Making Mayonnaise-An Edible Emulsion

Angela D. Patrick              Crown Community Fine Arts Academy
                               2128 S. St. Louis
                               Chicago IL 60623
                               (312) 534-1680

Objectives:

Main objective aimed for primary grades, but has extensions for the intermediate 
grade levels. 

Students will:
**Conclude that oil and water are immiscible and can not be mixed.
**Discover that an emulsifying agent can be added to two immiscible liquids to 
  stabilize the two liquids to each other when mixed.
**Conclude that the mixture of the three substances is called an emulsion.
**Explain why an emulsion is not a solution.

Materials Needed:

For the teacher:               Per student group:
large jar with lid             two egg yolks
measuring cups                 1/4 cup vinegar
salad oil                      1/4 cup salad oil
water                          one small clear jar with lid
food coloring (optional)       one small cup or bowl
                               four small hand lens
                               1/4 tsp prepared mustard
                               1/4 tsp table salt
                               data sheets
Strategies:

**Number One:
Have students see you measure out two - three cups of water and pour into the 
large jar.  Add about 4 - 6 drops of food coloring to the water and mix 
(optional).  Next measure out two - three cups of salad oil and pour into the 
jar. 

Have the students observe what happens.  Then pick a student volunteer to shake 
up the contents.  Again, have the students observe what happens.  Let the 
contents of the jar sit for 3 - 5 minutes. 

Questions to ask:
What is happening to the oil?
Why do you think that is happening?
Why is the oil in the top layer?

Explanation:
It is impossible for oil and water to mix together.  They are called immiscible- 
meaning that the two liquids can not be mixed together into a solution.  When 
shaken, the oil droplets become suspended in the water.  Eventually, the oil 
droplets enlarge to become a separate layer on top of the water.  The oil is on 
the top layer because it is less dense than the water. 

**Number Two:
Tell students that they are now going to make some mayonnaise, but they must now  
get some oil and water to mix.  Have them predict what would be needed to help 
the oil and water mix together.  (Optional:  create a prediction chart.)  Next, 
have each student count off to create groups of four. 

Experiment #1
Procedure for students:
1.  Pour the vinegar into your jar.
2.  Pour the salad oil into the jar and put on the lid.
3.  Shake the contents very hard for 3 - 4 minutes, then let it sit 3 - 4 
    minutes. 
4.  Compare your jar with your teacher's jar.

Questions for data sheet:
1.  What happened in your jar?
2.  Which liquid remained on top?
3.  How does this compare to your teacher's jar?
4.  What does this mean for the vinegar?

Explanation:
When shaken, the oil and vinegar seemed to mix.  But when the contents were 
allowed to sit, the two liquids formed two separate layers.  The oil remained in 
the top layer.  As a result, this means that the vinegar must be made mostly of 
water. 

Experiment #2
Procedure for students:
1.  Put the egg yolks into the small cup or bowl.
2.  Beat the yolks until they are liquid.
3.  Pour the beaten yolks into the jar with the oil and vinegar and close the 
    lid.
4.  Shake the contents very hard for 3 - 4 minutes.
5.  Observe what happens.

Questions for data sheet:
1.  Describe what happened in the jar now.
2.  Why do you think this happened?
3.  Put the hand lens to the jar.  What do you see?

Explanation:
The oil and the vinegar (water) mixed together much better when the egg yolk was 
added.  The lecithin, which is a protein in the egg yolk, acts as an emulsifying
agent.  Emulsifying agents have regions that associate with the oil and regions 
that associate with the vinegar (water).  Thus the emulsifier (the egg yolk), 
acts as a bridge between both the oil and the water. 

The lecithin molecules in the egg yolk formed a layer around the oil droplets 
and prevented the tiny droplets from coming together to make a separate layer.  
The egg yolk then enabled the oil to mix with the vinegar.  The mayonnaise that 
is being created is an emulsion of oil droplets in water.  This means that it is 
not a solution.  A solution is made when two or more liquids are totally mixed 
together.  Because the oil is still separate from the vinegar even with the egg 
yolk present, it can not be a solution.  

The emulsion created here is a dispersion of one liquid (the oil), inside 
another liquid (the water), with an emulsifier (the egg yolk) acting as a bridge 
to the two liquids for stabilization. 

Experiment #3
Procedure for students:
1.  Add the mustard and the salt to the mixture.
2.  Again, shake very hard 3 - 4 minutes.

Questions for data sheet:
1.  Did anything happen to the mixture now?
2.  Taste.  Describe the taste.
3.  How does this mayonnaise differ from the one bought from the store?

Performance Assessment:

Teacher will observe students watching the teacher demonstration with the oil 
and water, shaking and rolling the jar so that the students can conclude that 
oil and water are immiscible.  As the students follow the procedures for each 
mini-experiment, they are to compare their jars to the demonstration jar, and 
complete the data sheets with their observations to determine that another 
substance (an emulsifying agent) added to the immiscible liquids will allow them 
to be somewhat mixed.  The final assessment for this lesson is for the students 
to explain why the emulsion created is not a solution. 

Conclusions:

Students are to be able to conclude from the experiment that emulsions and 
solutions are different.

References:

Chemical Activities by Christie L. Borgford and Lee R. Summerlin.  The American 
Chemical Society, 1988.