest in taste

molecules, still poorly understood. Do these molecules exert the same effect in

foods as in water solution, where their properties have long been studied? At

the Institut National de la Recherche Agronomique Laboratoire de Recherches

sur les Arômes, in Dijon, Christian Salles, Erwan Engel, and Sophie Nicklaus

studied this question in connection with goat cheese.

As food is chewed, saliva conveys taste molecules to receptors in the papil-

lae. The Dijon physical chemists sought to analyze the behavior of these mole-

cules in water-soluble compounds, which is to say in the aqueous part of foods.

In the case of cheese this phase consists chiefly of lactose (a sugar), lactic acid

(formed from lactose by microorganisms in the course of fabrication), mineral

salts, amino acids, and peptides (short chains of amino acids). Although the

taste of most of these compounds is known, their effects in combination are

not. Some of the compounds in the aqueous part of the milk used to make

cheese mask the effect of other sapid molecules; others (known as enhancers)

augment their effect.

Salles and his colleagues first tested solutions containing only compounds

whose presence had been detected in the aqueous phase of goat cheese. Be-

206 |

cause peptides cannot easily be identified, the chemists isolated them from

the hydrosoluble part yielded by 20 kilograms (44 pounds) of cheese. After

centrifugation they separated out the juice by a series of ultrafiltrations using

membranes permeable by molecules of a mass lower than 10,000, then

lower than 1,000, and finally lower than 400. The unfiltered residue was

peptides.

All Except One

To evaluate the effect of the various compounds on each of the five basic

tastes, a jury of sixteen judges compared the reconstituted aqueous part to a

solution with the same components except for one or more compounds that

were deliberately omitted. These omission tests were performed under rig-

orous conditions, with anonymous products, individual booths, red light to

prevent bias due to color, and so on. Each taster was equipped with a nose clip

in order to eliminate the perception of odors. After training the tasters were

instructed to rank each of the five tastes by comparison with a specific refer-

ence solution for each sample presented.

Salts, Not Peptides

The first sensory evaluations came as a surprise. Although many studies

had offered glimpses of the sapid properties of peptides, suggesting that they

have a bitter taste, the peptides in goat cheeses turned out to have no dis-

cernible effect on taste, direct or indirect, regardless of their molecular mass.

Though not excluding the possibility that these compounds might one day be

shown to have a sapid effect in the case of other cheeses, the Dijon team dis-

counted an effect by peptides on the taste of goat cheese.

Comparing solutions containing lactose with ones from which it had been

removed, the researchers found that this compound had no effect on the sa-

pidity of the model solutions either. The amino acids likewise turned out to be

tasteless. However, lactic acid and mineral salts were found to powerfully con-

tribute to taste. The acidity of the cheese resulted principally from hydrogen

ions released by the phosphorus and lactic acids, an effect enhanced by sodium

chloride. In the presence of salt, then, the sour note is pronounced. Why? The

question has yet to be answered.

The Tastes of Cheese
| 207

The salt taste resulted from the effect of sodium, potassium, calcium, and

magnesium chlorides as well as of sodium phosphate. A part of the bitter

taste came from calcium and magnesium chlorides, although it was partially

masked by sodium chloride mixtures and by phosphates. As for the sweet and

umami tastes (the latter caused by monosodium glutamate, widely used in

commercial soups and sauces), they were so weak that the researchers were

unable to associate them with any of the hydrosoluble compounds tested.

An Overall Taste

The main conclusion to be drawn from these studies, apart from the de-

tailed information they yield regarding the various compounds contained in

the hydrosoluble part of the cheeses, is that no taste can be attributed to the

action of a single compound. Further complicating matters is the fact that the

different sapid compounds have both inhibiting and enhancing effects on one

another. On the other hand, we now know which compounds must be added

to cheeses in order to reinforce certain tastes or to mask others. Producers may

find it difficult to incorporate such compounds, however, not only for legal

reasons but also because a large proportion of the molecules dissolved in the

milk would be lost during drainage. Nonetheless, gourmets may now amuse

themselves by sprinkling their cheeses with various salts and raising toasts. To

your chlorides! To your phosphates! To your tartaric acid!

208 | investigations a nd mod el s

61

Yogurt

A smoother product can be obtained by modifying its milk composition and

fabrication process.

h o w d o e s o n e m a k e a g o o d y o g u r t ? The question is poorly posed,

for some like their yogurt runny and others like it firm. Ideally what one would

want to be able to do, then, is to balance the composition of milk and the meth-

od of fabrication in a way that will yield a specific texture and taste. Achieving

this objective will take some time, but already Anne Tomas and Denis Paquet

of the Danone Group, together with Jean-Louis Courthaudon and Denis Lori-

ent of the École Agro-Alimentaire in Dijon, have shown that the texture of

yogurt depends on the microstructure of the milk, which varies according to

the concentration of proteins and fats.

The best way to understand the difficulty of the problem will be to make

some yogurt ourselves. Put a tablespoon of commercial yogurt in some milk

and heat it gently for a few hours. The milk sets—or, as a physical chemist

would say, a gel has formed.

Milk is an emulsion, which is to say a dispersion of fat globules and aggre-

gates of casein (protein) molecules in water, in which various molecules such

as lactose are dissolved. When one adds yogurt to this emulsion, one is sowing

it with bacteria—
Lactobacillus bulgaricus
and
Streptococcus thermophilus
—that

transform the lactose into lactic acid. This process of fermentation acidifies

the liquid environment and triggers the aggregation of casein micelles in a

network that traps the water, fat globules, and microorganisms, which in the

meantime have proliferated.

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Textures to Order

Unless a great many precautions are taken, yogurt produced by this method

is disappointing. A bit of home experimentation will show why. Sowing two

identical samples of milk with different yogurts yields different textures and

tastes. Similarly, when the milk is curdled with the aid of glucono-delta-lactone,

a molecule that progressively acidifies the environment in which it is placed,

a still different result is obtained. What is more, causing the milk to curdle at

two different temperatures gives different results as well.

Entertaining though they may be, these experiments are not enough to sat-

isfy the needs of the food processing industry, which is obliged to make consis-

tently good products—hence the crucial question posed at the outset. For want

of an answer that would put an end to further research, the chemists at Danone

and in Dijon restricted their attention to the problem of composition: Given

that commercial producers make yogurt from milk that is fortified by the ad-

dition of powdered milk, condensed milk, and various milk constituents, how

does the composition of this milk determine its microstructure and therefore

that of the yogurt made from it?

Because of the variability of these products, the chemists examined emul-

sions of fixed composition that were prepared by processing a mixture of milk

fats and skimmed milk in a microfluidizer (which injects the mixture under

pressure into a clear small-diameter tube). Analysis of the light diffused by the

various emulsions indicated the size of the fatty droplets.

A Su‡iciency of Proteins

Contrary to what prior studies had suggested, the size of the fatty globules

did not change with the concentration of fat and protein; what had appeared to

be an increase in the size of the droplets, when the proportion of fat is raised,

turned out to be only an aggregation of globules of the same size. Naturally

the number of droplets grows when the concentration of fatty matter increases,

but the proteins are always sufficiently numerous to coat the fatty globules and

emulsify them.

Because proteins are not the only molecules that are tensioactive—that is,

capable of adhering to the surface of fatty droplets, so that one part is in con-

tact with the fat and the other with the water—the Danone and Dijon chemists

210 | investigations a nd mod el s

studied the changes produced by adding other kinds of tensioactive molecule

to emulsions that had already been formed and to mixtures that were subse-

quently emulsified with the aid of the microfluidizer.

It was expected that molecules with the greatest affinity for fat and water

would preferentially attach themselves to the surface of the fatty globules, but

experiments showed that this is not the case as long as the tensioactive mol-

ecules are put into the mixture before it is emulsified. When the tensioactive

molecules are added to an already constituted emulsion, the milk proteins that

coat the fatty droplets are not disturbed by these molecules, and their degree

of aggregation is unchanged. By contrast, when the tensioactive molecules are

introduced at the outset of emulsification, the distribution of the proteins is

altered and the degree of aggregation is reduced.

As a result of this research, the prospect of creating new kinds of diet

yogurt with the same smooth texture as high-fat ones no longer seems quite

so remote.

Yogurt
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62

Milk Solids

How to gelatinize milk without destabilizing it.

s l o w l y , o v e r c e n t u r i e s , c o o k s l e a r n e d to make solid foods from

liquid milk. Cheeses are milk “preserves,” made by destabilizing milk and

eliminating the water it contains in the form of whey. Yogurt is obtained by

heating milk fermented by the bacteria
Lactobacillus bulgaricus
and
Strepto-

coccus thermophilus
. These microorganisms transform the principal sugar in

milk, lactose, into lactic acid, which in turn acidifies its liquid environment and

causes a network to form throughout the liquid, creating a gel.

In recent years fermentation and curdling methods have been improved,

and the texture of yogurt is now known to be determined by the particular pro-

cedure used to solidify its milk constituents. This discovery has made it pos-

sible to create new milk products. With the aid of gelatinizing and thickening

compounds used to make sauces, for example, milk-based desserts have been

invented. But unexplained accidents have occurred: When gelatin is added to

hot milk, for example, the result often is a disagreeable lumpiness. Jean-Louis

Doublier, Sophie Bourriot, and Catherine Garnier, at the Institut National de

la Recherche Agronomique station in Nantes, have shown that excessive con-

centrations of thickening and gelatinizing agents of all kinds have the effect of

destabilizing milk.

At first sight this seems a surprising result, considering the varied char-

acter of these agents. Gelatin is an extract of animal bones, starches are pres-

ent in grains and tubers, carrageenans and alginates are derived from algae,

212 |

galactomannans (guar and carob gums) come from seeds, pectins come from

plants, and xanthan gum is obtained from fermented starch.

Destabilizing Sugars

The chemical analysis of these different compounds revealed common fea-

tures. With the exception of gelatin, all of them are polyosides, compounds of

the same chemical family as the sugars; the numerous hydroxyl (–oh) groups

found in these molecules are responsible for the thickening of solutions by