Oil and water don’t normally mix. But look at salad dressings: In many of them, oil and vinegar (which is made mostly of water) form a smooth, completely mixed combination—at least for a little while. What’s the secret? In this experiment, you’ll mix oil and vinegar on their own and then with some ingredients often found in salad dressings—will they help oil and water stay mixed?
Use masking tape and marker to label 1 jar as “Control,” second jar as “Mustard,” and third jar as “Mayonnaise.”
Add 6 tablespoons oil to each jar. Add 2 tablespoons vinegar to each jar. Screw lid tightly on jar labeled “Control.”
Hold 1 jar in each hand. Vigorously shake jars for 30 seconds. Set aside jars. Repeat shaking with remaining jar.
Make a prediction: In which jar do you predict that the oil and vinegar will stay mixed the longest: the control jar, the mustard jar, or the mayonnaise jar? Why do you think so?
Observe your results: What do the jars look like right after you finish shaking them? Check on your jars every 15 minutes until 45 minutes have passed. What do you notice? Which jar kept the oil and vinegar mixed the longest? The shortest?
Eat your experiment: Turn your emulsions into a salad dressing. Add contents of 1 or more jars to large airtight container with lid. For each jar you use, add ¼ teaspoon salt and ⅛ teaspoon pepper to container. Place lid on container and shake to combine ingredients. Dressing can be refrigerated for up to 1 week. Shake well before using.
(Don’t read until you’ve completed the experiment!)
In the Recipe Lab, we found that the mixture in the mayonnaise jar stayed mixed the longest. The mixture in the mustard jar came in a close second, and the mixture in the control jar separated back into two layers after less than 15 minutes. How long did the mixtures in your jars stay combined?
When you shake or whisk oil and vinegar (which is mostly water), the two liquids form what’s called an emulsion (“ih-MUHL-shun”). “Emulsion” is a scientific word for a combination of two liquids that don’t usually mix. But this emulsion won’t last long—the oil and vinegar retreat into two separate layers after just a few minutes. (An emulsion of oil and vinegar is called a vinaigrette, and it’s often used as a salad dressing or sauce.)
To get oil and water to STAY mixed, you need some help from an emulsifier. Emulsifiers are special molecules that bridge the gap between two liquids that don’t want to get along, such as oil and water. One end of an emulsifier molecule is attracted to water. The other end is attracted to oil (imagine that the emulsifier is holding hands with oil on one side and water on the other). Mustard and mayonnaise both contain emulsifiers, so a vinaigrette made with mustard or mayonnaise will stay combined for much longer than one made with just oil and vinegar. So which one should you use when making a vinaigrette? You pick—whichever you like best!