You and Your Students!
Vicki Cobb, Education World Science Editor
Discover the science in mayonnaise.
Setting the Scene (Background)
In the previous activity, Spheres of Influence, students learned about emulsions by shaking up water in oil. In this lesson, students will be shaking oil and vinegar, which are the basis for most salad dressings. Shaking up this mixture breaks up its vinegar (which is water-based) into tiny droplets that are temporarily suspended in oil.
As students will see in this activity, mayonnaise is nothing more than ordinary salad dressing with the addition of an emulsifying agent (egg yolk) to make the mixture more permanent.
I've set up this activity as a demonstration; but this is a fun activity for students to do in small groups. They will be making mayonnaise -- and coming to understand its chemistry as they do it.
Eventually, the mixture will become thick and creamy. Stop shaking it after all the oil is mixed in. To season the mixture, add a teaspoon of salt and half a teaspoon of pepper and stir well. You have mayonnaise!
Homemade mayonnaise is not as thick as mayonnaise from the store is. But some people think it tastes better. Keep it in the refrigerator. Cover it so a skin doesn't form on the surface of your mixture.
Behind the Scenes
Mayonnaise comprises tiny droplets of oil spread evenly through the egg yolk, which is the active emulsifying agent in eggs. Egg yolk coats the oil droplets as they form and prevents them from coming together and forming a separate layer. If you add the oil too fast, or if you add too much oil at a time, the droplets will come together before they can be forced into the egg yolks; the mayonnaise will curdle, or separate. If that happens, you can correct the situation by starting over with a fresh egg, but add the curdled mayonnaise to the egg rather than the other way around.
You can tell when the emulsion has formed because the mixture gets thick. This usually happens after about 1/3 cup of oil has been added.
Several cookbooks make the claim that mayonnaise is difficult to make on a rainy day or when a thunderstorm is threatening. But mayonnaise is made commercially on every working day, regardless of the weather. You might want to set your student-scientists to work doing experiments to test that claim.
See additional experiments with food in Vicki Cobb's book Science Experiments You Can Eat (HarperCollins, 1994).
Article By Vicki Cobb
Copyright © 2004 Education World