Eggs are a nutritious source of protein. The egg white is largely made up of protein, second only to the water content in the white (87% water, 11% protein by mass). Egg yolks also contain significant amount of protein (52% water, 16% protein by mass), but also significant amounts of lipids (27% lipids by mass). Overall an egg is just under 13% protein by mass. (You can visit USDA’s National Nutrition Database for more details.)
Cooking an egg (or many other protein-rich foods) largely involves denaturing the proteins, and cross-linking them with each other to form the rubbery, white substance we are familiar with eating. We think of cooking using heat; however, the proteins can also be denatured with a strong acid to disrupt the intermolecular forces chemically. This is the approach we will use today.
Corner Store Items
- Egg Whites
- 6M HCl
- Glass petri dish, beaker, or jar (glass is best when working with strong acids)
- Micropipette and Micropipette Tips (if you do not have access to a micropipette, you can use a glass Pasteur pipette with a rubber bulb)
The egg white should be in a glass container as to not risk unfavorable reactions between strong acids and some plastics. Always exercise caution when working with strong acids, and wear the appropriate personal protective equipment while handling strong acid (which should include safety goggles). If passing the sample around to students, ensure the sample is securely closed or that students are also wearing personal protective equipment.
- Begin with 3.0 g egg white in a small glass beaker (~50 mL beaker works well).
- Use a p1000 micropipette to take 0.5 mL of 6 M HCl, being careful of drips or other hazards that can come from pipetting strong acid.
- If you don’t have a micropipette, you can use a glass Pasteur pipette with a rubber bulb.
- Add the acid DROPWISE into the egg white, swirling slightly to observe the chemical changes happening over time.
- The slow addition of the acid is important for visualizing change.
- Continue to observe the beaker over the next half hour, and eventually you can poke gently with a pipette tip to study the final texture of your egg white.
This experiment can be done with acids of varying concentration, including distilled white vinegar that you probably have in your kitchen. Weaker acids will take longer to cause visual changes.
The acid is affecting the proteins in the egg white. Is the acid breaking bonds or IMFs? Is it forming bonds or IMFs? How do you know?
What conclusions can you draw about how acid affects protein structure?