THE SCIENTIFIC ASPECT OF COOKING, encompassing the physical and chemical changes that take place in food as it is cooked, is virtually ignored in cookbooks. It was an effort to explore this field that led me into the mysteries of sauce béarnaise, which I described in this department for December, 1979. Now I have been making meringue in order to learn something about the processes affecting egg whites as they are beaten and heated. Perhaps my limited achievements will inspire others to experiment in the field.

A meringue dish raises several questions about the protein chemistry of eggs and the physics of heat transfer For example, why do egg whites stiffen when they are beaten? Why does excessive beating separate water from the whites and ruin the dessert made with them? Why does the addition of a mild acid such as vinegar strengthen the whites? Why is salt undesirable if it is added before they are beaten?

Suppose the objective is a lemon meringue pie, in which the meringue is a fluffy topping consisting of beaten and cooked egg whites to which a bit of sugar has been added. The first step in the making of the meringue is to separate the whites from the yolks. Crack an egg and drop its contents into an egg separator. The central depression of the separator holds the yolk; the white slides off the yolk, over the rim of the separator and into a bowl. Yolk must be rigorously excluded from the meringue, and the surest way to do so is to first separate the white of each egg into a small bowl. If it is apparent that no yolk went along, the contents of the bowl can be added to a larger bowl containing the egg whites that have already been separated.

Now the egg whites are beaten with a metal whisk, an eggbeater or an electric mixer. The aim is to stiffen the whites and to work in bubbles of air. Keep beating until the material is opaque, glistening and fairly firm. At this point the sugar can be beaten in gradually, after which the mixture can be laid carefully on the lemon base and the pie can be put in the oven.

The heat of the baking expands and hardens the white topping. The expansion results from the enlargement of the trapped air bubbles as they are heated. The hardening comes from the cooking of the egg. The meringue is light and Huffy because the air beaten into it decreases its density, which declines further as the air expands in the oven.

The reason the egg whites must be free of yolk is that if they contain even a small amount of it, the beating operation will not add enough air. The resulting meringue will be flat and dense. The meringue can also fail from excessive beating. In my zeal to stiffen the whites I often overbeat them, causing the water to separate out. This error results in a flat and hard meringue.

Recently, after yet another embarrassing failure with a lemon meringue pie, I began to wonder what really happens to the egg whites when I beat them. What makes them change from a clear fluid to an opaque white mixture of egg and air bubbles? I also wondered precisely what happens to the mixture when I overbeat it.

These questions led me to wonder about other procedures. Some cooks add a small amount of acid, either vinegar (acetic acid) or cream of tartar (tartaric acid), to aid in stiffening the whites. Salt, on the other hand, is avoided because it is said to harden the whites. Sugar must not be added until the whites have been fully beaten, and then it must be put in slowly with a minimum of additional beating. Finally, some cooks insist that the best meringue is made with whites beaten in a copper bowl.

To explain what is happening to the whites in any of these procedures I shall first describe the protein in the egg white. What a good cook is doing is properly preparing that protein for the oven. What I am doing in my lack of experience is ruining the protein. The success of the meringue depends on how the protein behaves when it is beaten and combined with mild acids.

The whites of chicken eggs are composed of several types of protein, principally ovalbumin globulin and ovomucin. Proteins are huge molecules consisting of a chain (or several chains) of amino acid units linked by peptide bonds. These bonds are strong, covalent ones between a carbon atom in one amino acid and a nitrogen atom in the next amino acid in the chain. A generalized protein chain is shown in Figure 2. The symbol R represents the characteristic side chain of each amino acid. Ovalbumin consists of 386 amino acid units.

The particular sequence of amino acid units in a protein chain is the primary structure of the protein molecule. Such a chain is not straight but has a three-dimensional secondary structure. In some types of protein the chain forms a helix; in others the structure looks something like a pleated sheet. Ovalbumin and globulin may have a mixture of the two, along with regions where the chains coil more randomly. These secondary structures can be folded into a tertiary structure. Ovalbumin and globulin are folded so complexly that they resemble spheres rather than chains. Such proteins are called globular.

Several types of force hold a protein in its secondary and tertiary structures. Disulfide bridges are covalent bonds between sulfur atoms at separated points along the protein chain. Some points along the chain are joined by an ionic bond: an electrostatic attraction between two oppositely charged sites.

Neutral (uncharged) sites can attract each other through the force that is called the hydrophobic bond in biology and the van der Waals force in physics. One site forces a small separation of the centers of the positive and negative charges in the other site. The separation causes the positive charges of one site to be attracted to the negative charges of the other. A similar interaction takes place in polar groups. In such a group there is a permanent separation between the centers of positive and negative charge. The positive end of one polar group is attracted to the negative end of another polar group. Commonly the positive end is a hydrogen atom, in which case the attraction is called a hydrogen bond.

Of these interactions the peptide and disulfide bonds, which are both covalent, are the strongest. The ionic bonds are considerably weaker, and the hydrophobic and hydrogen bonds are weaker still. When a cook forces a whisk through egg whites, shearing the fluid, some of the weaker bonds are ruptured and parts of the tertiary structure of the proteins are destroyed. The cook does not totally disrupt the proteins because the forces holding them in their primary and secondary structures are comparatively strong. As a result of the whisking a protein is gradually unraveled from its initial spaghetti-like sphere. Any such altering of the structure of protein is called denaturing. One of the objectives in beating egg whites is to denature the proteins.

Once the protein molecules are partially unraveled they begin to attach themselves to one another to form a three-dimensional mesh, or gel. This attraction between proteins is unlikely before denaturation because the proteins are globular and relatively few of their sites for possible bonds are exposed. Moreover, the sites suitable for hydrophobic bonding are initially tucked away inside the globular protein. With part of the protein unraveled more sites are exposed for attachment to other proteins. As the whites are beaten air bubbles–air surrounded by a thin film of water–are formed and trapped in the gel mesh. The film is held together by the surface tension of the water and strengthened by the mesh of proteins in the film. Those proteins are not only attached to one another but also bonded (by hydrogen bonds) to water molecules in the film.

When the mixture is heated, the air bubbles expand and thereby expand the meringue. Moreover, the heat further denatures the proteins, unraveling them more and thus enabling the mesh to stretch as the air expands. The bonding between the proteins also becomes more extensive, coagulating the whites into a firm structure. Once the expansion has been achieved the trapped air bubbles are no longer required because the heat has hardened the whites in their expanded structure. The meringue is further heated only to dry it so that it is slightly crisp.

Excessive beating ruins the meringue because it unravels the proteins too much. As a result the mesh becomes too firm and water separates from the proteins. One sign that the mesh is too firm is that stiff peaks are formed when the beater is pulled out. If the mesh is too firm, the whites cannot expand properly during the cooking to match the expansion by the trapped air bubbles. The bubbles burst and the meringue collapses before the heat has had time to coagulate the protein. By the time the coagulation is achieved the meringue has become flat and the cook has nothing but baked egg whites on a lemon base.

A good cook knows when the whites are on the verge of being overbeaten by carefully watching the surface, waiting for the critical moment when the whites begin to lose their sparkle and water droplets are about to form. The unraveled protein is soluble in water because on the outside of the glob it forms are many hydrophilic (water-attracting) sites to which water molecules can bond with hydrogen bonds. Most of the hydrophobic (water-repelling) sites on the protein are tucked away inside the glob. As the protein is unraveled more of the hydrophobic sites are exposed and fewer hydrophilic sites are available for bonding water molecules.

Eventually, when the protein has been unraveled too much, the bonding of water to it is so greatly reduced that the protein is no longer soluble. If a small amount of protein was in water, the protein would precipitate out. In the bowl of egg whites it is the water that can be seen to separate and form droplets. They tell the cook that the protein has been denatured too much. They also reveal that the water is no longer as capable of holding small air bubbles in the gel mesh, because the water molecules are no longer attached to the proteins forming the mesh Thus the separation of water in the whites is a sign that the meringue will fail.

Fresh egg whites are transparent, but once their proteins denature and form a mesh they become opaque. I arranged a demonstration of the change by putting a beaker of egg whites on a hot plate and passing a thin beam of light from a helium-neon laser through them. Initially the beam was sharply visible in the whites partly because some of the light was scattered by the globular proteins. Additional scattering was evident because the whites were not homogeneous. Once the hot plate was warm and began to make the proteins coagulate into a mesh the beam became less distinct. The mesh scattered the light more, illuminating much of the interior of the whites in the beaker. When the coagulation was complete, most of the light was absorbed in the whites.

Research has shown that meringues can be made from mixtures containing only one or two of the proteins normally found in egg whites. With ovalbumin alone the meringue has almost the same volume but displays a coarser texture. It also takes longer to beat ovalbumin to the proper foam. With globulin and ovomucin alone the beating takes less time but the meringue collapses after baking, presumably because the mesh is not strong enough to support the weight. Globulin alone yields a foam with small bubbles, thereby imparting to the meringue a smoother texture than is achieved with ovalbumin alone.

I do not know why ovalbumin and globulin give rise to different kinds of foam, but I would guess it has to do with the ease with which the proteins are denatured. Since ovalbumin takes longer than globulin to produce a good foam, the ovalbumin is probably harder to unravel by beating. Most likely this quality is due to a tertiary structure that is somewhat stronger in ovalbumin than it is in globulin. Since globulin appears to unravel easier, a globulin mixture builds up unraveled protein faster than an ovalbumin mixture. With more unraveled globulin the gel mesh will be finer.

When a cook beats natural egg whites, the fine texture of the foam is due more to the globulin than it is to the ovalbumin. The ovomucin does not foam, but its mesh appears to help stabilize the foam that results from the denatured globulin and ovalbumin. Many cooks know that duck eggs make comparatively poor meringues. The reason may be that duck eggs contain less globulin than chicken eggs.

Before the whites are separated from the yolks the eggs should be at room temperature. Most cookbooks caution–. against working with eggs straight from the refrigerator; the whites from cold -eggs make a puny foam. The fault lies in their viscosity. When they are cold, the viscosity is high and the beater cannot properly mix air bubbles into the fluid.

When yolk is accidentally added to egg whites, the lipid (fat) molecules in it attach themselves to the hydrophobic sites on the protein by means of the van der Waals force. Hence those sites are no longer available for bonding with other proteins to create a gel mesh. It is then harder to trap air, and the volume that can be attained by beating the whites is smaller. When I beat egg whites properly, the volume they fill expands by five or six times because of the trapped air. When I add a tiny amount of yolk (or any other source of fat) to the same amount of whites, however, the expansion is dramatically reduced.

To demonstrate this effect yourself beat the whites from three Grade A eggs. When the surface is glistening suitably, mark the height of the whites on the side of the bowl. Then rinse and dry the bowl and separate the whites from three more eggs. This time add about 10 percent of one yolk to the whites and beat them as before. You will find that after prolonged beating the volume is about half that of the preceding batch and the mixture is noticeably runny, revealing the lack of a good gel mesh.

I have seen another sign of a poor or missing mesh. When I beat egg whites without any trace of yolk, the mixture climbs the central shaft of the beater of my electric mixer. This is known as the Weissenberg effect; it indicates that the fluid being stirred is non-Newtonian. (I described the effect in this department for November, 1978.) A good mesh is caught by the turning shaft and rises on it. When the whites contain yolk, the mesh is poorer and climbs the shaft weakly or not at all.

Some cooks believe adding a mild acid makes the job of beating the whites into a foam easier. The acid is either added directly in the early stages of mixing or is wiped on the interior of the bowl before the whites are put in. The belief is probably valid. When the acid is added, its molecules dissociate into positive and negative ions. The positive ions tend to collect around the negatively charged sites on the protein. Suppose those sites are involved in holding the protein in its raveled structure. Now, with a cluster of positive ions around them, they no longer can play a role in the structure and the protein partially unravels. Similarly, the negative ions may cluster around the positively charged sites on the protein and also help to unravel it.

The addition of acid has a more important effect, which is critical in the formation of a gel mesh. The proteins in freshly prepared whites have a net negative charge. Hence they form a mesh with some difficulty because of their mutual repulsion. When acid is added, the net charge on each protein is reduced because of the clustering of the ions around the protein's charged sites. The repulsion between the proteins is thus reduced and they can more easily bind to one another to form a mesh.

When salt (NaCl) is added to the egg whites before they are beaten, two things can happen. The salt dissociates into positive (Na+) and negative (Cl-) ions, which cluster around charged sites on the proteins. The sodium ions cluster around the negative sites, the chlorine ions around the positive sites. If those sites were partly responsible for the tertiary structure of the protein, the protein begins to unravel. The effect is small, since most proteins are not denatured by the salt.

The clustering of ions around the charged sites can also reduce the net charge on the proteins. Their mutual repulsion is then reduced and a mesh is easier to form.

In spite of these possible benefits from the addition of salt, its major effect is undesirable. It can remove water from the protein. As a result the water cannot form a film around the air beaten into the mixture and so is no longer attached to the mesh of entwined proteins you are attempting to create. Air is not trapped and a poor foam results. If salt is needed in a mixture of egg whites, it should be added only after the mixture has been beaten into a foam. Even then anything more than a small amount will tend to make the foam collapse.

What about the copper-bowl technique? There may be something to it, but I am not sure what it is. Aluminum works poorly because it discolors the whites. Copper might help if it somehow decreases the net charge on the proteins, making the formation of a mesh easier, but I am skeptical. You might experiment to see whether a copper bowl makes beating the egg whites easier, increases the volume of properly beaten whites or increases the volume of the meringue.

Many cookbooks maintain that egg whites are stiffened partly because of an electric field that develops when they are beaten. Recipes give instructions on how best to create such a field, or at least on how not to destroy one. For example, I have read that one should avoid beating the egg whites in a wet bowl lest the film of water eliminate the electric field.

The shearing effect of a moving whisk or beater might create an electric field much as one builds up when cellophane is pulled off a roll. If an electric field does develop in egg whites, it could help to unravel the proteins by breaking some of the weaker bonds of the tertiary structure.

I tried to find an electric field in egg whites that I beat with my electric mixer. I attached one probe of a voltmeter to the casing of the mixer and the other to a small length of wire I put in the whites. The probes would record any electric field that might lie between the beaters and the wire buried in the whites. No voltage registered on the meter, even at its most sensitive setting of .6 volt. (If you try this experiment, be sure to keep the wire from getting tangled in the beaters as they turn.)

Next I attached a wire to the first probe and lowered it into the whites close to the other wire. Again I watched in vain for a voltage difference between the two probes. If the mixing does generate any voltage, it must be less than .05 volt and must be localized along the whites actually being sheared at any given instant. As far as I can tell no field is set up throughout the mixture. The cook does not have any control over the field (if one exists) and so should not worry about it.

Sugar should be added to egg whites only after they have been properly beaten into a stable foam. Sugar added before the protein gel mesh has had a chance to form removes water from the protein and thickens the fluid, so that aerating the mixture is more difficult. Consequently good cooks add the sugar only after the foam is built up by the beating, and even then they add it slowly while continuing to beat the mixture. Ordinary granulated sugar will not do; one needs superfine sugar that will dissolve quickly into the water walls surrounding the air bubbles. Ordinary granulated sugar takes too long to dissolve and will form clumps in the meringue. Taste the mixture to see how the dissolving has progressed. A gritty texture reveals undissolved sugar. If necessary, a small amount of water can be added to complete the dissolving.

If any undissolved sugar remains in the meringue, droplets of syrup will appear on the surface as the meringue is baked. Such a pie, called a weeping meringue, also brings tears to the eyes of the cook who labored for a better product. Once the pie is baked it must be stored in a container or the sugar in the meringue will absorb water from the air, causing water droplets to form on the surface of the pie. The droplets not only add weight that can make the meringue collapse but also soften the protein gel mesh supporting the meringue. I have given up trying to make a lemon meringue pie during the humid midsummer of Cleveland. There is so much water vapor in the air that the meringue begins to fall almost as soon as I take it out of the oven.

The lemon base for a lemon meringue pie is made from egg yolks, sugar, cornstarch, salt, butter, lemon juice and grated lemon peel. Its preparation is begun by heating the cornstarch, the sugar and the salt in a small saucepan and then stirring in water. When the mixture is smooth and hot, the pan is removed from the heat source and half of the liquid is poured into a mixing bowl with the yolks, which should already have been lightly mixed. After a quick mixing the stuff is poured back into the pan. The entire mixture is then stirred and heated over medium heat until it has boiled for about a minute. Then the juice, the peels and the butter are stirred in and the mixture is poured into a baked pie shell.

What I have described is the standard procedure for making the base quickly. A good cook takes more care, particularly with the texture of the base. Let me simplify the mixture by concentrating on the yolks and the sugar. The cornstarch thickens the base and the lemon juice and the peels of course give the pie its flavor. The secret of the base lies in the yolks and the sugar.

When the yolks and the sugar are beaten together, they form a syrup. The beating lightens the mixture by denaturing the proteins in the yolks. Although the fat in the yolks interferes with the formation of a gel mesh of these denatured proteins, something of a mesh does form. The beating also whips air bubbles into the mixture. The sugar absorbs the water from the yolks and forms a syrup. The product is thick because of the syrup but is lightened somewhat because of the air beaten Into the denatured proteins. The expert can ascertain whether the mixture is properly beaten by examining how it flows from the whisk when the whisk is lifted from the mixture. If the mixture forms a ribbon on itself as it flows back into the bowl, it is ready for the pie shell.

Cookbooks contain a wide range of instructions for baking the pie. Some cookbooks suggest baking at a relatively high temperature of 400 degrees Fahrenheit for a short time, from seven to nine minutes. Others call for a much lower temperature (175 degrees F.) and a cooking time of several hours, after which the oven is turned off and the pie is left in it overnight. Still other books recommend intermediate temperatures and cooking times.

In general a lower temperature yields a crisper meringue, a higher temperature a chewier one. Those cookbook descriptions are a bit vague in my view; it would be more precise to describe the elasticity of the meringue. The elasticity is determined by the temperature. At a low temperature and a longer cooking time the meringue is slowly stretched by the expanding air bubbles and is also dried. It is not very elastic, partly because of the drying but mostly because the gel mesh is stretched for such a long time that it becomes rigid. At a higher temperature and a shorter cooking time the meringue is quickly stretched and perhaps is not as thoroughly dried. Then it is more elastic because the mesh was not stretched for enough time to become rigid.

All recipes call for heating from the bottom of the oven, never from the top, otherwise the top of the meringue would absorb the direct infrared radiation and quickly turn brown (or black). According to some cooks, the pie should be baked until the top begins to turn brown; others say browning should be avoided.

I wish I could say more about meringues, particularly about the forces that are most responsible for the protein gel mesh and how added ingredients affect it. In a future article I plan to continue the discussion of other egg dishes. In particular I shall turn to the secrets behind the superb omelets, soufflés and scrambled eggs turned out by my grandmother (who is also good at meringues and lemon meringue pies). Each of these concoctions is a challenge to my clumsy hands.

Bibliography

THE FUNCTIONAL PROPERTIES OF THE EGG WHITE PROTEINS. L. R. MacDonnell, R. E. Feeney, H. L. Hanson, Agnes Campbell and T. F. Sugihara in Food Technology, Vol. 9, No. 2, pages 49-53; February, 1955.

PROTEINS. Vicki Cobb in Science Experiments You Can Eat. J. B. Lippincott Company, 1972.

PROTEINS: THREE-DIMENSIONAE CONFORMATION. Albert L. Lehninger in Biochemistry. Worth Publishers, Inc., 1975.

MIRACLES IN A SHELL (EGG COOKERY). Madeleine Kamman in The Making of a Cook. Atheneum Publishers, 1978.

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