In the previous experiment, we did not discuss the main point that heating the egg white makes it turn
white. Isn’t it truly remarkable that a yellow material turns white on heating! Why? And isn’t it wonderful
that simultaneously on heating, a liquid material turns solid? Why?
Remember that an egg white is mainly composed of water: therefore the question of its hardening on
heating is even stranger!
A simple reasoning helps. When you heat pure water, it doesn’t solidify, but when you heat a mixture
of water and proteins it turns solid. This means that proteins are responsible for the solidification. But
proteins do not always cause solidification: heating a solution of gelatine dissolved in water does not
make a solid.
This simple observation makes us conclude that there are proteins of various kinds.
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Another question, at which temperature does the egg white solidify, or “coagulate”? Again, another
In order to know at which temperature egg white coagulation occurs, let’s begin by a crude
experiment. Put some egg whites in a drinking glass, and place the glass on a stove, heated by the
Slowly coagulation begins at the bottom of the glass, and the limit between the lower coagulated layer
and the upper liquid layer rises up.
Use a thermometer to measure the temperature over and under this limit (or “interface”): you can
observe that coagulation occurs between 60 and...
observe that coagulation occurs between 60 and 70°C. No point using boiling water –at the
temperature of 100°C- to cook eggs: 70°C is enough!
And, please, have a look to the consistency of coagulated egg white, from the bottom of the glass up
to the interface. What can you see?
Let’s now move to yolks. It is sometimes said that the yolk contains “fats”, but is it true? If you add
some oil to water, it does not dissolve, but if you add water to water, it dissolves well. Adding a small
quantity of oil to a ruptured yolk (remember that the membrane that would otherwise prevent mixing
has to be pierced) shows that the oil does not dissolve in the yolk; but after adding a small quantity of