Key concepts
Food science
Chemistry
Protein
Cooking
 
Introduction
When it comes to food, presentation matters. Creativity and science, combined, can result in fascinating ways to cook and present food; hard-boiled eggs are just one example. Some interesting chemistry happens when you cook an egg. For example, think about how an egg starts out as a thick, liquid, translucent substance. But after heating the egg it turns into a gel-like substance, which after further heating and cooling turns into a rubbery solid. How do the egg white and the yolk change during this process? When exactly does the egg turn hard?
 
In this science activity you will explore the flexibility of hard-boiled eggs, creating a delicious, fun-shaped Easter reward!
 
Background
Egg white is mainly made up of water (about 85 percent) and protein molecules (about 10 percent). The proteins are twisted, folded and curled up like tiny entangled balls of yarn floating around in the water. Heating causes the weak chemical bonds that keep these proteins entangled to break. The egg proteins then unravel and bump into other uncurled proteins, causing them to bond to one another, a process called coagulation. Water molecules are now trapped in a network of interconnected proteins. The egg white takes on a gel-like consistency, becoming a flexible solid. While the egg is still hot, the protein bonds can be molded into different shapes. Additional bonds between the proteins form until the egg cools. The longer the egg is left at a high temperature, the more protein bonds form, and the more rubbery the final egg will be. Once the egg has cooled down completely, however, the shape of the egg becomes permanent.
 
Materials

  • Plastic square box, one and a half inches wide by one and a half inches long and one and a half inches high. If you do not have one available, you can make one from an empty quarter-gallon (or larger) milk or juice carton, tape and scissors. (Use caution and adult help when cutting the carton).
  • Rubber band long enough to go around the square box
  • Cooking oil or spray
  • Two extra-large eggs (Less-fresh eggs will peel more easily.)
  • Saucepan
  • Water
  • Stove (Use caution and adult help when using the stove and handling hot items in this activity.)
  • Timer
  • Slotted or regular spoon
  • Plate
  • Paper towels or kitchen towel
  • Oven mitt
  • Kitchen knife

Preparation
  • If you do not have a plastic square box measuring one and a half inches on each side available, make one from an empty juice or milk carton. Here are the steps: First, clean out the empty carton, then cut it open so it lies flat. Draw the template of an open folded cubical box on the carton (making sure you draw an attached lid as well) with sides whose dimensions are one and a half inches in width, height and length. Cut out the template, fold it into a box and bind it together with tape, leaving one side open as a lid.
  • Grease the inside of your square box with oil.

Procedure
  • Place two eggs in the saucepan. Add enough water so there is half an inch covering the eggs. Put the saucepan on the stove.
  • Heat the water until it comes to a rapid boil and keep it boiling for 10 minutes. How do you think the contents of the egg are changing during this time?
  • Turn off the heat.
  • Use the slotted spoon to take one egg out of the water and carefully place it on a plate where it can cool completely.
  • Use the slotted spoon to take the second egg out of the saucepan. Do not discard the hot water; you will need it to keep the egg warm.
  • Place this egg on several paper towels or a kitchen towel and carefully wrap the towel(s) around the egg. Use an oven mitt to protect your hand while you handle the hot egg.
  • Gently tap the wrapped egg against the plate to crack the shell all over. Unwrap the egg and carefully peel away the shell. What consistency does the cooked egg have?
  • Place the egg back on the spoon to dip it back into the hot water to wash away small pieces of shell. This also helps keep the egg hot.
  • Gently push the hot egg that has had its shell removed pointed end first into the box without breaking the egg. How does the egg feel? Is it wobbly, gel-like, rubbery or stiff? Is it possible to fit the egg completely into the box?
  • The egg should just fill the box. If the egg does not completely fill the box, add some folded paper towels on top of your egg to fill the box completely.
  • Put the lid on the box and put a rubber band around it to keep the box closed.
  • Let both eggs—the unpeeled and the boxed egg—cool down for at least 30 minutes. Placing both eggs in the refrigerator can speed up the process.
  • Once the eggs are cool, open the box and let that egg slide out on the plate. How does the egg look? What is its shape? Touch it; does the egg feel differently than when it was hot?
  • Carefully peel the unpeeled egg by gently tapping it on the plate to crack its shell all over, then peel away the shell.
  • Gently push this egg, pointed end first, into the box. How does the egg feel? Is it wobbly, gel-like, rubbery or stiff? Is it possible to squeeze the egg into the box without damaging it?
  • Now that you have successfully molded the other egg into a cube, cut the cubed egg open with a knife to make some more observations. What shape is the egg yolk of the cubed egg? Is it any different from the egg yolk in hard-boiled eggs you have seen before?
  • Are you curious if a cubed egg tastes any different? This is your chance to try it out!
  • Extra: You just cubed an egg! Could you mold an egg in other forms, such as a pyramid, cylinder or ball? Use an empty juice box or milk carton to create other molds. Cookie cutters placed between two flat objects can lead to molds for even more shapes. Any ideas on what the constraints on the dimensions of an egg mold will be? Should it have a particular volume? How flat would you be able to mold an egg?
  • Extra: Try creating an egg mold with your hands. Let cold water run briefly over the egg as you hold it to cool it down abruptly. Does an egg instantly keep its form after abrupt chilling or does it need long, slow cooling to make the form permanent?
  • Extra: Experiment with different boiling times. Can you mold a soft-boiled egg? How long do you need to keep the egg at a high temperature for it to keep its new form?


Observations and results
Did you successfully mold the egg while it was still hot? Were you unable to force the cold hard-boiled egg into the cube without breaking it?
 
While hot, the egg should have felt like a wobbly, gel-like flexible solid. This is because bonds between proteins are still being formed. While hot, these bonds can be manipulated so the hot egg can be squeezed into a cube without breaking. As the egg cooled in the box, more protein bonds were formed, giving the egg white a more rubbery character. Once cold, however, the protein bonds were permanently set, like a solid, so it kept its cube form after you slid the egg out of the box.
 
The other hard-boiled egg cooled down with its eggshell as its mold. Once cool, it kept the permanent egg shape we are so used to. It felt stiff, and because the protein bonds were set permanently it was impossible to squeeze the cold hard-boiled egg into the cube without breaking it.
 
After cutting the egg open, you should have seen the shape of the egg yolk changed as well. The egg yolk of the cubed egg should have been cubed and the egg yolk of the egg that cooled in its shell should have remained spherical.
 
You can mold eggs into other geometric forms as long as the volume is that of the egg used. This is because the hot hard-boiled egg has a fixed volume, but not a fixed form; the cold hard-boiled egg has a fixed volume and a fixed form.
 
Cleanup
If you haven’t already, you can eat your boiled eggs or put them in the trash—and either way, you should compost the shells.
 
More to explore
The science of eggs, by Exploratorium
Heat Changes Protein Structure: Frying an Egg, from Sumanas, Inc.
Soft-Boiled Science: Egg-cellently Cooked Eggs, from Scientific American

This activity brought to you in partnership with Science Buddies

Science Buddies