EGGS-ACT-LY!
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 350°F. 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 350°F 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.
|
Control |
Oven
Variation |
2
eggs |
Microwave |
|
Consistency |
Consistency |
Consistency |
Consistency |
|
Texture |
Texture |
Texture |
Texture |
|
Flavor |
Flavor |
Flavor |
Flavor |
http://www.all-science-fair-projects.com/science_fair_projects/39/493/f41e6ddf1dad55b3457e2ee80d222225.html
*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://dictionary.reference.com/search?q=coagulate
http://www.deliaonline.com/cookeryschool/howto/how_0000000050.asp
http://pentesilea.com/menu/desert.htm
http://rds.yahoo.com/S=96062883/K=egg/v=2/SID=e/l=IVR/SIG=12720mf9q/EXP=1115300376/*-http%3A//www.memphilter.com/blog/archives/000385.html
http://rds.yahoo.com/S=96062883/K=egg+diagram/v=2/SID=e/l=IVR/SIG=12knjbbnv/EXP=1115299469/*-http%3A//www.borg.com/ ~svcselem/sauquoit/clubs/wingers/egg/egg.htm
http://www.all-science-fair-projects.com/science_fair_projects/39/493/f41e6ddf1dad55b3457e2ee80d222225.html
Christen,
Genevieve L. and J. Scott Smith. Food
Chemistry: Principles and Applications. California, 2000. Pages 422-424.