Re: What is protien denaturization?

Denaturization is a process that proteins undergo when they are subjected to 
stresses that will affect the structure of the molecule. So first, let's talk 
about protein structure.

Proteins are very long molecules containing 100 or more amino acids linked 
together. This long chain of amino acids is the primary structure of the 
protein but a long, straight chain is not stable; it requires a lot of energy 
to maintain it. So, proteins will form a coil, like a spring, that requires 
less energy to maintain. This coil is called the alpha helix and is the 
secondary structure of the protein. Once the protein is coiled, different 
groups that are attached to the amino acids in the chain become close to each 
other and may form bonds to further stabilize the molecure; this is called the 
tertiary structure. This final structure is very important when discussing the 
function of the protein in the body and in food preparation. Most proteins have 
either a globular (like a ball) or fibrous (long, kind of stringy) tertiary 
structure.

So now, back to denaturization. When a protein is stressed, like during heating 
(cooking an egg), stirring (whipping up egg whites) or exposure to UV light, 
the bonds that hold a protein in its tertiary structure will begin to break. 
When these bonds break, the protein starts to unfold and loses some its 
properties. For example, denatured proteins usually becomes less soluble, that 
is, it doen't dissolve in water as well. If the protein is an enzyme, it will 
lose its ability to function as an enzyme. If the stress that is causing the 
denaturation continues, other changes may occur. Now that the normal structure 
of the protein is gone, new bonds may be formed, giving it a different shape. 
Proteins next to each other may form bonds between them so now you have a big 
clump of proteins all hooked together. At this stage, its called coagulation. 
This is what happens when you cook an egg and why the color changes from clear 
(lots of space between protein molecules for light to show through) to white 
(not very much space for light to go through). Sometimes you want proteins to 
denature, like when you cook an egg and sometimes you don't want that to 
happen, like cooking milk so much that it curdles. The curdled parts are the 
proteins that have been denatured and coagulated into a big clump. This isn't 
very nice if you are making cream soup and you want it smooth and creamy, not 
lumpy.

A perfect exercise to do to demonstrate denaturization and coagulation is to 
whip egg whites. When you first start out, they are foamy but if you stop they 
will eventually sink back and look that same as when you started. But, as you 
continue to beat them, the proteins get more and more denatured, so they start 
to hold up even when you aren't beating them and become white. And if you keep 
doing it, they will begin to form clumps and the whole thing collapses. At this 
point, the egg white is coagulated and nothing you do can "fix" it - that is 
make it uncoagulated. These two processes are physical changes, not chemical 
changes to the structure of the protein. The same amino acids are still there 
as when you started; just in a different shape.

Now, all living things have protein in them but some more than others, so that 
for something like vegetables, what happens when they cook has more to do with 
the starches that make up the cell walls. Vegetables don't lose protein when 
you cook them, although some denaturation does occur, just like I described 
above. But the cell wall structure is so rigid in vegetables that you really 
have to cook them a long time (longer than even the most overcooked vegetable 
would be) before you end up with coagulated proteins.

This is a very long answer but I hope I explained it ok. The study of proteins 
and what happens to them when you cook or in the body is fascinating.

Jill Irvin, RD

resources:

Foods, Experimental Perspectives; Margaret McWilliams

Introductory Foods, Marion Bennion