A web page designed to help high school students understand the coagulation of egg proteins.


Some background on the egg...

An egg is a truly amazing structure. It has various layers, with each one contributing differently to the overall make up of the egg. Below is a diagram of the major parts of the egg.



1. The inner and outer shell membranes provide protection against intruding bacteria and surround the albumen.

2. The albumen has both thin and thick parts. The thin is closest to the shell and it protects the thick white in a high quality egg. The thick albumen is the major component of the egg. It is the major source of riboflavin and protein (which we'll dive into more).

3. The yolk is the yellow portion of the egg and serves as the egg's major source of vitamins, minerals, and fat.

4. The chalaza is a twisted, cordlike structure that anchors the yolk into the center of the egg for protection. A prominent chalazae indicates freshness.


A little more detail on protein...

Like I said earlier, the majority of the egg's protein can be found in the albumen. The range of protein content (9.7 - 10.6%) can be contributed to the age of the hen. The separation of egg white proteins can yield sixteen to nineteen different constituents, but the only one needing to be mentioned is ovalbumin. It is the major protein in the albumin and it coagulates on exposure to heat. The chemistry behind it is that the heat causes the protein to denature.

coagulate = To change from the liquid state to a solid or gel; clot.

Time to experiment...

Protein Coagulation

Purpose: To determine the influence of heat on thermal coagulation of egg proteins. This will be accomplished by preparing custards with varied heating conditions and egg protein amounts.

Ingredients: The amounts below will yield 2 custards. Vary your recipe according to your number of students.

1. 1 cup of milk

2. 25 g sugar

3. 1 egg (approx. 48 g)

4. 1 tsp vanilla

5. pinch of salt

General Procedure: The steps below should be done for each formulation.

1. Scald milk in double boiler.

2. Beat egg slightly (white and yolk should be thoroughly mixed but not foamy).

3. Stir sugar and salt gradually into egg.

4. Pour scalded milk into egg-sugar mixture, stirring constantly.

5. Add 1/4 tsp. vanilla and stir.

Pour the mixture into custard cups and cook according to the directions given below for the appropriate treatment.

Detailed Procedure:

There are 3 experiments to vary how the egg protein will coagulate. Each is detailed below.

The Control:

Preheat oven to 350F. Prepare custard mixture as directed above. Set custard cup in a Pyrex baker as deep as the custard cup. Fill the baker with hot water to the level of the custard in the cup. Place in oven. Bake until the tip of a sharp knife inserted halfway between the center and edge of the custard comes out clean. (Approx. 40-50 min.) Remove immediately from hot water and place on a rack to cool, then evaluate custard.

Oven Variation:

Into a 350F oven as in the Control but omit the water bath. Remove when a knife inserted into the custard comes out clean. Record the baking time.

Egg/Protein Variation:

Prepare custard according to the control using 2 eggs (96 g).

Microwave Variation:

Cover the custard cup with plastic wrap. Poke a few holes in the plastic to allow steam to escape. Adjust microwave time and power to get the best product you can.

*After reading each variation I would have your students write down their hypothesis to each.


Evaluation and Conclusion:

After preparing the various custards I would recommend having a mock sensory tasting to evaluate the various properties of the custards. Below is a chart to possibly aid in evaluations.



Oven Variation

2 eggs














*The consistency should be a continuous clotted mass (gel) firm enough to hold its shape and to hold fairly sharp angles when cut.

*The texture should be smooth and homogenous. No porosity or clumping.

*The flavor should be delicate with no pronounced egg flavor.

Obviously each variation on the custard is not going to abide by each of the standards of sensory. A few questions to ask your students would be:

1. Which custard was your favorite and why?

2. Least favorite and why?

3. Why did the water bath help the quality? Or did it at all?

4. Did another egg make a difference is quality? If so, why?

5. How did the microwave effect coagulation?

6. If you had to create a new variation on the procedure, what would you do? What results would you expect?

After carrying out this experiment, hopefully your students will understand better the effect heat has on the coagulation of egg proteins.

Sources:*-http%3A//*-http%3A// ~svcselem/sauquoit/clubs/wingers/egg/egg.htm


Christen, Genevieve L. and J. Scott Smith. Food Chemistry: Principles and Applications. California, 2000. Pages 422-424.