The Accidental Scientist: Science of Cooking

recipe: Pavlova]
What’s Going On?

Why do egg whites foam?

If you try to whip plain water into a foam, you won’t have much luck. Bubbles that form in plain water quickly pop. That’s because water molecules stick together. Water molecules are electrically attracted to each other. They won’t spread out to form a bubble film unless you add something that lessens the attraction.

An egg white is about 90% water and 10% protein. The egg-white proteins are long chains of amino acids that fold and curl into more or less spherical tangles. When you beat an egg white, these proteins uncurl and stretch out. (Click here to find out why they uncurl.)

When the proteins uncurl, they expose amino acids that were hidden in the center of the tangle. Some of the amino acids repel water; these are hydrophobic, or water-fearing amino acids. Some of the amino acids are electrically charged and are attracted to water; these are hydrophilic, or water-loving amino acids.

As you beat the egg whites, you also whip bubbles into the mixture. The water molecules and egg-white proteins bump around, jockeying for position. The water molecules are attracted to each other and to the hydrophilic amino acids on the proteins. While trying to get close to each other and to the hydrophilic amino acids, the water molecules squeeze the hydrophobic amino acids out. The best place for the egg proteins is on the surface of the liquid, with their hydrophobic amino acids sticking out into the air.

The surface of each bubble film becomes crowded with egg proteins. The water molecules are forced apart by these proteins. Since the attraction between water molecules decreases with distance, the water molecules don’t stick together quite as well—they can spread out and make a bubble film.

Separating the water molecules so that they spread out in bubble films is only one step in making a meringue. The chicken egg contains a mixture of proteins that makes meringue possible. Some of the proteins form bonds with each other to create a stable network that keeps the bubbles from popping. When the meringue is in the oven, another protein—ovoalbumin—forms bonds that cause the meringue to stiffen.

Why does the temperature matter?

Egg whites at room temperature can be beaten to a foam more easily than cold egg whites.

Why is this dessert called Pavlova?

This dessert is named after Anna Pavlova, a famous Russian ballerina. Born in St. Petersburg in 1881, Pavlova overcame humble beginnings and physical setbacks to become the most well-loved dancer of her time, worshipped all over the world. This delicate dessert was created in her honor. Her most famous role was in The Dying Swan, a ballet that was choreographed especially for her. She died at the age of 50, of pneumonia. She is famous for her last words. Moments before her death, she said, “Bring me my swan costume.”

Why can’t I use a plastic bowl?

Fat interferes with the formation of a good foam—and fat clings to plastic. No matter how carefully you clean a plastic bowl, odds are good that a bit of grease remains behind. It’s preferable—and easier!—to use a glass or stainless steel bowl to produce a fluffy meringue. Egg yolks also contain fat, so when you separate the eggs, try to make sure that none of the yolk ends up in your egg whites.

Is there any advantage to using a whisk?

We’ve gotten the best results from using a whisk with many thin wires. It may be because the whisk incorporates more air into the foam than a beater does.

Why not add sugar at the beginning?

Adding sugar at the beginning can double the time you have to whip the egg whites to get a foam. That’s because the sugar molecules get in the way of the egg proteins. With sugar molecules in the way, it takes longer for the proteins to find each other and form bonds.

When meringue is cooking, sugar helps keep it stable by bonding with water molecules and preventing them from escaping as water vapor. Delaying the evaporation of water from the foam helps keep the foam stable until it stiffens.

Why add vinegar?

Adding vinegar (or any other acid) can make the foam less likely to suffer the consequences of overbeating—lumpiness, loss of water, and collapse. These undesirable consequences result from too many bonds forming between the egg proteins.

When you add an acid to a mixture, you are essentially adding some positively charged particles. These positively charged particles are hydrogen ions—hydrogen atoms that have lost an electron. The hydrogen ions hop onto charged portions of the proteins and leave them uncharged. Proteins that are electrically neutral are less likely to react with other proteins.

Uh oh! My meringue shell broke! How can I save this dessert?

If your meringue shell gets too stiff and breaks apart, you can still make a tasty dessert. Just chop up the fruit that you had intended to put on top, whip the cream, break the meringue into large crumbs, mix it all together, and call it ambrosia!

Can overbeaten egg whites be salvaged?

“All is not lost. As long as you haven’t added any other ingredients, you can usually resurrect a foam by adding an extra egg white and beating just until the mixture looks glossy and forms peaks again.”

—Anne Gardiner and Sue Wilson, The Inquisitive Cook

 

 

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