Molecular Gastronomy: Exploring the Science of Flavor (25 page)

The pink color of trout filets, which results from the presence of carotenoid

molecules (astaxanthin and canthaxanthin) contributed by food, seems not to

be a reliable indicator of quality. Despite some genetic variability in the fixing

of these pigments, the color varies mainly according to whether the diet of the

fish contains these carotenoids (which cause their flesh to turn from yellow to

pink).

Why, then, does the shade of color vary between fish born of the same

parents? The answer undoubtedly has to do with the fact that young fish ma-

ture at different rates. Indeed, studies have shown that their muscles gain in

lipid content in the course of growth: The bigger (and therefore more rapidly

developed) the fish, the fattier it is. Because the red component of their charac-

teristic color increases with lipid concentration, it also increases with the rate

of growth. This means that a trout that eats a great deal, which is to say one

that fattens rapidly, absorbs more pigments than a small fish. In other words,

the quality of smoked trout filets would be uniform if the rate of growth could

be controlled.

The physical characteristics and sensory qualities of fish depend on the

macroscopic organization of their muscle cells, which is very different from

that of meats. Muscle cells in animals consist of very long fibers sheathed in

collagen, a protective protein, and collected in bundles, which are themselves

sheathed in collagen, and so on. The cooking of meat therefore involves a deli-

cate compromise between hardening, which results from the coagulation of

the proteins contained in these cells, and tenderizing, a consequence of separa-

tion and dissociation of the collagen molecules.

Tender to Cook

In the case of fish, by contrast, the cooking time must be short because

their flesh contains little collagen. The muscles are not individually sheathed

but are grouped together in sheets of which only the surface is supported by

collagen. It is the lipids, localized within the muscle sheets in anatomically

distinct adipose tissues, that play a predominant role in holding the sheets to-

gether and therefore in determining the texture of the flesh. Tests of resistance

Trout
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to compression have shown that the flesh of trout is firmer than that of other

freshwater fish such as carp and catfish.

To understand how trout filets are modified during cooking, the Nantes

researchers began by comparing two filets obtained from the same fish: One

was characterized at once in its raw state, and the other was analyzed after

poaching, at temperatures ranging from 10°c (50°f) to 90°c (194°f), depend-

ing on the experiment (the filet was placed in a sealed bag and the temperature

increased by 1°c per minute, then rapidly lowered by immersion in ice water).

These studies showed that although cooking did little to change the chemi-

cal composition of the filets, it did increase their mechanical resistance as a

direct result of the temperatures to which they were heated, which caused the

muscle proteins to coagulate. The overall composition of the filets was largely

unaffected because their constituent elements were lost in the same propor-

tions in the cooking juices. Because the juices contained a substantial quantity

of lipids, the red color contributed by dissolved pigments diminished—all the

more so because the luminosity of trout flesh increases as a result of protein

coagulation. All told, the loss of matter in the form of liquid, amounting to

10–20% of the total mass, increased as the poaching temperature rose.

Research continues into the effects of cooking (one preliminary result is

that trout filets seem not to become softer the longer they are cooked) and the

role of diet, which seems to affect the chemical composition of the flesh with-

out much affecting its gustatory qualities. Why should trout not be like fowl,

whose taste varies greatly depending on their diet? We do not yet know.

166 | investigations a nd mod el s

47

Cooking Times

A brief guide to cooking meat so that it will be tender and juicy.

h o w l o n g s h o u l d a j o i n t o f b e e f b e c o o k e d ? The problem is

an old one, as we know from Brillat-Savarin’s
Physiology of Taste,
and raises the

question of what laws govern the various methods that have been devised for

cooking foods. Putting aside certain exotic cases, such as the preparation of

fish with acids in the Tahitian manner, in which filets are marinated in lime

juice, the cook should keep in mind that cooking is fundamentally a transfor-

mation of foods by heat.

Heating with Hot Gas

This naturally leads us to ask another question: How is heat most efficiently

transmitted for the purposes of cooking? Traditionally foods have been heated

by means of gases, liquids, solids, and waves. Let us limit our attention here

to the first of these four sets of procedures, which includes smoking, drying,

braising, steaming, and oven roasting.

In the case of both drying and smoking the cooking is slow because the

temperature of the hot fluid is not much above room temperature. Steaming

is more efficient because the food receives both the kinetic energy of the steam

and the energy resulting from the condensation of the steam on the food.

Nonetheless, the upper limit of temperature in this case is 100°c [212°f]. In

an oven, by contrast, the air—filled perhaps with water vapor—is able to reach

| 167

much higher temperatures, whence the problem of determining the right tem-

perature for cooking, which cooks have resolved empirically by formulas such

as “twelve minutes a pound, plus ten minutes for the pot.”

What is the basis for such rules? Notice first of all that the maximum thick-

ness of a food determines how much time is needed for it to reach a given

temperature throughout. A sausage that is a mile long takes the same time to

cook as one that is only a foot long as long as their diameters are equal. On

the other hand, a doubling of thickness implies a quadrupling of cooking time

because both the distance the heat must travel and the quantity of matter to be

heated are doubled. For a spherical body, the cooking time is proportionate to

the mass raised to the power of two-thirds, a relationship described by a curve

that flattens out after an initially rapid rise. This approximates the old rules,

which cannot be applied to foods smaller than a certain size.

Old-fashioned braising, which also relies on a hot gas, is a remarkable oper-

ation. Surface microorganisms are destroyed by browning over high heat, and

the meat is then placed in a covered baking dish (classically a brazier, nowadays

a casserole) and left to cook with “ashes above and ashes below,” as the best

authors used to say. Although it is less than 100°c (212°f), the temperature of

the heating fluid is nonetheless sufficient to evaporate the ethyl alcohol of the

brandy customarily used for braising and the various aromatic compounds of

the accompanying vegetables. The meat therefore cooks in a fragrant atmo-

sphere, without losing its water, because the ambient temperature remains

below the boiling point.

The Fateful Threshold

This technique is fashionable in many restaurants, where it is known as

vacuum-packed low-temperature cooking. Foods are sealed in a plastic pouch

that has been emptied of air and then poached at a temperature lower than

100°c (212°f). The cooking takes a long time, as in the case of the old braziers,

but it makes it possible to prepare dishes in advance. This advantage was so

familiar to chefs in earlier times that M. Menon specified no cooking times in

his
Science du Maître d’Hôtel Cuisinier
(1750). He knew that as long as foods are

cooked over gentle heat, the results do not greatly vary: Meats remain remark-

ably tender and juicy because their juices have not evaporated.

168 | investigations a nd mod el s

In earlier times braising was a difficult procedure to master (one had to be

careful to guard against the embers suddenly bursting into flame). Today it

yields remarkable results as long as one uses a preset oven and keeps in mind

a few key temperatures: At 40°c (104°f) meat becomes opaque because the

proteins in it, initially folded into a ball, begin to unfold before they coagulate

(thus becoming denatured); at 50°c (122°f) the muscle fibers begin to contract;

at 55°c (131°f) the fibrillar part of myosin (a protein that, along with actin, is

essential for muscle contraction) coagulates, and collagen (a protein that gives

meats their toughness) begins to dissolve; at 66°c (151°f) various other proteins

coagulate; at 70°c (158°f) myoglobin no longer fixes oxygen, causing the inside

of meat to turn pink; at 79°c (174°f) actin coagulates; at 80°c (176°f) the cell

walls are ruptured and the meat becomes gray; at 100°c (212°f) water evapo-

rates; and at temperatures higher than 150°c (302°f) so-called Maillard (and

other) reactions produce brown and flavorful results.

What is the point of referring to these benchmarks? If the oven’s tempera-

ture control is calibrated properly, cooks can choose the exact degree of done-

ness that they want to achieve, without having to depend on unreliable em-

pirical indications and without having to worry about the flare-ups that used

to ruin braised dishes in the past. Naturally, meat that is cooked rare has its

devotees, but they should not forget that cooking meat at low temperatures fa-

vors the proliferation of dangerous microorganisms. Low-temperature cooking

is perilous, but the results are wonderful.

Cooking Times
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48

The Flavor of Roasted Meats

The avor of roasted meats depends on their fat content.

d o f a t s i m p a r t a d i s t i n c t i v e f l a v o r to meats? If so, which one?

It was long believed that lipids were capable only of dissolving odorant com-

pounds, many of which are water insoluble. They have also been accused of

giving meat a bad taste, turning rancid, or oxidizing during cooking. Nonethe-

less cooks have long known that the flavor of meat is affected by the fats it

contains or the fats that are added to it during cooking. Today chemists can

confirm that fats play a decisive role in Maillard reactions, whose products are

the chief aromatic components of heated foods.

There are hundreds of odorant compounds, which vary according to the

type of meat, the age of the animal, its diet, and the mode of cooking. More-

over, compounds present in minute quantities may be aromatically preponder-

ant. One of the principal reactions responsible for generating tastes is the Mail-

lard reaction between sugars (such as glucose) and amino acids. Named after

Louis-Camille Maillard, a chemist in Nancy who first identified the reaction in

1912, it contributes to the flavor of bread crust as well as the roasted aroma of

meats, beer, and chocolate, among other foods. This reaction also leads to the

formation of the dark compounds called melanoidins, which give cooked foods

their characteristic color.

Chemists have been investigating the precursors of the volatile compounds

of meats for several decades. They first observed that these compounds have

a low molecular mass. In addition to the reactive agents typical of Maillard

170 |

transformations (amino acids and sugars), they found phosphate sugars, nu-

cleotides, peptides, glycopeptides, and organic acids.

The role of lipids, in particular, long resisted explanation. It was known that

phospholipids (fatty acids linked to a hydrosoluble group that are very sensitive

to oxidation) were responsible for the appearance of fatty and rancid notes,

but in 1983 Donald Mottram and his colleagues at the afrc Meat Research

Institute in Bristol, England (now the Institute for Food Research), were the

first to observe that they are also indispensable to the development of the char-

acteristic taste of cooked meat. In 1989, their colleague Linda Farmer showed

that lipids are involved in the unfolding of Maillard reactions, not only through

their degradation products but also on their own account, changing the odor-

ant profiles of roasted meats.

The first studies showed that extracting triglycerides (molecules consist-

ing of a glycerol molecule bound to three fatty acid molecules) from a meat

did little to change its odor after cooking, whereas eliminating phospholipids

replaced its characteristic aroma with one of roasted meat and biscuit. It is

thought that triglycerides are scant in polyunsaturated fatty acids, which gives

them a relative degree of chemical stability; many phospholipids are rich in

polyunsaturated fatty acids, however, which explains their sensitivity to oxida-