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Salad Dressing Science: Emulsions
Science Friday Education
Science Friday Education
Background 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. No matter how hard you try to shake, stir, or whisk oil and vinegar together, they eventually separate. This happens because oil and vinegar are made of 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 the atoms that make up the molecule have different electronegatives. This means that negatively charged electrons are distributed unevenly, and part of the molecule experiences a slightly negative charge wile a different part of the molecule results in a slightly positive charge. Polar molecules are generally attracted to other polar molecules because their slightly negative poles have an affinity for the slightly positive poles on another molecule. Polar molecules are attracted to polar water molecules and are hydrophilic, The more electronegative oxygen which means “water loving.” in a water molecule pulls electrons Oils are a different story. Oils are a type of fat (like away from the two hydrogens, butter, shortening, and lard) and are considered creating an uneven distribution of nonpolar. Fats and oils are composed primarily of long charge within the molecule. 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, so they are neither negatively nor positively charged (although fatty acids do contain small regions of polarity, just not enough to make the whole molecule polar.) Polar molecules are attracted to each other and have no interest in interacting with nonpolar molecules, so when nonpolar molecules are mixed with water, the water pushes the nonpolar molecules out of the way and the nonpolar molecules golm together. You can observe this phenomenon by placing a few drops of oil on the surface of a bowl of water—eventually the
The carbon (black) and hydrogen (white) in this nonpolar fatty acid molecule share electrons evenly and are neither negatively nor positively charged.
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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 you bring together polar and nonpolar molecules to make something delicious like mayonnaise (which is essentially a combination of water and oil) or salad dressing? You 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 nonpolar 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. In this experiment, you will test a few common household ingredients to see what the most effective emulsifiers for making salad dressing are. Procedure 1. Set out four clean 20-mL test tubes in a test tube rack. 2. Use a scale to measure 2 g of each of the emulsifiers you will test. 3. To three test tubes, add 2 g of an emulsifier, putting a different emulsifier in each test tube. Label each test tube with the emulsifier that was added, and label the empty one “control.” Enter the emulsifiers you will test into the data table. 4. Using a graduated cylinder, add 8 mL of vinegar to each test tube and swirl to fully mix in the emulsifier. 5. Using a clean graduated cylinder, add 8 mL of oil to each test tube. Observe the oil and vinegar layers as they avoid mixing with one another. This is what separation looks like, a process you’ll need to be familiar with to collect data in the next step. 6. Using your thumb or a stopper, cover the opening to the control test tube, and shake it up and down for 30 seconds (time it with a stopwatch). At the end of 30 seconds, place it back in the test tube rack and time how long it takes for the liquids to separate with the stopwatch. 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 in your data table in the column marked “separation time.” 7. Repeat step six for each of the additional test tubes, making sure not to contaminate one test tube with the emulsifier from another. If an emulsion has not separated after 5 minutes, write “>5 mins.” 8. After you have mixed and observed all of the emulsifiers, go back and check to see if any of the emulsions that didn’t separate earlier have now separated. Record any additional observations in the data table. Data/Observations Emulsifier
Separation time (min:sec)
Control (no emulsifier)
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Observations
Analysis 1. Did the mixtures with the emulsifiers take more or less time to separate than your control? Is this what you expected?
2. How would you expect the separation time to change if you added more emulsifier? Why? What about if you added more oil than vinegar?
3. Lemon juice is mostly citric acid and water. Would you expect it to mix better with olive oil or vinegar? Why?
4. Look for recipes for salad dressings or vinaigrettes online. For each, identify which ingredient is the polar molecule (hydrophilic), which ingredient is the nonpolar molecule (hydrophobic), and which ingredient is the emulsifier.
Conclusion Which emulsifier would you recommend using for making salad dressing, based on your data? Why? American Association of Chemistry Teachers
