proteins change when you heat them, beat them, or mix them with
other ingredients. Understanding these changes can help you understand
the roles that eggs play in cooking.
are made of long chains of amino acids. The proteins in an egg white
are globular proteins, which means that the long protein molecule
is twisted and folded and curled up into a more or less spherical
shape. A variety of weak chemical bonds keep the protein curled
up tight as it drifts placidly in the water that surrounds it.
you apply heat, you agitate those placidly drifting egg-white proteins,
bouncing them around. They slam into the surrounding water molecules;
they bash into each other. All this bashing about breaks the weak
bonds that kept the protein curled up. The egg proteins uncurl and
bump into other proteins that have also uncurled. New chemical bonds
formbut rather than binding the protein to itself, these bonds
connect one protein to another.
enough of this bashing and bonding, the solitary egg proteins are
solitary no longer. Theyve formed a network of interconnected
proteins. The water in which the proteins once floated is captured
and held in the protein web. If you leave the eggs at a high temperature
too long, too many bonds form and the egg white becomes rubbery.
with heating eggs by hard cooking eggs,
by making deviled eggs, or
by making flan.
you beat raw egg whites to make a soufflé or a meringue,
you incorporate air bubbles into the water-protein solution. Adding
air bubbles to egg whites unfolds those egg proteins just as certainly
as heating them.
understand why introducing air bubbles makes egg proteins uncurl,
you need to know a basic fact about the amino acids that make up
proteins. Some amino acids are attracted to water; theyre
hydrophilic, or water-loving. Other amino acids are repelled
by water; theyre hydrophobic, or water-fearing.
proteins contain both hydrophilic and hydrophobic amino acids. When
the protein is curled up, the hydrophobic amino acids are packed
in the center away from the water and the hydrophilic ones are on
the outside closer to the water.
an egg protein is up against an air bubble, part of that protein
is exposed to air and part is still in water. The protein uncurls
so that its water-loving parts can be immersed in the waterand
its water-fearing parts can stick into the air. Once the proteins
uncurl, they bond with each otherjust as they did when heatedcreating
a network that can hold the air bubbles in place.
you heat these captured air bubbles, they expand as the gas inside
them heats up. Treated properly, the network surrounding bubbles
solidifies in the heat, and the structure doesnt collapse
when the bubbles burst.
with foaming egg whites by making Pavlova.
knows that, left to their own devices, oil and water dont
mix. But for many recipes, you mix oil-based and water-based liquids—and
want them to stay that way. Often, egg yolks come to your rescue
by creating an emulsion.
food emulsions are known as the oil-in-water type, which means that
oil (or fat) droplets are dispersed throughout the water. Put oil
and water in a jar, shake it vigorously, and youll disperse
the oil. To prevent the oil droplets from coalescing, however, a
substance known as an emulsifier is required. Egg yolk contains
a number of emulsifiers, which is why egg yolks are so important
in making foods such as hollandaise and mayonnaise.
proteins in egg yolk can act as emulsifiers because they have some
amino acids that repel water and some amino acids that attract water.
Mix egg proteins thoroughly with oil and water, and one part of
the protein will stick to the water and another part will stick
to the oil.
is another important emulsifier found in egg yolk. Known as a phospholipid,
its a fatlike molecule with a water-loving “head”
and a long, water-fearing “tail.” The tail gets buried
in the fat droplets, and its head sticks out of the droplet surface
into the surrounding water. This establishes a barrier that prevents
the surface of the fat droplet from coming into contact with the
surface of another fat droplet.
with mixing oil and water by making hollandaise