|MadSci Network: Chemistry|
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
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