Why do eggs turn hard when you boil them?

Eggs are rich in protein, and this protein causes eggs to turn hard when they're boiled - but how does it actually work? Tune in as Marshall Brain breaks down the science behind hard-boiled eggs.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from house stuff works dot com where
smart happens. I am Marshal Brain with today's question, why
do eggs turn hard when you boil them? Eggs are
rich in protein, especially the egg whites. It's this protein

(00:23):
that causes eggs to become hard when you boil them.
Here's how it works. Protein is a chain of amino acids.
These amino acid strings fold back of themselves, and the
way the strings fold determines the proteins chemical and biological properties.
The proteins are held in place by weak bonds, that is,

(00:44):
non covalent bonds between different parts of the amino acid string.
When you break those strings by various methods, you are
doing what's called denaturing the protein. You can do nature
a proteinen in several different ways. Here are two ways
to do it with eggs. The first is heat. When

(01:06):
you heat an egg, the proteins gain energy and literally
shake apart the bonds between the parts of the amino
acid strings, causing the proteins to unfold. As the temperature increases,
the proteins gain enough energy to form new, stronger bonds,
in this case covalent bonds with other protein molecules. When

(01:28):
you boil an egg, the heat first breaks or unfolds
the proteins and then allows the proteins to link back
together with other proteins, so the egg white turns from
this kind of liquid e mass into something that's much
more like plastic. The second way to do nature proteins
is to use chemicals. You can break the weak bonds

(01:50):
between protein molecules with chemicals, and then they can link
back together in the same way. Heat causes this to happen.
According to an experiment done by the bat BC Science Shack,
if you put vodka on an egg white, the alcohol
in it breaks weak hydrogen bonds in the protein. If
you put vinegar on an egg white, the acid in

(02:11):
it breaks ionic bonds. If you mix the two, the
vodka and the vinegar together, you break both types of
bonds and quite effectively de nature the protein. You do
something similar when you whisk egg whites. By exerting mechanical
energy in the whisking process, you cause the protein bonds

(02:32):
to break and subsequently to reconnect. Once these new strong
bonds are formed, the eggs stays in that state. The
proteins have formed a network of strong, permanent cross links.
A cooked, chemically altered, or well beaten egg will never
go back to its original state. When cooking eggs, use
moderate heat. High heat causes the protein and eggs to

(02:55):
become tough and rubbery. Be sure to check out our
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