Molecular Gastronomy: Exploring the Science of Flavor (32 page)

bonding with water molecules.

But in milk polyosides interact with various dissolved proteins in addition

to the casein proteins, which are gathered in bundles called micelles. These

bundles are either dispersed in the milk or attached to the surface of the fatty

droplets. Why, then, should destabilization be observed when attractive forces

seemingly ought to bind the polyosides to the micelles and the fatty droplets?

Doublier and his colleagues explored this question with the aid of laser

scanning confocal microscopy, which reveals internal structures without any

need for a thinly sliced sample. Using both casein and polyoside labels, they

sought to identify areas that were rich in polyosides and proteins, even in mix-

tures where no destabilization was visible to the naked eye. They discovered

that high concentrations of each of the polyosides produce a phase separa-

tion: The polyosides come together in certain areas of the solution and the

casein proteins in others. In the case of some polyosides, however, this separa-

tion sometimes occurs at a very low concentration, although not immediately,

because the more viscous the environment the slower the phase separation.

Commercial producers who ignore this phenomenon therefore run the risk

that their products will separate between the time they are made and the time

they are consumed.

Instabilities Arising from a Tendency to Equilibrium

Two Japanese physical chemists, S. Asakura and F. Oozawa, have identi-

fied a mechanism known as depletion–flocculation, which occurs in particle

suspensions and seems to take place in milk. Particles are in equilibrium

when they repel one other (by electrostatic forces between the electrically

charged molecules on their surface) more than they attract. Nonetheless, this

Milk Solids
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equilibrium may be disturbed by the presence of a polymer so large that it can-

not insert itself between adjacent particles; the polymer concentration is said

to be null in this space, which is called a depletion zone. In solution, as a con-

sequence of molecular diffusion, the concentrations of each type of molecule

tend to become equal. Because the polymers cannot migrate to the depletion

zone, its concentration there is always null, with the result that water leaves

this zone in order to reduce the polymer concentration outside it. When the

water diffuses in this way the particles are moved closer together. In milk, the

casein particles thus form a flocculent mass, forming the dreaded lumps.

This very general phenomenon explains why one finds the same instabili-

ties in milk to which various polyosides have been added. The only way to avoid

them is to use as few polyosides as possible. Sailors are fond of saying that a

boat can never be too strong; the hull, the rigging, the mast, and the spars

should all be reinforced as far as possible in order to avoid rupture. When it

comes to food processing, however, this is worthless advice.

214 | investigations a nd mod el s

63

Sabayons

The foam of a sabayon is stabilized by the coagulation of the egg.

t r y t o i m a g i n e w h a t t h e p e r f e c t, ideal, Platonic sabayon would

be like. A sabayon (the name is derived from the Italian
zabaglione
) is made

by mixing egg yolks and sugar, beating them with a whisk, and adding sweet

wine. Heating the mixture while continuing to whisk it, one observes that it be-

comes foamy, forming a sauce that is served with thinly sliced fruits or sipped

as a digestive. The danger, then, is that for one reason or another the prepara-

tion won’t foam up. Can chemistry and physics be harnessed to guarantee a

successful result?

There are many published recipes. Marcelle Auclan, in
La Cuisine
(1951),

gives a basic proportion of “1 egg yolk, 1/2 egg shell filled with sugar, two 1/2

egg shells of wine,” assuming two yolks per person: “Mix the ingredients to-

gether over a very low flame in an enameled saucepan. Stir, stir some more—

and keep on stirring. This does not take very long, nor is it at all difficult, but

you need to pay attention, and to rely on some sixth sense to know when the

sabayon is beginning to set and to become smooth. From the moment—stir-

ring all the while—that you sense a greater density in the liquid, open your

eyes and watch very carefully, for it takes only a second for it suddenly to be-

come creamy, all by itself, without having come to a boil.”

Mystery lingers, alchemy hovers, and failure threatens. If we use Auclan’s

proportions as a basis for experiment, translating them first into precise values

and then slightly altering them, we can try combining four egg yolks and 200

| 215

grams (about 7 ounces) of sugar to make two sabayons. Let’s put 30 centiliters

(about 10 ounces) of water in one of them but only 10 centiliters (a bit more

than 3 ounces) in the other. We will observe that the first one foams, whereas

the second one does not.

Water Needed

Why is this so? Keep in mind that egg yolks can be used to thicken sauces

and to harden the walls that separate the air bubbles of a foam—as long as the

foam forms in the first place. In order for these walls to appear, there must be

a sufficient quantity of water. Our second sabayon seems not to have foamed

because it didn’t have enough water. How can we be sure of this? If we add

20 centiliters (not quite 7 ounces) of water and cook the sabayon a bit longer,

whisking as before, we will find that it expands. This is proof that there was

not enough water to begin with, but it also provides the cook with a valuable

tip: If your sabayon doesn’t foam, you can fix the problem by increasing the

quantity of liquid.

What role does sugar play in all of this? In addition to contributing to the

taste of the dish, its sucrose molecules, which are highly hygroscopic, bind

with the water molecules, with the result that the water should no longer be

able to form the walls on which a foam’s bubbles depend. To test this predic-

tion, compare the first sabayon with two others prepared using four egg yolks,

only 100 grams (3 1/2 ounces) of sugar, and 10 and 30 centiliters (roughly 3 and

10 ounces) of water, respectively. The first one rises, but less than the preceding

ones, because it contains a lower proportion of water to egg yolk. The second

one rises as well, but it reliquifies because it contains too little egg yolk.

Air Bubbles, Not Steam

Let us turn now from composition to cooking. How should a sabayon be

heated? Observation, unsupported by theory, seems to show that sabayons ex-

pand properly only if the heat is progressively increased. Some chefs believe

that steam forms at the bottom of the pan and is trapped there by the coagula-

tion of the egg. If so, one ought to be able to record a temperature of 100°c

(212°f) at the bottom of the pan when the sabayon begins to foam. However,

216 | investigations a nd mod el s

other chefs believe that a sabayon has to be brought to a temperature of only

80°c (176°f) in order to foam. Which assumption is correct?

Let’s try to reason our way to an answer. The point of using a whisk to

prepare a sabayon, obviously, is to introduce air into the sauce. And the only

physicochemical transformation that occurs during cooking is the coagulation

of the egg yolks, which begins at 68°c (154°f). Therefore one would expect

that there is no need to heat the sabayon to the boiling point, or even to 80°c

(176°f), in order for it to foam (the white of an egg foams at room tempera-

ture) and that a temperature of 68°c (154°f) is enough to stabilize the foam by

rigidifying the walls that separate the air bubbles through the coagulation of

the proteins in the egg yolk.

Experiment confirms these expectations. The question remains which of

these three cooking temperatures is to be preferred. Leading pastry chefs agree

that a perfect sabayon should not have the taste of cooked egg. For this reason

a sabayon cooked at the lowest temperature would be the best. True, it will

take a bit longer to make, but what gourmet would not be willing to wait for a

Platonic sabayon?

Sabayons
| 217

64

Fruits in Syrup

How to optimize the sugar concentration of syrups for preserving fruits.

a u t umn d r a w s n e a r , and with the coming of cold weather summer’s

fruits will soon vanish. How can they best be preserved to last through the

winter? Several methods are common, especially canning and freezing. In

each case one seeks to prevent the proliferation of germs, which at room tem-

perature occurs rapidly in foods that contain sufficient quantities of water and

organic matter to favor the development of microorganisms.

Anyone who has tried to preserve fruit in syrup is familiar with the need

to steer a safe course between the Charybdis of fruit that swells to the point

of falling apart and the Scylla of fruit that shrivels up. The observant cook will

suspect that the first danger is encountered when the syrup contains too little

sugar, and the second presents itself when the syrup has too much sugar. What

does one need to know to get the amount of sugar right?

Experience and Science

Answering this question involves analyzing a phenomenon that occurs dur-

ing cooking. Cookbooks generally recommend using fruit that has not com-

pletely ripened. Once the pieces have been carefully pierced (use a needle or

the tip of a sharp knife), they are to be put in a “20° syrup” and heated in

glass jars packed tightly together with cloth and covered by boiling water. The

218 |

cooking time is said to depend on the size of the fruit: only two minutes for

currants, for example, but five for apricots.

The reason for doing this is to destroy any microorganisms that are pres-

ent by heating, a technique that has been widely used since the earliest days

of canning. But cooks today deserve more than simply being told to follow

the instructions of an old recipe. They ought to be told, in particular, that the

degree to which microorganisms are eliminated is proportional to time and

temperature. In this case, with the temperature fixed, time is what determines

the degree of sterilization. Classic recipes therefore are mistaken in prescrib-

ing different times depending on the size of the fruits because all types of

fruit must be cooked for the same length of time to obtain the same degree

of sterilization. But this means that the smallest fruits are in danger of being

overcooked, which will not come as welcome news to gourmets.

On the other hand, it is experience rather than science that explains the use

of cloth in packing the glass jars together. When the jars are heated in a pre-

serving pan, the boiling of the water violently shakes the jars, which are likely

to shatter if they are not protected.

The Strength of Syrups

Now we come to the problem of the syrup. Recipes often call for a syrup

of 20°, without further explanation. The degrees in this case obviously are not

degrees Celsius or Fahrenheit, but one has to know something about the art

of confectionery to realize that there is a possible confusion between degrees

Baumé (equal to 145 – 145/S, where S is the specific mass of the sugar) and de-

grees Brix (or Balling), which represent the proportion of sugar to mass. Why

don’t cookbook authors simply speak of the quantity of sugar to be dissolved

per liter of water? In the case of fruits in syrup, as we will see, the cook can

safely ignore such complications.

The main problem in preserving fruits in syrup, assuming the sterilization

process has been carried out properly, is that they swell up in syrups that are

too light and shrivel up in syrups that are too concentrated. What accounts for

these phenomena? Fruits are modified by osmosis: Preserved for a sufficiently

long time in syrup, they evolve toward equilibrium, which is to say a balance

between the concentrations of water in the fruit and in the syrup.

Fruits in Syrup
| 219

We can verify this by looking in a microscope: A plant cell in the presence of

a sugar crystal is emptied of its water, which diffuses through the cell wall and

membrane, equalizing the concentrations of water inside and outside the cell.

Given that a fruit is itself an assembly of cells, the same phenomenon occurs

on a macroscopic scale in canning.

If the syrup is light, the fruit—having a lower concentration of water—ab-

sorbs the liquid outside it, expands, and finally bursts. Conversely, a syrup

that is too concentrated draws out the water inside the cells. One observes the

same phenomenon when gherkins and eggplants “sweat” in the presence of

salt, for example.

Nonetheless, this explanation does not tell us how to gauge the right

strength for syrup. Let’s prepare syrups using 10 grams of sugar per liter, 20