Objective: To make an oil-in-water emulsion

Background: Some liquids cannot mix by themselves. When combined they tend to separate. We call these liquids immiscible, for example oil and water. Being immiscible makes it difficult to produce food products containing these two liquids without affecting quality and appearance. However we can fix the problem by using an emulsifier to create an emulsion. An emulsion is formed by first breaking up one of the liquids and dispersing it homogeneously into the other. After that, an emulsifier is added to prevent the liquids from separating. A common emulsifier used for this purpose is lecithin. Lecithin is found abundantly in egg yolks and soybeans. It has a unique structure that allows it to bind both water and oil. This is called an amphiphilic property, i.e., it is both hydrophilic (water loving) and lipophilic (fat loving).

There are two main type of emulsions. Water-in-oil and oil-in-water. Oil-in-water emulsions are more common. Examples are mayonnaise, salad dressing and milk, where tiny droplets of oil is dispersed in water. The oil is called the dispersed phase and the water is called the continuous phase. It is interesting that the tongue senses the continuous phase and not the dispersed phase. That is why although mayonnaise have much more oil to vinegar, it does not taste oily. An example of a water-in-oil emulsion is butter and margarine, where the water is dispersed in the oil. The oil is the continuous phase, so it produces a greasy texture on the tongue. The figures below illustrates an example of an oil-in-water and a water-in-oil emulsion.

In this lab, you will be making an oil-in-water emulsion. The emulsion that you will make is the same procedure for making mayonnaise. I have kept the procedure simple to reduce the ingredients required and the need to pasteurize the egg. Therefore, the product may not be safe for consumption. If you want to make a batch of mayonnaise that you can eat and serve your family, you will have to pasteurize the egg first.

Materials

  1. Cooking oil
  2. Eggs
  3. Wisk (or mixing spoon, or electric hand mixer)
  4. Water
  5. Tablespoon
  6. Mixing bowl

Formulation 

  1. 2 cups cooking oil
  2. 2 table spoons water
  3. 2 egg yolks 

Procedure

  1. Add two egg yolks (yellow part of egg) to the mixing bowl
  2. Add two table spoons of water to bowl
  3. Now slowwwwwwllly and gradually add oil to the bowl while mixing rigorously. I suggest adding the oil in small batches at a time and making sure all the oil is thoroughly mixed out before adding more. The liquid will be runny at first, but as you progress it should thicken to a mayonnaise-like texture. Too much oil at a time will break the emulsion causing oil and water to separate permanently. If this happens you will have to start over. The finished product should look similar to this:

Courtney Simons
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Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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