6 - 8
15 min - 1 hr
If you’ve ever tried to make salad dressing from scratch, you know that one of the biggest challenges is getting the oil and the vinegar to mix properly. No matter how hard you try to shake, stir, or whisk oil and vinegar together, they eventually separate. This happens because vinegar and oil are made of very different types of molecules that are attracted to their own kind.
Most vinegars are solutions of acetic acid and water (plus some other acids and alcohols, depending on the type of vinegar you are using). Water, acetic acid, and alcohol are all examples of polar molecules—molecules that have a slightly negative charge at one end, or pole, and a slightly positive charge at another end. These slightly charged poles arise because one or more atoms in the molecule are electronegative, meaning that they tug electrons—which are negatively charged—towards them, creating an uneven distribution of charge within the molecule. Polar molecules are generally attracted to other polar molecules because their slightly negative poles have an affinity for their slightly positive poles. Polar molecules are attracted to water molecules—which are also polar—and are called hydrophilic, which means “water loving.”
Oils are a different story. Oils are a type of fat (like butter, shortening, and lard) and are considered non-polar. Fats and oils are composed primarily of long molecules called fatty acids (usually bound together by glycerol molecules into groups of three called triglycerides). Most of the atoms in a fatty acid molecule share electrons evenly and are neither negatively nor positively charged (although fatty acids do contain small regions of polarity—just not enough to make the whole molecule polar.) Non-polar molecules love other non-polar molecules and will glom together when mixed with water. You can observe this phenomenon by placing a few drops of oil on the surface of a bowl of water—eventually the drops will form a single large oil slick. Oils repel polar molecules such as those found in vinegar. Because oils also repel water, they are called hydrophobic, which means “water-fearing.”
How can we bring together polar and non-polar molecules to make something delicious like mayonnaise (which is essentially a combination of water and oil) or salad dressing? We need an emulsifier. Emulsifiers are the hand-holders of the molecule world. They contain both hydrophobic and hydrophilic regions and are able to attract and “hold hands” with polar and non-polar molecules simultaneously, pulling them together to form a special type of mixture called an emulsion. For instance, after adding an effective emulsifier to oil and vinegar and mixing thoroughly, separation of the oil from the vinegar will take much longer or won’t happen at all.
Salad Dressing Science Experiment
Vegetable or olive oil
At least three of the following emulsifiers:
Stopwatch or clock (preferably both)
Tablespoon and ¼ teaspoon measure
Whisk, spoon, or other stirring implement
Emulsifier data sheet
Four small glasses, bowls, or jars with clear sides—one for a control, and one for each emulsifier to be tested
While the materials used in this activity are used in home kitchens every day, please be aware of the following:
- The dry mustard and vinegar used in this activity can cause respiratory irritation if inhaled. Avoid swallowing, shaking, inhaling, or sniffing these products during the activity.
- Glass can pose a risk. Clear plastic vials or cups will work for this activity, but they will be much harder to clean and reuse.
- Unfortunately, if you work in a space or with equipment where non-food safe materials are handled (such as a laboratory), you should not taste your results.
For classes, it may be helpful to designate a measuring spoon to each of the emulsifiers or pre-measure individual allotments for each student/group. You may wish to modify this procedure with volumes that are appropriate for the lab equipment you use. Print out the lab protocol and materials list for use in a lab with test tubes, pipettes, and a scale.
- In a clean kitchen space used only for food safe materials, set out four small, clean glasses with clear sides on a stable work area.
- To each of three glasses, add ½ tsp of an emulsifier to be tested, putting a different emulsifier in each glass. Label each glass with the emulsifier that was added, and label the empty glass “control.” Label the data sheet with the emulsifiers you will be testing.
Unseparated (left) and separated (right) mixtures of olive oil and balsamic vinegar.
- To each glass, add four tablespoons of vinegar, and swirl to fully mix in the emulsifier.
- To each glass, add four tablespoons of oil. Take a moment to observe the layers of oil and vinegar as they avoid mixing with one another. This is what separation looks like, a process you’ll need to be familiar with in order to collect data in the next step.
- Using a fork or a whisk, vigorously stir, whisk, or whip the oil and vinegar in the “control” glass for 30 seconds (time it with a clock or stopwatch). At the end of 30 seconds, start the stopwatch and watch the sides of the glass for 1-5 minutes for signs of separation. When you see that most of the oil has separated from the vinegar, stop the stopwatch and record how long the process took on your data sheet.
- Repeat step 5 for each of the glasses containing an emulsifier, rinsing off and drying the whisk after each mixing. If an emulsion has not separated after 5 minutes, write “over five minutes” and the time of day on your data sheet.
- After you have mixed and observed all of the emulsifier combinations, go back and check to see if any of the emulsions that didn’t separate earlier have now separated. Record your observations on your data sheet in the column marked “separation time.”
- You can repeat steps 1-6 to try out other herbs or spices, such as salt and pepper, to see how they effect the separation time of your dressings. You can also experiment with other vinegars or oils to see how their separation times differ, or investigate the effect of temperature on separation time. When you are done, grab a carrot or piece of lettuce and give your emulsions a taste! [Refer to safety precautions described above.]
- Did the mixtures with the emulsifiers take more or less time to separate than your control? Is this what you expected?
- Based on your observations of separation time, which emulsifier would you recommend using for making salad dressing?
- How would you expect the separation time to change if you added more emulsifier? Why? What about if you added more oil than water?
- Lemon juice is mostly citric acid and water. Would you expect it to mix better with olive oil or vinegar? Why or why not?
- Look for recipes for other salad dressings or vinaigrettes online. For each, try to identify which ingredient is the polar molecule (hydrophilic), which ingredient is the non-polar molecule (hydrophobic), and which ingredient is the emulsifier.
NGSS DCI PS1-A: Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.
NGSS MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures.
NGSS HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.