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Oct. 07, 2011

Science Dad on Unfolding Proteins

by Vince Harriman

Click to enlarge images

This week we have a special guest in our house -- Beckett’s Aunt Melanie is visiting from Anchorage, Alaska. We thought it would be fun if she could help with one of our science projects, and since she is a chef, and very creative in the kitchen, we decided to 'cook up' some science. As creative as she is, her first suggestion was that we find out about poaching eggs and make Hollandaise sauce to learn about protein folding. Beckett liked the idea of learning about proteins, but wasn’t so excited about poaching eggs. So...we decided to learn about protein folding by making cookies. That's right. Science and chocolate cookies!

The reason why eggs get hard when you boil them is because when eggs are raw and untouched, the proteins are folded and compact. When you heat them or beat them or add water, the proteins in the egg unfold and some parts of the protein change and retain the new shape. What this means to scientists is that protein strands can have different structures depending on the temperature and the amounts of air (oxygen) or water present.

When a protein is folded, it looks like a messy ball of yarn. If you want to play around with understanding how proteins can have these multiple structures, try twisting a shoe lace or large rubber band. Have a friend hold one end and twist the other end gently until the lace or rubber band begins to kink and fold. Don’t hold it too tightly -- you’ll want it loose so that it can fold over on itself. If you are careful and patient you can twist up a nice ball or clump. Unfold it and it has a different shape and structure than it had originally. You can even now tie knots in it, or fold it regularly on itself. This is what proteins do. It looks like this:

 

When the protein is unraveled, the amino acids that make up the protein present more binding sites. Some of these sites absorb water and some sites repel water. Most of the water binding sites are on the outside of the clumped up protein, and the water repelling sites are inside the clump. You can see how unraveling the protein and exposing the different amino acids would change the nature of the protein. When the water binds to the amino acid, the structure changes and with it the properties the protein had.

Since we didn’t want to have poached eggs for dinner, we decided to whip eggs with air and make chocolate meringue cookies instead. We started by separating the whites from the yolks of three large room temperature eggs. You can use cold eggs, but like so many things in science and especially chemistry, temperature matters. We put the eggs in a stainless steel bowl (you can use glass, but don’t use plastic or wood-the eggs will stick and won’t whip as well) and added a bit of cream of tartar. You can also add a bit of vinegar -- the acid helps control the reaction a bit. Set the blender on low and mix the eggs with the acid for a minute or so, then increase the speed gently until the whisk is rotating at full speed.

Beating the egg whites

Depending on your eggs and mixer, you will blend the eggs for several minutes. As the egg whites begin to foam, turn off the blender and poke the mixture with a spoon. The eggs will go from liquid, to non-Newtonian liquid, to solid state as you beat them. When the mixture stands up peaks on its own, you are done beating. Next, you want to add the sugar slowly and beat it in.

Adding sugar

For three large eggs you probably need around half a cup of sugar. You’ll want to fold this in with a spoon gently -- our chef Aunt Melanie says that the sharp edges of the sugar crystals will pop the bubbles in the foam and you’ll have to start over again. After you have the sugar folded in, you’ll want to fold in your flavor -- in our case we added two tablespoons of cocoa powder and a cup of mini chocolate chips.

Adding chocolate

Finally, you spoon the mixture on to a non-stick pad or baking parchment and bake at 325 degrees for 20-30 minutes depending on the size of your cookies. If the cookies don’t hold their shape uncooked on the sheet before you put them in the oven, you popped too many bubbles and your meringue won’t hold. The cookies should look like this before they go in the oven:

The cookies are already stiff before baking

Don't bake them too long -- remember that the proteins are already changing and forming bonds and structure. The structure in this case will be how crunchy and edible the cookies are. You can experiment with this by boiling two eggs: one for five minutes and one for twenty minutes. One egg will be good to eat and one will be like a golf ball! Watch the cookies carefully -- when the edges just start to brown, pull them out of the oven.

Ready to eat!

An important note about baking your own meringue cookies -- if you've ever had store-bought meringue cookies, they were probably hard as a rock. This is for two reasons. First, they were cooked much longer, and just like the boiled egg example above, the longer you cook them the harder they get. Cookies baked for sale (and transport) need to be strong and not collapse. Your cookies should have a delicate shell with a soft and foamy inside. If you want to experiment with this, bake two batches by putting them on two trays and bake them at different temperatures for different times.

 

Our recipe was simple:

3 large egg whites
1/4 teaspoon cream of tartar or white vinegar
1/2 teaspoon vanilla
1/2 cup sugar
2 tablespoons powdered chocolate
1 cup mini chocolate chips

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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