Molecular Gastronomy: Exploring the Science of Flavor (10 page)

their water content and the ease with which this water is released), and texture

of meats that were boiled after having first been marinated in these solutions

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were modified by five ingredients: alcohol, organic acids, glucose, amino acids,

and salt.

Decisive Marinades

In 1995 Okuda and Ueda extended this study by analyzing samples of beef

that were boiled after having been marinated in white wine, in red wine, and

in solutions containing only certain components of wine. The samples used

(cubes of meat weighing about 50 grams, or a bit less than 2 ounces) were

marinated for three days, then boiled for ten minutes or so. The outer part and

the inner part were analyzed separately. The water content and mass of the

samples marinated in red wine were slightly greater than those of the samples

marinated in white wine, but the masses of dry matter were about the same.

In other words, red wine marinades did a better job of preserving the tender-

ness of the meats.

Furthermore, the maximal resistance to internal compression was clearly

lower in the case of the samples marinated in red wine, which is to say that

red wine marinades also did a better job of tenderizing the meats. Finally, after

cooking, both the inner and outer parts of the samples marinated in red wine

were more tender than the ones marinated in white wine and also more tender

than samples cooked without having been marinated.

How are these advantages to be explained? Okuda and Ueda previously

showed that the effects of sugars, amino acids, and inorganic salts are weak

but that red wines contain more polyphenols than white wines because they

are generally more tannic and highly colored (tannins and anthocyanins, the

natural color pigments in wine, are polyphenols). Because polyphenols react

chemically with proteins, the two researchers tested their effect by marinating

the same meats in trial solutions containing fixed concentrations of tannic acid

(representing the polyphenols), organic acids, and ethanol.

Solutions composed of water, ethanol, and organic and tannic acids (such

as the ones found in red wine) modified the meat in the same way as red wine

alone, suggesting that red wine marinades act primarily through organic acids

and tannic acid; ethanol and, to a lesser degree, organic acids are important

during cooking.

The molecular details of these reactions are being studied, but it appears

that proteins react with the polyphenols found in red wines in such a way as to

54 | secrets of the kitchen

seal the juices of the meat by hardening, or caking, its surface. Classic recipes

therefore are justified in recommending that a combination of red wine and

acidic liquids, such as vinegar, be used for tenderizing meats. It now remains

to elucidate the mysteries associated with the roasting of marinated meats and

the proper use of parsley.

Wine and Marinades
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12

Color and Freshness

How to prevent discoloration in fruits and vegetables.

t h e v i b r a n t c o l o r s o f f r u i t s a n d v e g e t a b l e s are a sign of their

freshness. Alas, no sooner have avocados, salsifies, apples, pears, and mush-

rooms been sliced or chopped than they turn brown. Can this degradation be

avoided? Can fresh-squeezed apple juice make it from the kitchen to the table

without turning dark? Cooks have long recommended using lemon, whose

juice they believe prevents the appearance of colors associated with overripe,

damaged, or rotten organic matter. Is this recommendation sound?

Let’s put it to the test. If we compare avocado slices exposed to the oxygen

in the air with slices that have first been sprinkled with lemon juice, the dif-

ference is plain after a few hours. This confirms the wisdom of customary

culinary practice but does not explain why lemon juice has a protective effect.

If acidity were responsible, it ought to be possible to substitute vinegar. But this

is easily disproven by experiment.

And so? Lemons contain ascorbic acid, or vitamin C, an antioxidant com-

pound. Pure ascorbic acid of the kind one finds in tablet form at the pharmacy

ought to be more effective than lemon juice, and experiments show that this

is indeed the case. By investigating the role of oxygen in the darkening of

vegetables, modern food science has been able to add to the empirical list of

remedies that cooks have compiled, which includes not only the juice of certain

citrus fruits (lemons, oranges, limes) but also various salty brines.

56 |

Vitamin C Versus Enzymes

The darkening of vegetables is caused by enzymes called polyphenol oxi-

dases, which alter the structure of the polyphenol molecules of fruits and

vegetables. These molecules have a benzene center surrounded by six carbon

atoms at the apices of a hexagon, with either a hydrogen atom or a hydroxyl

(–oh) group associated with each carbon atom. In the presence of oxygen the

polyphenol oxidase enzymes replace the hydroxyl groups with oxygen atoms,

producing quinones whose reaction generates brown pigments of the same

family as melanin (the pigment that is formed in our skin when it tans under

the sun). This enzymatic darkening, thought to defend plants against ravaging

birds and insects, is observed in most fruits, leaves, and many mushrooms

that have been cut; it is not commonly found in uncut vegetables because the

enzymes and polyphenols are separated by membranes.

Various methods are used to prevent the darkening of vegetables and fruits

that have been sliced or chopped—often their lot in the kitchen. Freezing and

refrigeration slow but do not prevent the action of enzymes. Pasteurization,

a more radical procedure that inactivates the enzymes, cannot be applied to

all fruits and vegetables, for it often degrades their texture and color. Finally,

vacuum packing—sealing fruits and vegetables in containers from which the

oxygen has been drawn out—prevents the appearance of brown compounds;

alternatively, nitrogen and carbon dioxide atmospheres sometimes are used in

the food processing industry.

Inhibitors are also found in nature that, in minute proportions, work to pre-

vent enzymatic darkening. For example, a very weak dose of salicylhydroxamic

acid completely inhibits the formation of polyphenol oxidases in apples and

potatoes. Bentonite, a protein-absorbent clay, reduces the activity of enzymes

as well. Gelatin, activated charcoal, and polyvinyl pyrrolidone can also be used

also extract soluble phenols from wines and beers, but they modify the proper-

ties of these beverages.

Sulfites are used in the food processing industry to prevent darkening be-

cause they bond with quinones and form colorless sulfoquinones. Sulfur di-

oxide and sodium metabisulfite are commonly used by wine producers, for

example, but they too have secondary effects that worry health authorities (in

addition to migraine headaches caused by excessively sulfited wines, asthma

Color and Freshness
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attacks and outbreaks of hives, nausea, even anaphylactic shock have been re-

ported). Research into other equally effective but less harmful inhibitors there-

fore is needed.

Other inhibitors have been discovered or synthesized whose safety remains

to be proved. One that is now being studied is cysteine—an amino acid con-

taining sulfur—and its derivatives, as well as natural compounds (found in

honey, figs, and pineapple) and synthetic ones.

New Protective Agents

Jacques Nicolas, a researcher at the Conservatoire National des Arts et Mé-

tiers in Paris, is exploring the use of cyclodextrins. Working with researchers

at the Institut National de la Recherche Agronomique station at Montfavet,

Nicholas first analyzed the antidarkening effect of these molecules in trial so-

lutions containing one or two phenols and some polyphenol oxidases from

the Red Delicious apple. For the time being, until commercial applications of

these results are developed, if you want to make apple juice without resorting

to inert atmospheres you will need to clarify the juice. Polyphenol oxidases in

the cellular chloroplast of apples form solid fragments that darken, aggregate,

and fall to the bottom of the liquid as sediment. So let the juice rest awhile, and

then decant the clear amber liquid.

58 | secrets of the kitchen

13

Softening Lentils

The virtues of sodium bicarbonate.

c o n s i d e r t h i s p a s s a g e from an anonymous work published in 1838

under the title
Le cuisinier parisien:
“Beans, peas, and lentils, and many other

vegetables cook well only in very pure and light water; [water] from rivers and

streams is always the best; that from wells is worthless. In places where only

well water is to be had, it can be made suitable for cooking vegetables by adding

to it a little carbonate of soda, dissolved in water to the point that it no longer

whitens the water. It leaves a small deposit; one takes the clear liquid and uses

it to cook the vegetables.” Why should the quality of the water determine the

tenderness of vegetables? And why should carbonate of soda (or wood ash, also

recommended by the same author) be useful?

Basic Softening

Ash and bicarbonate have in common the property of making water basic,

or alkaline. To determine whether this alkalinization acts on vegetables, we

may begin by cooking lentils in three identical pans over the same heat. In the

first pan let’s use distilled water as the cooking medium; in the second, water

made basic by the addition of sodium bicarbonate; and in the third, water that

has been made acidic by adding a bit of vinegar.

The difference in tenderness between the three samples is so clear that no

laboratory instrument is needed to measure it: When the lentils in pure water

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are just cooked, the ones in acidified water are still as hard as pebbles, whereas

the others in water enriched by sodium bicarbonate are falling apart. Thus

the relative acidity of the cooking medium determines the rate at which the

vegetables are softened.

Why should this be? Vegetables are composed of cells held together by pa-

rietal tissue, which is composed of pectin and cellulose. To soften this tissue,

one must therefore modify its pectic “cement.” In an acid environment, pec-

tin molecules are neutralized: Their –coo– carboxylate groups capture the hy-

drogen atoms, yielding electrically neutral –cooh groups. Because the pectin

molecules are no longer subject to electrostatic repulsion, the lentils remain

hard. By contrast, sodium bicarbonate triggers the ionization of the –cooh

carboxylic acid groups into –coo– groups, with the result that electrostatic re-

pulsions between the pectin molecules cause them to separate, breaking down

the parietal walls and thus softening the lentils.

The Hardness of Water

Is the change in the acidity of the water used to cook the lentils the only

effect of sodium bicarbonate?
Le cuisinier parisien
indirectly suggests that the

hardness of the water, caused by the presence of calcium ions, plays a part as

well. To test the effects of these ions, let’s once again use two identical pans,

adding lentils and putting them over the same heat, then pour distilled water

into the first pan and water that has been artificially hardened by the addition

of calcium carbonate into the other one. After about 45 minutes the lentils

cooked in pure water are done, but the lentils cooked in the hard water are still

as hard as wood. This time, the effect results from the calcium ions, whose two

positive charges bind them with phytic acid molecules and pectin molecules,

reinforcing molecular cohesion rather than weakening it. Monovalent ions,

such as sodium, do not establish such bonds.

These phenomena are of particular interest to lentil producers, who are

looking for ways to make their products easier to prepare. Can lentils be pre-

cooked, for example? At the Institut National de la Recherche Agronomique sta-

tion in Montfavet, Patrick Varoquaux, Pierre Offant, and Françoise Varoquaux

have studied this question with a view to identifying the optimal conditions

for softening such seeds without releasing either the starch they contain or, by

60 | secrets of the kitchen

rupturing the molecular structure of the integument, cellular fragments. Steam

cooking would be a good way to achieve this result if it did not take so long.

The Montfavet researchers cooked lentils at different temperatures for dif-

ferent lengths of time and measured the firmness of the seeds. They observed

first that the firmness diminished with the length of cooking time, in inverse

ratio to temperature. This came as no surprise, but further quantitative analy-

sis proved to be instructive. The curves charting the firmness of the seeds