GENERAL CHEMISTRY LABORATORY
Module #5 Egg Science
Task list
Read course and learning outcomes
Read study guide prior to class attendance
Read required learning resources and watch the videos; refer to terminologies for jargons
Perform easy to do experiments at home
Proactively participate in classroom discussions
Participate in weekly discussion board (Canvas)
Answer and submit course unit tasks
Learning Outcomes
At the end of this unit, the students are expected to:
1. Demonstrate the protein denaturation as a type of chemical change
2. Know the principles behind cooking proteins
Study Guide
Open Educational Resources:
Denaturing a protein
https://youtu.be/Zk_4WAcY44g
Egg Cooking Basics- Egg Anatomy
https://youtu.be/UzQXw9EsR3k
This Photo by Unknown Author is
The Egg’s Shell
It makes sense to start from the outside of the egg and work our way in, so let’s begin with the egg’s
shell. It’s made primarily from calcium carbonate, the chemical compound which also makes up the
majority of sea shells, as well as chalk and limestone. Nanoparticles of calcium carbonate are arranged
into ordered crystals by proteins, eventually forming the calcite mineral that makes up the shell. The
shell isn’t actually completely solid – it has thousands of tiny pores, around 9,000 on average, which
allow gases to pass in and out. As we’ll see later, this can have implications for cooking.
The colour of egg shells can also vary; chicken eggs tend to be somewhere on a colour spectrum
between white and brown, but the eggs of other avian species can also encompass blue or green hues.
This colouration is due to the deposition of pigment molecules on the eggshell whilst it is being formed
in the chicken’s oviduct. One pigment, protoporphyrin IX, gives shells a brown colour. This pigment is a
precursor of haemoglobin, the oxygen-carrying compound found in blood. Other pigments, such as
oocyanin which gives blue and green colours, are side-products from the formation of bile. White egg
shells have an absence of pigment molecules.
The Egg White
Once inside the egg, we first come to the egg white, or albumen. The egg white is formed of a number
of different layers, and is in fact mostly composed of water (90%). A range of proteins make up the
majority of the remaining 10%, serving a number of varying purposes. Some, such as ovalbumin, are
thought to provide nourishment for the developing chick, whilst also blocking the action of digestive
enzymes. Another, conalbumin or ovatransferrin, binds iron atoms tightly, both to prevent their use by
bacteria and hence help prevent infection, but also to ensure a supply of iron for the developing
chick. Finally, one of the most important proteins in the albumen in terms of the egg white’s consistency
is ovomucin. This protein helps to thicken the egg white and give it its gloopy consistency.
The Egg Yolk
The egg’s yolk is made up of a number of spherical compartments. Unlike the egg white, which
contains very little fat, the yolk contains a significant amount of fatty acids such as oleic acid, palmitic
acid, and linoleic acid, as well as high level of cholesterol. It also contains fat soluble vitamins (A, D, E,
and K).
The colour of the yolk is a consequence of the presence of two chemical compounds: lutein and
zeaxanthin. These are both compounds known as xanthophylls, and can also be classed as carotenoid
compounds; they are hence members of the same chemical family to which beta-carotene, the
chemical that gives carrots their orange colour, belongs. A chicken’s feed can influence the colour of
the yolk, and for this reason beta-carotene-containing substances, or even marigold petals, can be
added to chicken feed to enhance the colour. Interestingly, including the main colour-creating
compounds in red peppers, capsanthin and capsorubin, in chicken feed can cause the yolks to appear
a deep orange or even red.
Egg proteins change when you heat them, beat them, or mix them with other ingredients.
Understanding these changes can help you understand the roles that eggs play in cooking.
Proteins are made of long chains of amino acids. The proteins in an egg white are globular proteins,
which means that the long protein molecule is twisted and folded and curled up into a more or less
spherical shape. A variety of weak chemical bonds keep the protein curled up tight as it drifts placidly in
the water that surrounds it.
Heat ’em
When you apply heat, you agitate those placidly drifting egg-white proteins, bouncing them around.
They slam into the surrounding water molecules; they bash into each other. All this bashing about
breaks the weak bonds that kept the protein curled up. The egg proteins uncurl and bump into other
proteins that have also uncurled. New chemical bonds form—but rather than binding the protein to
itself, these bonds connect one protein to another.
After enough of this bashing and bonding, the solitary egg proteins are solitary no longer. They’ve
formed a network of interconnected proteins. The water in which the proteins once floated is captured
and held in the protein web. If you leave the eggs at a high temperature too long, too many bonds form
and the egg white becomes rubbery.
Experiment with heating eggs by hard cooking eggs, by making deviled eggs, or by making flan.
Beat ’em
When you beat raw egg whites to make a soufflé or a meringue, you
incorporate air bubbles into the water-protein solution. Adding air bubbles to
egg whites unfolds those egg proteins just as certainly as heating them.
To understand why introducing air bubbles makes egg proteins uncurl, you
need to know a basic fact about the amino acids that make up proteins. Some
amino acids are attracted to water; they’re hydrophilic, or water-loving. Other
amino acids are repelled by water; they’re hydrophobic, or water-fearing.
Egg-white proteins contain both hydrophilic and hydrophobic amino acids.
When the protein is curled up, the hydrophobic amino acids are packed in the center away from the
water and the hydrophilic ones are on the outside closer to the water.
When an egg protein is up against an air bubble, part of that protein is exposed to air and part is still in
water. The protein uncurls so that its water-loving parts can be immersed in the water—and its water-
fearing parts can stick into the air. Once the proteins uncurl, they bond with each other—just as they did
when heated—creating a network that can hold the air bubbles in place.
When you heat these captured air bubbles, they expand as the gas inside them heats up. Treated
properly, the network surrounding bubbles solidifies in the heat, and the structure doesn’t collapse
when the bubbles burst.
Mix ’em up
Everyone knows that, left to their own devices, oil and water don’t mix. But for many recipes, you
mix oil-based and water-based liquids—and want them to stay that way. Often, egg yolks come to
your rescue by creating an emulsion.
Most food emulsions are known as the oil-in-water type, which means that oil (or fat) droplets are
dispersed throughout the water. Put oil and water in a jar, shake it vigorously, and you’ll disperse
the oil. To prevent the oil droplets from coalescing, however, a substance known as an emulsifier is
required. Egg yolk contains a number of emulsifiers, which is why egg yolks are so important in
making foods such as hollandaise and mayonnaise.
Many proteins in egg yolk can act as emulsifiers because they have some amino acids that repel
water and some amino acids that attract water. Mix egg proteins thoroughly with oil and water,
and one part of the protein will stick to the water and another part will stick to the oil.
Lecithin is another important emulsifier found in egg yolk. Known as a phospholipid, it’s a fatlike
molecule with a water-loving “head” and a long, water-fearing “tail.” The tail gets buried in the fat
droplets, and its head sticks out of the droplet surface into the surrounding water. This establishes
a barrier that prevents the surface of the fat droplet from coming into contact with the surface of
another fat droplet.
Home Experiment:
A. Sunny-side up egg:
1. Crack the egg open.
2. Put the egg on a plate. Observe.
3. Put oil in a frying pan and fry an egg. Observe.
B. Hard boiled egg (3eggs):
1. Prepare a pot of boiling water and a bowl of cold water.
2. Put one egg to boil for 3 mins.
3. Get the egg out of the pot (you may use a strainer. Be very careful!!) and gently immerse it into
the bowl of cold water.
4. Repeat step 2 for the 2 eggs (for 6 minutes and 9 minutes respectively) and step 3 as well.
5. Peel the hard boiled eggs and cut it into half. Observe.
C. Mayonnaise
1. Crack the egg and put the egg into a bowl (preferably glass/metal)
2. Put coconut oil about twice the volume of the egg. Add a dash of salt and pepper. Add 1
teaspoon of vinegar/calamansi extract
3. Whisk the egg constantly and vigorously until the mixture becomes thick and the oil is
incorporated on the egg.
4. Optional (you may add chopped pickles/bacon to your mayo)
Terminologies
Denaturation: the change of folding structure of a protein (and thus of physical properties)
caused by heating, changes in pH, or exposure to certain chemicals
Each protein has its own unique sequence of amino acids and the interactions between these
amino acids create a specify shape. This shape determines the protein’s function, from digesting
protein in the stomach to carrying oxygen in the blood.
Changing the Shape of a Protein
If the protein is subject to changes in temperature, pH, or exposure to chemicals, the internal
interactions between the protein’s amino acids can be altered, which in turn may alter the shape
of the protein. Although the amino acid sequence (also known as the protein’s primary structure)
does not change, the protein’s shape may change so much that it becomes dysfunctional, in
which case the protein is considered denatured. Pepsin, the enzyme that breaks down protein
in the stomach, only operates at a very low pH. At higher pHs pepsin’s conformation, the way its
polypeptide chain is folded up in three dimensions, begins to change. The stomach maintains a
very low pH to ensure that pepsin continues to digest protein and does not denature.
Enzymes
Because almost all biochemical reactions require enzymes, and because almost all enzymes
only work optimally within relatively narrow temperature and pH ranges, many homeostatic
mechanisms regulate appropriate temperatures and pH so that the enzymes can maintain the
shape of their active site.
Reversing Denaturation
It is often possible to reverse denaturation because the primary structure of the polypeptide, the
covalent bonds holding the amino acids in their correct sequence, is intact. Once the denaturing
agent is removed, the original interactions between amino acids return the protein to its original
conformation and it can resume its function.
However, denaturation can be irreversible in extreme situations, like frying an egg. The heat
from a pan denatures the albumin protein in the liquid egg white and it becomes insoluble. The
protein in meat also denatures and becomes firm when cooked.
Figure: Denaturing a protein is occasionally irreversible: (Top) The protein albumin in raw
and cooked egg white. (Bottom) A paperclip analogy visualizes the process: when cross-
linked, paperclips (‘amino acids’) no longer move freely; their structure is rearranged and
‘denatured’.
Unit Tasks
Study Questions
1. What are the parts of the egg and their functions?
2. What is the nutritional value of an egg?
3. What happens when the egg is exposed to heat or kinetic energy?
4. What is happening when the egg is beaten and whisked vigorously?
5. What happens when a protein becomes denatured (e.g. enzymes)?
6. Is denaturation an irreversible change to the protein structure?
References
(n.d.) Science of Eggs. Retrieved from:
https://www.exploratorium.edu/cooking/eggs/eggscience.html
(2016) Chemistry of Eggs and Egg Shells. Retrieved from:
https://www.compoundchem.com/2016/03/26/eggs/
