Download Egg proteins change when you heat them, beat them, or mix them

Name __________________________
Assn ___________
The Science of Cooking with Organic Molecules
From “The Science of Cooking” at www.exploratorium.edu
The Science of Eggs
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
[ball] shape. A variety of weak chemical bonds keep the
protein curled up tight as it drifts placidly in the
water that surrounds it.
Suppose this is a chain of amino acids:
Draw what it might look like when twisted up into a
spherical protein:
Uncooked Egg Proteins
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.
Draw two uncurled proteins, and draw lines between
them representing new bonds:
Cooked Egg Proteins
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.
Add some water molecules to your drawing above,
inserting them in the web of protein bonds. Draw
the water molecules showing one oxygen and two
hydrogen atoms, like this:
I’m making an omelet with 3 eggs. Explain why I
have to cook them to get them to stick together:
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I have cooked my eggs too long, and they are like
rubber! Explain why this happened:
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Beat ’em
When you beat raw egg whites to make a soufflé or a
meringue, you incorporate air bubbles into the waterprotein 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 waterloving. Other amino acids are repelled by water;
they’re hydrophobic, or water-fearing.
Hydrophilic means ___________________________
Hydrophobic means ___________________________
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.
If protein is in water, where will the:
hydrophobic amino acids want to be?
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hydrophilic amino acids want to be?
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Draw a curled up protein with water molecules
outside of it.
Label where the hydrophobic and hydrophilic amino
acids are.
Protein in water with hydrophilic and
hydrophobic amino acids labeled
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 waterfearing 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.
Draw a web of connected proteins, with air bubbles
trapped in the web:
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.
Explain why heating this in my oven will make the
egg whites expand (get bigger):
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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 [spread out] the
oil. To prevent the oil droplets from coalescing
[clumping back together], 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.
Oil belongs to which group of organic compounds:
proteins, lipids, carbohydrates, or nucleic acids?
__________________
Name a food that depends on oil and water being
held together with eggs: ______________________
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.
Draw a sketch that shows how egg proteins can keep
oil and water molecules mixed together:
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.
Lecithin is another molecule that holds fat and
water together. It is called a “phospholipid,” so
it must belong to which group of organic compounds?
___________________________
The Science of
Candy
What is sugar?
**Extra Credit**
The white stuff we know as sugar is sucrose, a molecule
composed of 12 atoms of carbon, 22 atoms of hydrogen,
and 11 atoms of oxygen (C12H22O11). Like all compounds
made from these three elements, sugar is a
carbohydrate. It’s found naturally in most plants, but
especially in sugarcane and sugar beets—hence their
names.
Sucrose is actually two simpler sugars stuck together:
fructose and glucose. In recipes, a little bit of acid
(for example, some lemon juice or cream of tartar) will
cause sucrose to break down into these two components.
Sucrose is made of a molecule of ________________
and a molecule of ________________ bonded together.
Sucrose has ____ carbon atoms, _____ oxygen atoms,
and ____ hydrogen for a total of _______ atoms.
What happens when you heat a sugar solution?
When you add sugar to water, the sugar crystals
dissolve and the sugar goes into solution. But you
can’t dissolve an infinite amount of sugar into a fixed
volume of water. When as much sugar has been dissolved
into a solution as possible, the solution is said to be
saturated.
The saturation point is different at different
temperatures. The higher the temperature, the more
sugar that can be held in solution.
When you cook up a batch of candy, you cook sugar,
water, and various other ingredients to extremely high
temperatures. At these high temperatures, the sugar
remains in solution, even though much of the water has
boiled away. But when the candy is through cooking and
begins to cool, there is more sugar in solution than is
normally possible. The solution is said to be
supersaturated with sugar.
How does cooking ingredients to a high temperature
allow us to make a sweeter candy (hint: “sweeter”
= more sugar)?
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Supersaturation is an unstable state. The sugar
molecules will begin to crystallize back into a solid
at the least provocation [disturbance]. Stirring or
jostling of any kind can cause the sugar to begin
crystallizing.
Why are crystals undesirable in some candy recipes—and
how do you stop them from forming?
The fact that sugar solidifies into crystals is
extremely important in candy making. There are
basically two categories of candies - crystalline
(candies which contain crystals in their finished form,
such as fudge and fondant), and noncrystalline, or
amorphous (candies which do not contain crystals, such
as lollipops, taffy, and caramels). Recipe ingredients
and procedures for noncrystalline candies are
specifically designed to prevent the formation of sugar
crystals, because they give the resulting candy a
grainy texture.
How do crystalline and noncrystalline (amorphous)
candies differ?
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One way to prevent the crystallization of sucrose in
candy is to make sure that there are other types of
sugar—usually, fructose and glucose—to get in the way.
Large crystals of sucrose have a harder time forming
when molecules of fructose and glucose are around.
Crystals form something like Legos locking together,
except that instead of Lego pieces, there are
molecules. If some of the molecules are a different
size and shape, they won’t fit together, and a crystal
doesn’t form.
A simple way to get other types of sugar into the mix
is to "invert" the sucrose (the basic white sugar you
know well) by adding an acid to the recipe. Acids such
as lemon juice or cream of tartar cause sucrose to
break up (or invert) into its two simpler components,
fructose and glucose. Another way is to add a
nonsucrose sugar, such as corn syrup, which is mainly
glucose. Some lollipop recipes use as much as 50% corn
syrup; this is to prevent sugar crystals from ruining
the texture.
Explain how adding cream of tartar to my cooked
sugar mixture will prevent crystals from forming:
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Fats in candy serve a similar purpose. Fatty
ingredients such as butter help interfere with
crystallization—again, by getting in the way of the
sucrose molecules that are trying to lock together into
crystals. Toffee owes its smooth texture and easy
breakability to an absence of sugar crystals, thanks to
a large amount of butter in the mix.
Explain how adding butter to my cooked sugar
mixture will prevent crystals from forming:
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