Proteins and Hard Boiled Eggs

    Kids, did you ever wonder why eggs get hard when you boil them? It’s because they have lots of protein, especially in the egg whites. Here’s how it works. Protein is a polymer chain of amino acids that is flexible enough to fold up on itself in different ways based on their chemistry. It’s all wound up like a loose ball of string and held in place by weak bonds that are fairly easy to break apart. When that happens, the protein is called “denatured”.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Have an adult help you to hard boil an egg. Imagine what’s going on inside the shell. When heated, the protein molecules gain enough energy to shake apart the weak bonds and the proteins begin to unfold. With time and more heat, new and stronger bonds are formed between different protein molecules. Another way to break the weak bonds is through chemical action. If you put a raw egg white in vinegar, the acetic acid will break some bonds in the egg. Use a dark bowl to help see it better. The egg white will start to set right away and get sort of pickled. When using an alcohol like vodka instead, the ethanol will break the weakest bonds in the protein. A lot of alcohol is needed, so really cover up the egg white. You should see some white strands form, but don’t be tempted to stir for this will just make a mess. You can see the greatest effect when both the alcohol and vinegar are used together. Notice the differences between these three different solutions and their effects on proteins. Mechanical energy will also work; whisking egg whites will unfold proteins and cause new bonds to form, and it stays in a new low-density “fluffier” state. A cooked, chemically-altered, or well-beaten egg white will never to go back to its original wet and gooey state.

    The yolk of the egg holds up better to both the mechanical energy and to the alcohol or vinegar attack. While there is a lot of protein in the yolk, there is also a lot of fat and other molecules that make it more difficult to denature. When hard-boiling eggs the recipe always calls for using a moderate heating process. High heat causes the proteins to get really tough and rubbery, and a chemical reaction between the yolk and the white leaves a green film around the yolk. Did you ever see this, maybe in an Easter egg? That film is actually iron sulfide, made from iron in the yolk and hydrogen sulfide from the white. It doesn’t hurt you of course and has no taste, but it doesn’t look too appetizing!

    Next check out these websites: www.hows gives the scientific reason for the answer to the question of which came first, the chicken or the egg, and www.howstuffwork describes how a chicken makes an egg using some really cool inorganic chemistry. 


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    August 2001


    Reference : Marshall Brain’s “How Stuff Works” website at /question616