SPU 27
Practice Questions for First Exam
Fall 2013
Problem 1: Unsalted vs. Salted Butter (by Denise)
It’s your friend’s birthday and you have all gathered in a lake house to celebrate. Your friend
has heard about your famous apple strudels and asks you to make some for her birthday. You agreed
and she went ahead and got all the ingredients you asked her for (you are doing all the work, after
all). A recipe for 6 strudels reads as follows:
2 tbsp.
3 tbsp.
1
⁄4 tsp.
1
⁄3 cup + 1 tbsp.
1
⁄2 tsp.
1 stick
11⁄2 cups
1
⁄2 cup
2 lbs.
Strudel Filling
apple juice
raisins
ground cinnamon
sugar
salt
unsalted butter, melted, divided
fresh bread crumbs
coarsely chopped walnuts
tart cooking apples, peeled, cored
Strudel Dough
1 ⁄3 cups unbleached flour
1
⁄8 tsp.
salt
7 tbsp. water
2 tbsp. vegetable oil
1
⁄2 tsp.
cider vinegar
1
You decide to prepare the strudel filling first and realize that she accidentally bought salted butter
instead of the unsalted variety! You begin to worry until you remember that the filling has some salt
anyway.
1. What is the mass of salt required for the strudel filling? (The density of table salt is 2.16 g/cm3 ,
1 teaspoon = 5 mL.)
2. Using the nutritional information for salted butter given at
right, calculate how much table salt (NaCl) there is in a stick
of butter (1 stick = 8 tbsp., 1 tbsp. of butter = 14 g). The atomic
mass of sodium is 23 amu and the atomic mass of chlorine
is 35.5 amu.
3. Given the amount of salt already in the butter, how much
extra salt (in grams) would you need to add to match the
total amount in the original recipe?
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SPU 27
Practice Questions for First Exam
Fall 2013
You realize your friend made another mistake. There’s no salt! You mumble to yourself and
decide that the show must go on. The strudel dough instructions read:
1. Combine the flour and salt in a bowl.
2. Mix the water, oil and vinegar and add mixture to flour slowly.
3. Knead the dough until you have a soft ball with rough surface.
4. Shape into a ball and leave covered for 90 minutes.
5. Flour a large surface and roll out dough until it’s about 2 ft. by 3 ft. in area.
4. Give a plausible reason with an explanation why you think the recipe calls for leaving the ball
for 90 minutes before rolling it out.
You go ahead and make the dough as you always do but without the salt. Once it’s time to stretch it
out, you can’t get the dough to stretch out to 2 ft. by 3 ft. in area.
5. Develop a testable hypothesis for why not adding the requisite amount prevented the dough
from stretching out fully.
6. Design a controlled experiment to test your hypothesis. Indicate how you would interpret
your results.
Problem X: Dry Aging (by Sami)
In your problem set, you examined the elasticity changes involved in the dry-aging of meat in
a cold meat locker ( 4○ C).
1. A beef sirloin steak weighs 85 grams and contains 25 grams of protein and 8 grams of fat.
Assuming that the density of the steak is the same as water and that all of the protein is myosin
(MW = 500,000 amu) what is the initial molarity of protein in steak?
2. Beef is dry aged for 20 days, during which time it loses one third of its weight. What is the
final protein molarity, assuming the protein remains unchanged?
3. As we learned, the proteins DO change. They are broken down by enzymes called calpains.
Calpain in beef, when measured at room temperature, can degrade 1 nmol protein per second
per gram of steak. How long would it take for all of the protein in one steak to be degraded?
4. Comment on how your answer compares to the actual aging length of beef. Can you come up
with a reason why this value might be too low or too high?
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SPU 27
Practice Questions for First Exam
Fall 2013
Problem X: Cheese (by Tim)
Cheese-making is essentially a process that increases crosslinking between milk proteins to
create a solid or semi-solid substance. Harder cheeses such as parmesan have very high elastic
moduli, while softer cheeses such as brie do not.
1. Knowing that cooking many foods can result in increased bond density by protein denaturation and aggregation, you try to “toughen” a wheel of brie cheese so that you can make a
fancy dish using the brie cheese as a tiny plate to hold a bread soup. However, upon heating,
the brie melts instead of toughening. You hypothesize that this is largely due to the high fat
content of brie cheese relative to other cheeses.
(a) Assuming your hypothesis is correct, how do you expect the fatty acids in brie triglycerides compare to those found in other cheeses?
(b) Design an experiment to test your hypothesis. Assume that you have access to any common grocery store ingredients and any equipment found in the labs we’ve run in class
so far. Your answer does NOT have to be in essay format - a numbered list of steps with
clearly noted controls and variables is acceptable. Include an interpretation of potential
results.
2. One of the reasons that cheeses don’t behave like meats when cooked is due to the fact that the
protein denaturation/re-aggregation of meat cooking is the same basic process used to make
most cheeses. In the ricotta cheese lab, we used both heat and a low pH to coagulate cheese
proteins. Were you to heat the ricotta further, you would find that it would not melt. However,
many cheeses are made using rennet, a mixture of enzymes that can selectively cleave κ-casein
molecules at the surface of casein micelles in a way that eliminates their negative charge.
(a) Given the above information, draw a picture of what you might expect the final network
to look like for a rennet-derived cheese.
(b) Treatment of milk at very low pH leads to complete dissociation of casein micelles. Draw
a picture of what you might expect the final network of a cheese made using only milk
acidification to look like.
(c) Which of these two types of cheeses would you expect to have a higher elasticity? Justify
your answer.
Problem X: Steam Room (by Greg)
You want to make your kitchen into a home steam room, but right now your kitchen is a mess.
The sink is broken, and the (1,000 watt) stove will only stay on for 20 minutes. Luckily you do have
a freezer full of ice, kept at -5 degrees Celsius. You want to use just the right amount of ice to heat
on the stove to maximize the amount of steam. We will figure this out in steps. Assume that all of
the energy used by the stove goes into heating the ice.
1. Using the variable x to denote the mass of ice in grams, write the amount of heat needed to
raise it from its original temperature to 0○ C.
2. Write the amount of heat needed to turn our x grams of 0○ C ice into 0○ C water.
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SPU 27
Practice Questions for First Exam
Fall 2013
3. Write the amount of heat needed to raise that water to 100○ C degrees Celsius (in terms of x).
4. Write the amount of heat need to turn our 100○ C water into 100○ C steam.
5. Add all of these and compare to the amount of heat one could get from the stove in twenty
minutes to determine the optimal amount of ice to use in order to make as much steam as
possible.
Problem X: Phase Diagram Interpretation (by Greg)
Above is a phase diagram for a substance much like water.
1. Label the places where you would expect there to be solid, liquid and gas.
2. Which could you use to move up vertically on the graph (circle all that apply)
(a) pressure cooker
(b) rotovap
(c) sous vide
3. Which could you use to move vertically down on the graph (circle all that apply)
(a) pressure cooker
(b) rotovap
(c) sous vide
4. Is it possible to use a pressure cooker to turn this substance from a liquid into a solid?
5. Is the same true for water? (hint: think about where the ice is when a pond begins to freeze)
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SPU 27
Practice Questions for First Exam
Fall 2013
Problem X: Lobster Bisque (by Miriam)
You’re a French chef cooking for the court of Louis XVI. King Louis has demanded you cook
a meal for the royal banquet. You decide to make lobster bisque.
Ingredients:
Food
Water
Lobster
Vegetables
Cream
• 10 cups (2.4 liters) of lobster stock water
.
• 1.5 pounds ( 680 grams) of lobster meat
• 3 cups (700 g) of mixed vegetables
• An unknown amount of heavy cream
Specific heat capacity
4.18 J/g⋅K
3.43 J/g⋅K
3.5 J/g⋅K
3.77 J/g⋅K
.
1. To make the bisque, you first heat the water, lobster, and vegetables (the cream comes in later).
What is the specific heat of this mixture?
2. Assuming the bisque starts at room temperature (23○ C), how much heat do you need to add
to get the bisque to 80○ C?
3. Once the lobster has cooked, it’s time to add the cream. However, you’re not exactly sure how
much to add. You take the bisque off the fire, where it was at 80○ C, and quickly add the cream,
which is at 4○ C. What will the final temperature of the bisque be, expressed in terms of the
mass of the cream you put in?
Last week Louis insulted your cooking in front of the whole court, and you’ve decided it’s time
to exact your revenge. You are going to add a poison derived from snake venom to the bisque.
Unfortunately, the active proteins in the snake venom denature a little above 60○ C, so you need to
cool the bisque before putting the venom in.
4. How much cream should you add such that the temperature of the bisque is just below 60○ C
before you add the venom?
5. The molarity of the active proteins in the snake venom is 1 mM. The poison needs to have
a concentration of 1 nM to remain lethal. How much venom should you add to ensure that
Louis never questions your cooking again?
6. Using this amount of venom, how many protein molecules will there be in the entire pot of
bisque?
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SPU 27
Practice Questions for First Exam
Fall 2013
Problem X: Tiramisu (by Nefeli)
Ingredient List:
Directions:
• 6 egg yolks
1. In a medium saucepan, whisk together egg yolks and
sugar until well blended. Whisk in milk and cook over
medium heat, stirring constantly, until mixture boils.
Boil gently for 1 minute, remove from heat and allow
to cool slightly. Cover tightly and chill in refrigerator 1
hour.
• 3⁄4 cup white sugar
• 2⁄3 cup milk
• 11⁄4 cups heavy cream
• 1⁄2 teaspoon vanilla extract
• 1 pound mascarpone cheese
• 1⁄4 cup strong brewed coffee,
room temperature
• 2 tablespoons rum
• 2 (3 ounce) packages ladyfinger
cookies
• 1 tablespoon unsweetened cocoa
powder
2. In a medium bowl, beat cream with vanilla until stiff
peaks form. Whisk mascarpone into yolk mixture until smooth.
3. In a small bowl, combine coffee and rum. Split ladyfingers in half lengthwise and drizzle with coffee mixture.
4. Arrange half of soaked ladyfingers in bottom of a 7 x 11
inch dish. Spread half of mascarpone mixture over ladyfingers, then half of whipped cream over that. Repeat
layers and sprinkle with cocoa. Cover and refrigerate 4
to 6 hours, until set.
1. Why does the recipe require cooking the egg yolks, milk and sugar on the stovetop? What
happens to the mixture while it is being cooked?
2. Why does the recipe indicate to boil gently for 1 minute and remove the pan from heat shortly
after the mixture reaches boiling?
Ingredient
Milk
Egg Yolks
Sugar
Specific Heat (J/g⋅K)
3.93
0.012
1.24
Quantity
1 cup
1
1 cup
Mass Conversion (g)
240 g
18 g
200 g
3. What is the specific heat of the custard mixture?
4. What is the energy required by the refrigerator to bring the temperature of the custard down
from its boiling point of 77○ C to 5○ C?
5. How long will it take to cool the custard down to 5○ C given that the fridge uses 600W of
power?
6. Compare your answer to the recipe recommended time of 1 hr. Is it smaller or larger? Why?
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SPU 27
Practice Questions for First Exam
Fall 2013
Problem X: Hot Iced Tea (by Kathy)
Imagine that the first attempt at making hot and cold tea was done using a block of ice (-50○ C)
that filled half a 200 mL cup. It was thought that the hot tea would melt the block of ice and you
would be able to experience the two different temperatures at once.
1. If the hot tea was at 90 ○ C when it was poured into the half ice filled cup, what was the equilibrium temperature that was reached by the solution in the cup?
Heat of vaporization of H2 O
Heat of fusion of H2 O
Specific heat of water
Specific heat of ice
2260 kJ/kg
334 kJ/kg
4.18 kJ/kg⋅K
2.10 kJ/kg⋅K
2. Draw a phase diagram depicting the change that occurs for the ice in the cup. Be sure to label the axes and the key points on the graph, including major temperatures and phase changes.
3. On paper, you’ve found that it would take 3 minutes for the ice to melt and for the temperature
of the system to equilibrate. However, when you poured the hot tea into the cup containing
the ice, you found that it took almost 5 minutes. What are some reasons that it might have
taken longer to melt in real life? What is one method of testing your hypothesis?
Problem X: Popcorn (by Noam)
Popcorn is formed when a small droplet of water trapped in a corn kernel boils quickly.
1. Estimate the mass of a popcorn kernel. Assuming it is 14% water by mass, determine the
volume of water in the kernel.
2. What is the volume of water vapor released when the kernel pops? (assume a density of 0.6
g/L).
3. How does that volume compare to the volume of a corn kernel?
4. Sometimes some kernels don’t pop. What are two possible explanations for why they fail to
pop?
Problem X: Joan Roca’s Chicken (by Matthieu)
Joan Roca’s idea of a perfectly cooked chicken was to make it jump from space and land in your
plate. As final project, you decide to study if this process would have actually cooked the chicken or
if anything different might have happened.
Atmospheric pressure quickly drops with altitude and at 100,000 m it is almost down to 0 Pa.
Temperature also varies a lot with altitude. Both variations are summarized in the joint graph and
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SPU 27
Practice Questions for First Exam
Fall 2013
you also have a phase diagram graph of water. Pressure units are in millibars (1 mbar = 102 Pa).
Let us assume that the chicken is made of 100% of water for the moment.
1. Initially the chicken will jump from 80,000m where we have a pressure of 0.001% of the sealevel pressure. What are the conditions of temperature and pressure at this altitude? What
would be the state of the water in the chicken at this altitude?
2. At the end of the mesosphere, what will be the conditions of temperature and pression? What
will happen to the water during the flight in the mesosphere? Where does it go? At approximatively what altitude will this phenomenon happen?
Due to liberation of the water in the previous phenomenon, we estimate that the chicken has lost
80% of its water by the time it reaches 9,000 m above sea level, the cruising altitude of planes in the
stratosphere.
3. What would someone in a plane see if looking at the chicken falling? Reference the pressure
and temperature at this altitude when giving your answer.
4. At approximately what altitude will something new happen to the water in the chicken? Justify
your answer.
5. Joan’s restaurant is currently at 5 0m above the sea level and the average temperature there is
20○ C. How would the chicken arrive on the table?
6. How would you define a cooked chicken? Would this chicken be considered cooked when
landing? Would it look good? Cooking a chicken would imply having protein denaturing
due to heat (or pH). Here, we had no temperature high enough to denature proteins so the
chicken would not be considered cooked.
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SPU 27
Practice Questions for First Exam
Fall 2013
7. Based on the past question, plot the entire trajectory of the chicken on the phase diagram
above. Indicate the mesosphere, stratosphere, and troposphere part. Indicate the plane and
the restaurant positions.
Problem X: Ice Cream (by Kathy)
Tim and Blair are testing the best ways to make ice cream at home and decide to go to great
lengths to test different methods.
1. Explain why each of the methods would or would not be successful, and indicate the change
that occurs to the phase diagram of ice cream.
(a) Rather than sticking the ice cream base in the freezer, they pour liquid nitrogen over it.
(b) Rather than sticking the ice cream base in the freezer, they stick it in a pressure cooker
and then pour the liquid nitrogen into the vestibule.
2. After testing the ice cream product from the two methods above, Tim and Blair have found
that the ice cream in part 2 retained a creamier texture. Develop a hypothesis for why this
occurred and how you would test it.
Problem X: Candy Making (by Emily)
After making his candy apple, Bill Yosses recruits you to help him gain a better understanding
of the candy making process. He knows that sugar water goes through different stages as it’s heated,
but he doesn’t know if you can relate that to the elasticity.
1. He gives you some sugar water mixtures cooled from different stages in the heating process.
You start by cutting them into cubes with sides of 10 cm. You then place a 1 kg weight on
each cube and measure how much the cube compresses. Use the values below to calculate the
elastic modulus of each stage.
Stage
Hard-Ball
Hard-Crack
Maximum
Temperature
130○ C
149 ○ C
Change
in height
1430 µm
1.65 µm
Elastic modulus
2. What is the approximate percentage by weight of sugar at these temperatures?
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SPU 27
Practice Questions for First Exam
Fall 2013
3. You remember from class that the bond energy between sucrose molecules is Uint =2.7⋅10−17 J.
Using your measurements of the elastic modulus, calculate the bond length (`) for your two
samples. Express your answer in nm.
4. You want to use hard-crack cubes as building blocks to create an elaborate dessert. To maintain structural integrity, your bottom cube shouldn’t compress more than 30 µm. Each cube
has a mass of 1600 g. How many cubes could you stack?
Problem X: Tofu (by Tim)
Tofu is a common food derived from soy milk. First, the soy milk is treated at high heat and
exposed to chemicals that help its proteins coagulate. These soy curds are then allowed to set in a
mold before being cut and packaged.
1. One of the major differences between different kinds of plain tofu that you find in stores is
related to the amount of pressing and draining that the soy curd undergoes while in the mold.
What effect, if any, would you expect pressing and draining to have on the elasticity of the
tofu? Justify your answer.
2. Out of curiosity, you decide one day to measure the elasticity of a piece of extra-firm tofu you
have in your fridge. You find that adding a 0.5 kg weight to the surface of a 5 cm by 5 cm
square of tofu causes it to lower in height from 2 cm to 1.8c m. Assuming that the tofu has
equilibrated to room temperature (25○ C), what is the average crosslink distance, `, between
proteins in the network? Assume that Uint ≈ kB T.
3. Tofu is often cooked through frying, either in a pan or a deep vat. One of the properties of
tofu that changes most notably as a result of frying is the elasticity. Would you expect the
elasticity of tofu to increase or decrease as a result of frying?
4. What other parameter would you expect to affect the elasticity of tofu? (Hint: What does the
tofu making procedure remind you of?)
Problem X: Philly Cheesesteak (by Emily)
Professor Brenner doesn’t eat meat, but he knows a thing or two about Philly cheesesteaks. To
test your understanding of elasticity, he asks you to make one in a very particular way.
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SPU 27
Practice Questions for First Exam
Fall 2013
1. You cook your 8 oz. steak to medium and find that it has an elastic modulus Esteak =18,700
Pa. The interaction energy of crosslinks in the 57○ C steak is approximately equal to thermal
energy. What is the crosslink distance in your steak?
2. Michael claims the cheese should have an elastic modulus about 10 times greater than Esteak .
You pull out all the cheeses you can find in the fridge and start testing them. To determine
the elastic modulus, you decide to place a weight on the surface of each type of cheese.
(a) The only weight you have is your On Food and Cooking textbook (≈ 1400g). How much
force can you exert on your cheeses in Newtons?
(b) Use your force from (a) and the measurements below to calculate the elastic modulus of
each cheese.
Cheese
Parmesan
Mozzarella
Cheddar
Force
Applied
Surface Area
20 mm × 25 mm
20 mm × 40 mm
20 mm × 35 mm
Original
height
20 mm
18 mm
19 mm
Change
in height
1 mm
4 mm
2 mm
Elastic
modulus
(c) Which cheese should you use to make the sandwich?
3. Michael asserts that the steak should be put on top of the cheese, but he only wants the change
in height of the cheese to be 0.1 mm (“barely noticeable!”). How thick should you cut the
cheese if you want it to have the same area as the steak (9 cm x 15 cm)? (Remember that you
have an 8 oz steak and 1 oz ≈ 227 g)
4. You really want to use feta cheese in the sandwich even though it has a higher elastic modulus
than what Professor Brenner wants (Efeta =470,000 Pa). He says he’ll consider the change if
the cheese deforms elastically in the sandwich. You put the feta under the steak and notice
that some of the cheese crumbles. Will Professor Brenner accept your substitution? Explain
your rationale.
Problem X: Gummy Bears (by Michelle)
Your friend recently returned from a trip to Europe with a gift for you. He brought you a
bag of authentic German “Goldbären” (gummy bears). He claims that they are way better than the
American version. So, you decide to buy a bag of the American candy and give them a side-by-side
taste test. You remember the lecture on elasticity in your SPU 27 class and decide to do some more
scientific tests too.
1. You want to make sure that the bears are actually similar enough to test. To do this, you want
to measure the density of each. You look at the nutrition facts and see that one serving size of
the German gummy bears is 40 g for 20 bears. The American bears are 45 g for 21 bears. Both
bears are the same size - 2 cm x 1 cm x 1cm. What is the density of each type of bear? What is
the percent difference between the two? Can we compare them?
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SPU 27
Practice Questions for First Exam
Fall 2013
2. From your taste test, you find that the German gummy bear seems much chewier, so you
decide to test the elasticity. You apply 0.7 N to the top surface (2 cm x 1 cm) of the American
gummy bear. It deforms 0.5 cm. What is the elastic modulus of the American gummy bear?
3. You repeat the same test with the same force on the German gummy bear. It only deforms
half as much as the American one. What is the elastic modulus of the German gummy bear?
4. Draw the stress/strain curves for both gummy bears on one axis. Make sure to label the slope.
Include axis labels and label the lines.
The list of ingredients for each bear is shown below:
German: Glucose syrup; sugar; gelatin; dextrose; fruit juice from concentrate: apple, strawberry, raspberry, orange, lemon, pineapple; citric acid, lemon, pineapple; citric acid; fruit and plant
concentrates: nettle, apple, spinach, kiwi, orange, elderberry, lemon, mango, passionfruit, blackcurrant, aronia, grape; flavorings; glazing agents: white and yellow beeswax; carnauba wax; elderberry
extract; fruit extract from carob; invert sugar syrup
American (Turkish): Corn syrup, sugar, gelatin, dextrose, citric acid, starch, artificial and
natural flavors, fractionated coconut oil, carnauba wax, beeswax coating, artificial colors: yellow 5,
red 40, blue 1
5. What ingredient is responsible for the texture of the candy? How does it cause the “gummyness”?
Problem X: White Bread (Michelle)
It’s the night before the midterm and after long hours of studying about food and science,
you’re really hungry. You decide to get some sandwich-making materials at CVS to carry back to
your dorm. You pick up some white bread and your favorite lunch meat. When bagging, you accidentally put the loaf of white bread vertically into the bag, so all the weight is resting on the bottom
slice. Are you going to lose out on a sandwich because the bottom slice is too squished? Find out by
answering the questions below.
One slice of bread is 10 cm long, 10 cm wide, and 10 mm thick. The whole loaf has 24 slices
and has a mass of 672 g.
1. What molecule is responsible for the elasticity in your white bread?
2. The elastic modulus of white bread is about 20 kPa. If the interaction energy of this molecule
is 4 × 10−19 J, what is the average crosslink distance?
3. If we hold the loaf vertically, how much force is on the bottom slice? (Assume uniform shape).
4. What is the stress on the bottom slice?
5. What is the strain on the bottom slice?
6. What is the total deformation of the bottom slice? Is it noticeable?
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SPU 27
Practice Questions for First Exam
Fall 2013
Problem X: Consider your teeth (by Anjali)
The protective outer part of teeth is made up of tooth enamel, the hardest substance in the
human body. Even though it’s so hard, tooth enamel is easily damaged or lost. If you’re wondering,
enamel is 96% minerals, of which a significant portion is calcium phosphate.
1. Common sources of tooth enamel erosion include soft drinks and citrus fruits. What is it
about soft drinks that makes them so deleterious to tooth enamel? Explain how they might
cause this erosion.
2. Erosion of tooth enamel occurs during and even after drinking soft drinks. Why does the
process continue even after you’ve stopped drinking soda? Is this more similar to direct or
reverse spherification?
3. Dentists recommend that you wait at least 30 minutes after consuming soft drinks before
brushing. Why might this be? If the diffusion constant for citric acid, a major component
of sodas, is D = 0.885 × 10−5 cm2 /s, how far will the unbound acid move during the waiting
period?
4. Citric acid is particularly troublesome for your dental health because it bonds to the calcium
in your teeth and saliva. Given what you know about diffusion, how might you slow or try to
reverse this process?
5. Michael’s son has just lost a baby tooth. Luckily the tooth fairy has delivered the tooth to you
for testing of your erosion prevention method. Assuming that baby teeth are the same as adult
teeth in composition, design a controlled experiment to test your answer to part 4.
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