Kitchen Mysteries_ Revealing the Science of Cooking Part 7

The Seasoning Finally, a word regarding the use of this water-in-oil (or oil-in-water) emulsion for salads.

Oil adheres better to the surface of vegetables than water does, but both substances do harm to the color. They penetrate the surface thanks to fissures in the waxy cuticle that coats vegetable leaves, like salad greens, and they drive out the air that, by refracting the light, gives the leaves their beautiful green color.

Whether your vinaigrette is an oil-in-water emulsion or a water-in-oil emulsion, toss it with your greens only at the last minute if you want to serve a very green salad.

Yogurt and Cheese Acid and Rennet I warned you in my introduction, my dear guest of gastronomical literature, that I would only lead you where your own cooking resources would suffice. So do not take me to task for offering too little advice when it comes to cheeses. These appear at the table in just the state in which they were acquired. At best, through a little elementary care, you might continue the maturing process accomplished by your cheese maker.

Nevertheless, the good gourmand will be tempted to make cheese and will need some information. It is important to know that cheese is obtained through the coagulation of milk. I will only add that milk coagulates because the micelles of casein (the protein that represents 85 percent of the proteins in milk, which const.i.tute 4 percent of the total milk) aggregate when conditions lend themselves to it.

In addition, the gourmand should know that the fatty substances in milk are formed during the lactation of the cow, sheep, or goat. The mammary cells have a surface that forms a protuberance from which are released fat globules about 2.5 micrometers in diameter. The interior contains fats but also vitamin A and cholesterol. The outer membrane contains surface-active molecules, which ensure the fat emulsification.

When milk is made to curdle, either by adding rennet (extracted from the fourth stomach of calves), salt, or acid, the casein coagulates, while the curds retain some of the fats and a few proteins in solution. The effect is the same as the one that allows us to fry an egg sunny-side up successfully (see page 46). In the presence of ions provided by the acid or salt, the casein molecules no longer exert the electrical forces on one another that are responsible for their repulsion, and the casein micelles aggregate.

Have you already done the experiment of salting or acidifying hot milk?

Careful Attention Coagulating the milk is only the first step in making cheese. It is followed by a draining process, a salting process, and then a maturing process, which takes place with the help of selected microorganisms.

During the draining process, manufacturers use lactic bacteria with the rennet, which acidify the environment (by releasing lactic acid). The salting takes place through immersion in a brine. Then comes a sprinkling of microorganisms to start the maturing process, which gives each cheese its unique character.

Why Does Cheese Smell?

Cheese smells because a considerable share of the fatty acids is in a free form (that is, not incorporated in the triacylglycerols) because of the lipase enzymes in the microorganisms used for the maturing process. The Penicillium camemberti Penicillium camemberti microorganism, for example, has an important function in attacking and transforming the fats. Localized especially near the rind, the bacteria hydrolyzes the triacylglycerols, weakens the periphery of the cheese (in well-ripened camembert), and releases the gas we call ammonia. It is this smell that repulses some, depriving them of the immense pleasure of tasting, with well-ripened Camembert cheeses, the flavor of France. microorganism, for example, has an important function in attacking and transforming the fats. Localized especially near the rind, the bacteria hydrolyzes the triacylglycerols, weakens the periphery of the cheese (in well-ripened camembert), and releases the gas we call ammonia. It is this smell that repulses some, depriving them of the immense pleasure of tasting, with well-ripened Camembert cheeses, the flavor of France.

The presence of ammonia around cheese in the process of maturing seems to contribute to its favorable evolution. A word of advice, if you have acquired some insufficiently aged Camembert, put it in a tightly closed bag in a high place in your kitchen.

Preparing Yogurt How does yogurt form? The recipe is simple: place a spoonful of yogurt in a pot full of warm milk and heat slowly for a long time (many hours)-for example, in a double boiler or in the oven. The milk forms into a ma.s.s. It is a kind of multiplication of the little yogurts.

The principle molecule in this process is lactic acid, which can be considered as half a glucose molecule, the fuel for our bodies. Lactic acid forms through the fermentation of glucose and other sugars in the absence of oxygen.

Milk, which contains sugars, is rapidly colonized by the bacteria that act on the milk sugar, lactose, and break it down, releasing lactic acid. The lactic acid coagulates the milk according to the same phenomenon used in making cheese. The coagulation of casein produces a gel that traps the water and fat droplets.

Fruits of the Harvest Why Do Apples Turn Brown When They Are Cut?

When an apple is cut or peeled, its surface, which is initially white, turns brown within minutes. Apricots, pears, cherries, and peaches do not brown, but, even worse, they blacken! Bananas and potatoes turn pink before turning brown. Lemons and oranges, on the other hand, do not brown. Does their natural acidity protect them?

Absolutely. If certain fruits brown when they are cut, it is because the knife damages some of their cells, releasing their contents and especially some enzymes that were enclosed in special compartments.

More precisely, the enzymes, called polyphenolases, oxidize the colorless polyphenol molecules of the fruits into orthoquinone compounds, which are rearranged, undergoing oxidation and polymerizing into colored molecules that are cousins of melanin (melanin is the molecule that makes us turn a beautiful bronze color when we have been exposed to the sun).

Acidity slows these reactions, because it limits the action of the enzymes. In addition, the as...o...b..c acid in lemons and other fruits of the same family (oranges, grapefruits, etc.) is an antioxidant. These are two reasons it is a good idea to squeeze lemon juice over cut fruit if you want to retain its original color. You can also use pure as...o...b..c acid (vitamin C) from the drugstore if you want to avoid the taste of lemon.

How Much Sugar for the Syrup for Preserved Fruit?

Those who understand the physical phenomenon of osmosis, already discussed with regard to braising, can succeed in preserving fruit in syrup. The word "osmosis" is a mouthful, but the phenomenon is simple. In a liquid, a drop of ink disperses gradually so as to occupy all the liquid; its concentration is equalized. There is nothing mysterious about that. The molecules in a liquid move incessantly, so that, through chance collisions, the molecules of the drop are distributed throughout the liquid.

In fruits preserved in syrup, the identical phenomenon is at work. When fruit is cooked in plain water, the sugar from the fruit will tend to pa.s.s into the water in order to equalize the concentration of sugar, and the water in the external environment will pa.s.s into the fruit cells to dilute the sugar found there. As sugars are large molecules, only the water moves, so the fruit swells with water and then explodes.

On the other hand, if fruit is cooked in a solution in which the concentration of sugar is higher than the concentration found in the fruit, the water in the fruit tends to be released from the plant cells in order to lower the concentration of sugar in the solution. The fruit shrivels. Consequently, cooking the fruit in a sugar solution equal in concentration to the fruit's will best preserve the fruit's natural appearance.

The same phenomenon takes place in preparing marrons glaces. First, the chestnuts are cooked for a long time in water in order to soften them thoroughly. Then they are peeled, and, when they have cooled, slowly (so as not to break them) immersed in increasingly concentrated syrup (flavored with vanilla). Gradually, the sugar saturates the chestnuts.

Ices and Sorbets How Must Ices Be Agitated?

The scourge of ices and sorbets is the ice crystal. When it seems to be absent, the dessert is delicious, velvety smooth, and melting in the mouth. But when it is present, a horrible sensation of broken gla.s.s in the mouth ruins the pleasure that the dish's thousand s.h.i.+mmering reflections had promised.

Physicists and especially crystallographers are well acquainted with crystals. They know that in order to obtain large ones, the parent solution must not be moved and as slow a growth as possible must be encouraged.

The cook, who desires the opposite effect, must thus agitate such solutions as much as possible in order to prevent large ice crystals from forming. At the same time, the cook wants to introduce air bubbles in order to obtain a light consistency.

How and when should this be done? At the beginning, agitation serves no purpose: the preparation must first cool. As long as the temperature is above 0C (32F), no ice crystals can form. Furthermore, introducing air bubbles at this stage is useless because the preparation is still too liquid to retain them. And, finally, the cream that is present in the preparation is in danger of turning to b.u.t.ter if it is agitated too much.

But once you round the Cape of Zero, heave-ho!

Must the Preparation Be Placed in the Freezer Hot or Cold?

What a question! Common sense tells us that the freezer will work more efficiently if the preparation is already cold. Common sense is not always right, however. Hot water freezes more quickly than cold water.

This effect was studied by Ernesto Mpemba in Tanzania, who prepared a recipe that called for heating the milk, incorporating the sugar, letting the mixture cool to room temperature, and freezing it. One day, he forgot to let the mixture cool and discovered that his preparation froze much more quickly than when he faithfully followed the recipe. He published the results of his subsequent studies in the Journal for Physical Education Journal for Physical Education.

Why does hot water freeze more quickly than cold water? It has been claimed that it warms the receptacle in which it is deposited, which melts the ice, and thus a better thermal contact is then established with the freezer. This explanation is insufficient, because the effect occurs even if the receptacle is insulated with little wooden chocks or polystyrene foam, for example.

In fact, three different effects seem to come into play. First, convection: that is, the movement of liquid when its temperature at the top is not the same as at the bottom; the difference in density produces flows that h.o.m.ogenize the solution. Second, cold water dissolves more gas than hot water; with the gases removed, hot water cools more quickly. Third, a hot solution loses water through evaporation, so that, in the end, there is less water to cool.

Instant Ice Cream In 1901, at the Royal Inst.i.tution of London, Agnes B. Marshall invented an ideal method for preparing ice cream or sorbet. It is ideal because, using her process, the ice crystals are tiny, as desired, and the preparation is extremely light because of the countless air bubbles introduced into it. And last but not least, the preparation can be made at the table, before your guests, in a few seconds. What is this marvelous contribution to gastronomy?

Agnes Marshall proposed abandoning the cla.s.sic, old-fas.h.i.+oned ice cream maker for liquid air, or, more precisely, liquid nitrogen. This transparent liquid, present in all chemistry and physics laboratories, is nothing other than nitrogen from air that has been cooled to -196C (-320F). I do not have to tell you that that is very cold.

When it is (slowly) poured into a preparation for ice cream or sorbet, it vaporizes immediately, absorbing the preparation's heat and instantly freezing it. Penetrated by the cold, the preparation becomes filled with tiny ice crystals, while the liquid air pa.s.ses into a gaseous state; the air bubbles are trapped in the ice cream or sorbet.

The whole thing takes place in an impressive cloud of white mist, the same kind that is used in shooting films when the director asks for fog. A guaranteed success!46

Cakes LIGHT AND MELTING.

A Base Both Robust and Light To become a good cook, Escoffier said, you must first try your hand at pastries, because that is the best school for learning correct proportions. Let us add that pastries are also a wonderful domain for the physical chemist ... and for the gourmand. Isn't that where we find whipped cream, mousses, candied fruit, and a thousand other preparations that science can help us make successfully without mistakes?

Many cakes begin with a solid base that supports the rest of the creation. How to obtain one that is airy and melting? Spongy or foamy textures are essential. The walls of the bubbles or cells, like the walls of a honeycomb, provide a kind of strength that modern engineers have learned to use in their work. A structure full of bubbles retains a tenderness that harmonizes with the cream, often whipped, that it supports.

Nevertheless, the foam of beaten egg white is too fragile to support a whole cake, and the foam in a souffle has the disadvantage of collapsing after being cooked. What to do? It must be reinforced.

Let us examine two types of reinforcement a.n.a.lyzed by Peter Barham, my friend from Bristol, whom I mentioned earlier. The first, used in meringues, rigidifies the walls of the bubbles in the foam. The second, borrowed from the building industry, adds a load (an edible one, of course): flour or sugar.

Meringue Foam When plain water is beaten, a few bubbles form and then collapse. On the contrary, when an egg white (which contains 90 percent water) is beaten, an excellent foam is obtained, stable for many hours. The reason? "Surfactants" is the key word here, as we have seen many times.

Egg white foam, as we have already seen with regard to souffles (see pages 50-51), is formed by trapping air bubbles in a liquid. In plain water, air bubbles rise to the surface; beating egg whites, however, produces the forces that will stabilize them. We benefit from the presence of surface-active molecules in the egg, that is, molecules having a hydrophilic part (which binds to the water molecules) and a hydrophobic part (which resists being in the water and thus positions itself instead in the air).

Small bubbles are more sensitive to the surface forces and to the forces provided by the surface-active molecules. They will thus form a more stable foam and better withstand any pressure applied to them because this pressure will be distributed over a greater number of bubbles.

To make a meringue, powdered sugar, which dissolves easily in the bubble walls, must be added. This is the cla.s.sic recipe: two tablespoons of sugar per egg white for a soft meringue and four tablespoons for a firm meringue. The mixture is dried in a warm oven, and when some of the water has disappeared, only the rigid structure remains, composed of surface-active molecules, sugar, and water, which is bound to this structure and will evaporate only after prolonged heating, which, naturally, must be avoided.

In the oven, the heat expands the bubbles and vaporizes the water, which makes the meringue swell. Simultaneously, the heat from the oven coagulates the various egg white proteins, which permanently rigidifies the formed bubbles. Ideal baking will create a firm crust with a soft, supple interior. Bake for forty minutes at 120C (248F) first and then for two hours more at 100C (212F) for meringues with a soft inside, or one hour more at 110C (230F) for harder meringues.

When Should the Sugar Be Added?

When should the sugar be added to the egg whites when preparing a meringue? Before beating them or after?

All cooks are sure about this: the sugar must only be added when firm peaks have already formed. Why? Because it dehydrates the proteins, especially if it is very fine and, as in the form of a sugar glaze, very well dispersed. The physical phenomenon is, again, a matter of diffusion. If sugar, which contains no water, comes into contact with proteins, with properties dependent on their bonds with water, the water tends to leave the proteins to dissolve the sugar. If the sugar is added too early, the foam cannot form, and the whites whip up poorly.

Opening the Oven Door For a meringue as for a souffle, do not open the oven door while it is baking. The bubbles of air and vapor, which swell as they heat, are in danger of rapidly deflating. If this happens, as the meringue continues to bake, the egg whites solidify before they have time to reinflate. Instead, use an oven with a gla.s.s door and prepare to be patient.

The Scylla and Charybdis of Meringues: Overbeating and Egg Yolk Be careful not to beat too much. If you denature the proteins too rapidly, air will be introduced in insufficient quant.i.ties at the time when the bonds between the proteins should be established. And if you continue beating after these bonds are established, the number of bonds between the molecules will continue to increase in the foam, which will expel the water that is normally bonded to the molecules. You will see it beading on the surface.

Now that you know the Scylla of meringue, let us examine the Charybdis. It is frequently said, mistakenly, that it is impossible to obtain a good foam if you beat the whites with even a trace of yolk in them. It is true that it is much more difficult to obtain a stable foam when yolk is present, but it is not impossible. We know that the cholesterol in egg yolk is a molecule that tends to bond to the hydrophobic groups of the denatured proteins and thus prevent those groups from partic.i.p.ating in the formation of the foam. Because of this, when yolk is present in the whites, a greater quant.i.ty of proteins must be denatured to bond with the cholesterol, and beating takes much longer than with egg whites alone. Furthermore, the speed of beating required to denature a molecule increases as the viscosity diminishes (stress is proportional to viscosity); thus adding a pinch of sugar or salt, as recommended by cooks, makes the beating easier by increasing the viscosity.

A Soft Base While some cakes contain meringue, others have a spongy base. The principle is a.n.a.logous to that of meringues, but instead of baking stiffly beaten egg whites mixed with sugar in order to obtain a rigid surface structure, a softer texture is retained by adding a load: flour.

As with the sugar in meringue, the flour is only added when the whites are very firm; otherwise, the fine starch particles in the flour will capture the air bubbles and make the foam collapse. Thus the preparation is mixed like a souffle, by folding the foam over the flour with the help of a spatula that is manipulated as though one were cutting a tart. As soon as the color is uniform, mixing stops.

Likewise, a fatty substance is added, generally melted b.u.t.ter, to provide a silky texture and slow down the recrystallization of the starch. As before, the foam is folded over the warm, melted b.u.t.ter, because the fat molecules in the b.u.t.ter tend to bond to the hydrophobic groups of proteins and make the foam collapse.

Finally, to prevent the foam prepared in this way from collapsing, it is baked. This increases the denaturation of the proteins and leads to the formation of permanent intermolecular bonds, transforming the semiliquid mixture into a rigid sponge.

In the oven, many simultaneous reactions harden the interface of the bubbles and thus allow them to withstand the pressure caused by the expansion of the air and the formation of vapor. In practice, the speed of hardening and the formation of vapor are not equal, which means that the bubbles grow and the volume of the cake increases by 10 percent. A good way to test if a cake is baked is to insert a knife in the foam and see if it sticks to it. If the foam sticks to the knife, the cake has not finished baking.

When the cake is taken out of the oven, it cools, the gas in the bubbles contracts, and the vapor condenses, which reduces the internal pressure and causes the cake to collapse if it is not completely baked.

It is quite easy to avoid this inconvenience. You can deliberately make some of the bubbles burst by dropping the cake, in its pan, from a height of about ten centimeters (four inches). The result will be less beautiful than what comes out of the oven, but you will not have the displeasure of watching the base collapse unevenly under the weight of your various garnishes.

Whipped Cream Finally, the ultimate step remains: preparing the garnishes, often composed of a mixture of whipped cream and dark or red fruits. Whipped cream is a type of foam, once again, but its stabilization is the result of a different effect. Actually, milk and natural cream are composed of small globules of fat, and their suspension in water is stabilized by surface-active molecules, such as casein.

In milk, the proportion of fatty substances is about 7 percent. In thin cream, it reaches 18 percent, and in thick cream, it rises to 47 percent. In b.u.t.ter, on the other hand, the proportion of fatty substances is 83 percent, but the emulsion is reversed. Here droplets of water are dispersed in the fat.

In whipped cream, we are looking for a dispersion of air and water in the fatty substance. This inversion of the emulsion that const.i.tutes cream is obtained by beating it.

The same stabilization phenomena are at work in whipped cream as in stiffly beaten egg whites. Viscosity is important for stabilizing the foam. Thus, as thick a cream as possible should be used, and the ingredients should be refrigerated before beating them to increase their viscosity further. To avoid the formation of b.u.t.ter, beating can be done in a bowl that is sitting on crushed ice.

When the cream is stiff enough to support its own weight and also that of the garnishes, that is, when the fat globules have been divided enough to coat the air bubbles and stabilize them, alternate layers of sponge cake, meringue, fruit, and whipped cream will be piled on top of one another. A drop of alcohol will charm adults and give children a taste of good things to come. And a dusting of chocolate, for example, will add the finis.h.i.+ng touch to your creation.

Pastry Dough TART, SHORTBREAD, AND PUFF PASTRY.

Why Must Dough Be Allowed to Rest Before Baking It?

Everyone knows that pastry doughs are basically flour, water, and b.u.t.ter. Nevertheless, tart dough is nothing at all like puff pastry, which differs considerably from sweet shortbread dough. Why do the same ingredients produce such different results? Because the hand of the pastry cook comes into play. Let us examine how.

The simplest dough is prepared with just flour and water, mixed in proportions that yield a substance with the consistency of thick putty that does not stick to the fingers. In the making of this dough, water is introduced between the countless starch granules in the flour, and it binds them into a coherent ma.s.s, linking to proteins (we shall later explore "gluten"; please be patient). In effect, as soon as water comes in contact with flour, it penetrates between the granules through capillarity.47 If one rolls out this dough into a round two or three millimeters thick (about one-sixteenth of an inch), one will have an unleavened flat bread. If one rolls out this dough into a round two or three millimeters thick (about one-sixteenth of an inch), one will have an unleavened flat bread.

While it bakes, the temperature of the water increases, and the granules inflate and form starch that gradually binds together as the water evaporates. A single ma.s.s of hard dough is formed.

Pastry dough differs from this because it contains b.u.t.ter (or margarine, or some other fatty substance) that separates the particles of flour from one another and thus helps the dough retain a certain suppleness. As before, mixing the flour and the water forms a starch, but the fat separates the individual starches. After baking, the dough is still crumbly because the starch granules have remained fairly separate. The cohesion is the result of the b.u.t.ter, which, in cooling, forms a sort of smooth cement. The proof of this? Warm pastry dough is more crumbly than the same pastry dough when cool. Thus it is better to take cakes and tarts with a pastry crust out of their pans only after they have cooled.

The preceding examination of these two simple doughs teaches us an important culinary lesson. Since the goal is to obtain the jellification of starch, the preparation of the dough must not be rushed. The water must have time to migrate between the granules and then to penetrate them in order to make them swell. That is also why recipes tell us to let the dough rest before baking it.

Limited Kneading Good cookbooks warn against kneading pastry dough too much. The reason behind this advice? Because flour is not composed only of starch. It also contains proteins, some of which form what is called gluten.

The term "gluten" was used as early as 1742 by the Italian chemist Giacomo Bartolomeo Beccari (1682-1766). Wanting to investigate the composition of flour, he kneaded flour with water to make a dough and then kneaded that dough in water. The white powder that escaped with the water was starch; the elastic yellow ma.s.s that remained was gluten, made of some of the proteins of flour.

Gluten proteins are thirsty for water and in its presence form a very tough, though elastic, network. Prolonged kneading, which makes the gluten proteins coagulate, produces a very tough dough.

It is worth noting that when the dough is being used to make brioche, it is better first to mix the b.u.t.ter with the flour in order to coat the particles of flour with the fatty substance. Water added subsequently will form the necessary starches, and, if the kneading is limited, it will not produce the tough elastic network of gluten that would inhibit the brioche from rising.

A Thousand Layers ...

Now, puff pastry is "exponential." If you fold one layer of dough once into three (one turn), you form an ensemble of three stacked layers. If you fold this into three, you get nine layers, and if you make six successive turns, always separating the layers, you get 3 3 3 3 3 3 layers, or 729 altogether! In French, this is called pate feuilletee pate feuilletee, foliated pastry, a name it certainly deserves. The b.u.t.ter keeps the layers separate, and that is how they can bake without merging together. In addition, the folding of the dough traps air, which expands in baking and helps to separate the layers further, as does the vapor that forms from the water present in the dough. The pastry becomes lighter as it expands in baking.

Having established these principles, I would like to perform a public service by offering the recipe for puff pastry here. Let me explain. Having obtained very different results with the various recipes given in cookbooks, I methodically compared them and finally arrived at the following.

Make a dough by mixing flour and water. How much of each? For 250 grams (8.8 ounces, or a bit more than a cup) of flour, use a maximum of 1.5 deciliters (slightly more than 5 ounces, or a little less than two-thirds of a cup) of water. The proportion of water can vary considerably according to the flour, which can contain more or less protein, and also to temperature. When you have obtained a uniform soft ball of dough, knead 250 grams of b.u.t.ter until it is the consistency of the dough. Roll the dough into a fairly thick square, about 20 centimeters (about 8 inches) on each side, and on top of it deposit the b.u.t.ter in a square about 10 centimeters (about 4 inches) on each side, in such a way that the corners of the b.u.t.ter square come to the centers of the sides of the dough square. Fold the four corners of the dough over the b.u.t.ter to form an envelope around it. Then roll the dough in just one direction to obtain a rectangle, which you will fold in three. Having once again formed a square, you will turn it one-quarter turn, and you will roll it once more into a rectangle, which you will once more fold in thirds.

Let the dough rest in a cool place for twenty minutes, and then repeat the operation of rolling, folding in three, turning a quarter turn, and folding again in three. Return the dough to a cool place, and, before baking, repeat the operation of rolling, folding in thirds, rotating a quarter turn, rolling, and folding in three. Bake for about 40 minutes at 180C (392F), after tr.i.m.m.i.n.g the edges and making light incisions on the surface brushed with an egg yolk mixed with milk. Your puff pastry will be superb.

Sweet Dough The pastry doughs I have examined consist of flour, water, and b.u.t.ter. Sweet pastry dough, used for a dry shortbread pastry that breaks and crumbles in the mouth, is again obtained by mixing flour and b.u.t.ter, but added to these ingredients are sugar and egg yolks. The dough, made without water, is crumbly and difficult to roll out; to be successful, do not apply too much pressure on the rolling pin. Then bake.

During the preparation of the dough, the b.u.t.ter and egg yolk penetrate between the granules of starch and sugar. However, since sugar captures more water than starch, a dispersion of starch granules becomes dispersed in a syrup, preventing gluten formation. The (weak) cohesion in sweet pastry doughs is due to the egg yolks, which coagulate into a network that traps the various granules.

The Bubbles in Sponge Cake Sponge cake dough does not contain any water, either. Its lightness, once again, is due to the absence of jellified starch in the uncooked dough. To make it, egg yolks and castor sugar (a fine-grain sugar somewhere between granulated and powdered sugar) are whisked together, incorporating as much air as possible. The egg yolk forces itself between the sugar granules, which, in the fatty medium, remain intact and separated by the millions of air bubbles.