Learning Activity Series
Investigating Chocolate
www.nano-cemms.illinois.edu
Copyright © 2012 Board of Trustees, University of Illinois. All rights reserved.
Investigating Chocolate
Description:
The instructor uses a PowerPoint presentation to
discuss chocolate, how chocolate can change in
certain storage conditions, and an instrument used
in nanotechnology called atomic force microscopy.
Using a worksheet to guide the hands-on activity,
students view AFM pictures of five samples of
chocolate, examine the samples’ appearance, and
then taste the five samples. Four of the samples
have been stored in different conditions and one is
a control. Students aim to correlate crystal size,
color, and taste.
Prerequisites:
Student should have an introductory knowledge of
nanotechnology (which can be provided by the
Introduction to Nanotechnology kit).
Instruction Time:
Approximately 30 minutes
Contents
2
Instructional Method
2
Background Information
3
Materials
3
Safety
3
Preparation
5
Presentation Details
A-1
Student Worksheet
A-2
AFM Scans
14
Notes
Audience:
High school students
National Science Education Standards:
Content Standard A: Abilities Necessary To Do
Scientific Inquiry.
Content Standard B: Structure and Properties of
Matter.
Content Standard E: Understandings about Science
and Technology.
Illinois State Learning Standards:
11.A.4b Conduct controlled experiments or
simulations to test hypotheses.
12.C.5b Analyze the properties of materials in
relation to their physical and/or chemical
structures.
13.B.5b Analyze and describe the processes and
effects of scientific and technological
breakthroughs.
1
Univ
Instructional Method:
The instructor uses a PowerPoint presentation to
introduce the topic. Students then do a hands-on
activity, guided by a worksheet. They study AFM
pictures of four samples of chocolate stored in different
conditions (plus one control) and then they examine and
taste the samples.
Background Information:
Chocolate
Cocoa beans come from seeds of the Theobroma cacao
tree, which is native to Central and South America.
Chocolate can be traced back to the Olmec civilization,
the first human civilization in that area. This civilization
existed from about 1500 to 400 B.C.E. The Maya
civilization likely established the first cocoa plantation,
which they used to produce a drink. The Spanish
brought cocoa to Europe in 1544, where it became very
popular in the Spanish court and then spread through the
European aristocracy to the rest of Europe.
The production of chocolate, though it is a natural product, is a very complicated process. First, the cocoa
beans are harvested by hand after 5-6 months of growth. Then beans are then piled together and allowed
to ferment. After 5-7 days, the beans are allowed to dry and then roasting takes place. After fermentation
and roasting, chocolate has been found to contain around 400 compounds. The shell of the bean is then
removed from the kernel or “nib” in a procedure called “winnowing.” Grinding of the nib occurs and a
non-alcoholic liquid called chocolate liquor is formed, containing 55% fat (cocoa butter), 17%
carbohydrates, 11% protein, and remaining tannins and ash. The liquor will also contain caffeine, a
nervous system stimulant. Theobromine, a smooth muscle stimulant, may also be present. The
solidification of chocolate liquor forms unsweetened (bitter) baking chocolate. The liquor may be
pressurized and ingredients like sugar, cocoa butter or solids, and condensed or dried milk will be added
to produce chocolate confectioneries. After the ingredients are homogenized, they are pulverized to
produce particle sizes around 20 micrometers (0.02 mm), which have a smoother “mouth feel” than larger
particles sizes. Conching, the next process, creates changes in both flavor and texture. Then, an
emulsifier is added. The careful process of tempering is the final stage of the production of chocolate,
resulting in its crystallization. Chocolate can crystallize in 5 or 6 ways, but only one of those creates its
“characteristic snap, gloss, and texture.”
“Chocolate bloom” is the whitish appearance that chocolate can get under certain conditions. It is not
mold; rather, it is fat that has risen to the surface of the chocolate. Bloom can occur if the cocoa butter is
not of the correct chemical structure or if it is exposed to the sun and allowed to recrystallize.
2
Univ
Today, each American consumes about 12 pounds of chocolate each year and the market is worth about
13 billion dollars. But we are ranked only 11th in chocolate consumption in the world. The Swiss
consume over 22 pounds of chocolate per person each year. (Summarized from G. Tannenbaum’s
“Chocolate: A Marvelous Natural Product of Chemistry. Journal of Chemical Education, Vol. 81, no. 8.
August 2004.)
Atomic Force Microscopy
A light microscope is limited in what it can image by the wavelength of light. Things smaller then the
wavelength of light can’t be seen, no matter how much you try to magnify them. The wavelength of light
is so much bigger then the physical world of atoms. Visible light is about 400 to 700 nanometers, but
atoms are only about ¼ of a nanometer. This means there are lots of interesting things to see that you
can’t use a light microscope to image. An atomic force microscope, or AFM, is a microscope that can
resolve objects much smaller then a light microscope can image. (In the picture above, the small box on
the right is the actual AFM.) This is because an AFM uses a completely different method to form the
image. Imagine yourself in a completely dark room, one with no light at all. Although you can’t get
around with your eyes, you can still discover things about your environment by touching. You can feel for
a wall and move along until you find a bulge. You can feel the contours of the bulge and determine it is a
door knob. An AFM images a sample in much the same
way, by touch.
An AFM uses a probe. This very small “finger” that feels
its way along the surface is atomically thin. Although the
tip of the probe is might really be quite blunt on the
atomic scale, and be many atoms across, one atom will be
slightly lower than all the rest. This one lowest atom
becomes the tip.
The tip glides along the surface, raising and lowering as
the surface changes. As the tip raises and lowers, the angle of the top of the tip changes. This is recorded
by shining a laser at the top of the tip and detecting where that light reflects. As the angle of the tip
changes, so does the location where the light reflects to. This is recorded in a computer and the computer
interprets that location as a height of the sample at that point.
3
Univ
The computer records the height at various points on the sample and displays this information. The higher
the point, the lighter the color displayed. Although we might assume that this height data would for a
black and white image (white at high points, black at low points), traditionally brown and yellow are used
instead, although any color range could be used. This can help us distinguish an AFM image from an
SEM image. Both the SEM and the AFM display height data, but the SEM, which is the older technology,
already used black and white, so the scientists using an AFM decided to use brown and yellow.
Materials:
•
•
•
•
•
•
•
•
1 evaluation
30 assessments
30 handouts
Power Point presentation
AFM pictures
Plate toppers
30 plates
30 samples of each type of chocolate
Teacher needs: LCD or overhead projector; access to a clean incubator is optional
Safety:
Make sure there are no allergies to the ingredients in chocolate before you begin. Otherwise, involvement
in this activity poses minimal safety issues.
Preparation:
Read the Background Information and the Presentation Notes. Prepare chocolate one month
before you would like to do the activity with students. Buy disposable plates and print copies of
the plate toppers and cut them to fit your plates. Laminate the plate toppers if you would like to
use them with multiple classes. Print copies of the AFM pictures and prepare to do the
presentation.
Preparation of Chocolate:
Dove® Dark Chocolate, in individually wrapped pieces, works well for this activity. You will
need to buy 4 pieces of chocolate for each student to start. For example, if you have 30 students,
buy 120 pieces of chocolate. To create the four storage conditions, unwrap the chocolates and
divide them equally amongst 4 plastic storage bags and seal. Put the bags in the following places:
• A freezer at -20° C
• A refrigerator at 4° C
• Anywhere at room temperature at 20° C
• A clean incubator at 32° C (If you do not have access to a clean incubator, you may
omit this storage condition.)
4
Univ
After one month of storage, buy fresh chocolate (for example, 30 pieces for 30 students) and take
the chocolate out of the storage conditions. You are now prepared to do this activity.
•
Presentation Details:
Slide 1 (Investigating Chocolate): Today we will be investigating chocolate.
Slide 2 (Background): The production of chocolate is a big industry--in fact, a 15 billion dollar industry.
In the United States, the average American consumes about 12 pounds of chocolate each year. But we
don’t consume as much as other parts of the world. In Switzerland, the average consumption is 22 pounds
per person.
Slide 3 (Background): Chocolate is in fact healthy for you. It contains an antioxidant called a flavanol. It
has more antioxidants then even vitamin C does. In addition, it may help decrease blood pressure,
increase circulation, and improve the quality of blood platelets. The problem is, we like our chocolate
with lots of sugar and fat from cream, and that isn’t so healthy. The stronger the chocolate and less sugar
and fat, the better. If you can stomach it, you might almost be able to claim that you are eating dark
chocolate for the health benefits.
Slide 4 (Background): However, chocolate does seem to have a shelf life. After a while, it blooms.
Although nutritionally it is the same, it isn’t as desirable.
Slide 5 (What is bloom?): What is bloom? You have probably all seen it. It is when chocolate gets that
whitish look. It isn’t bad, it just doesn’t look as good. Some people claim it doesn’t taste as good, and the
chocolate crystals may become larger – so large that our tongues can detect them and the chocolate tastes
gritty.
Slide 6 (Some questions): Lia Andrae worked in Nikki Engles’ food science lab at the University of
Illinois. Lia’s research was looking at the storage conditions of chocolate and crystal size variation under
these different storage conditions. With an understanding of what happens to chocolate and the crystals as
they are stored, perhaps chocolate could someday be engineered to extend its shelf life.
Slide 7 (Focus for today): We are going to do three tests on chocolate stored under four different
conditions. We will look at the crystal size, the color, and then actually taste the chocolate to see what can
be learned about how chocolate changes with varied storage conditions. We have chocolate that has been
stored for 1 month at four different temperatures.
One sample was stored in a freezer at -20°C. One was stored in a refrigerator at 4°C. Another was stored
at room temperature at 20°C, and the fourth piece was stored at a slightly elevated temperature,
something like a warm room, at 32°C. We also have a fresh piece to use as a control.
Slide 8 (Testing for crystal size): Our first test is to determine crystal size. These crystals are small, too
small to see with the unaided eye. We will need some sort of microscope to see them. But what kind of
microscope should be used?
5
Univ
We can’t use a traditional light microscope, the kind you are probably familiar with, because the crystals
might be too small. A light microscope is limited by the wavelength of light. Objects smaller than the
wavelength of light cannot be seen with a light microscope, and it is hard to see things just a little bigger
then the wavelength of light. (The image shown is brain tissue.)
Slide 9 (Testing for crystal size): Another type of microscope is a scanning electron microscope. This is
the microscope that gives us those interesting black and white images of insect heads you may have seen.
(The image shown is a the tip of a fly foot.) This type of microscope doesn’t use light but rather electrons
to create the image.
Electrons are smaller then the wavelength of light, so an electron microscope can image things much
smaller then a light microscope. Because this microscope shoots electrons at the object, the object needs
to be coated with a thin layer of metal to conduct the electrons away. A sample needs to be put into a
machine called a sputterer. It coats a sample with a thin layer of either gold or palladium by spraying
atoms of the element on the sample. If this is done with a soft material like chocolate, it will deform the
chocolate, ruining the crystals we are trying to image. We can’t use a SEM with our chocolate samples.
Slide 10 (Testing for crystal size): There is another kind of microscope called an atomic force
microscope. This microscope can potentially image individual atoms. The nice thing about this
microscope is that no special preparation needs to be done before you look at the sample like you have to
do with the SEM.
Slide 11 (How does an AFM work?): Like feeling your way in the dark, an AFM creates an image of
the sample by touching the surface. It runs a “finger” over the surface called a probe. The tip of this probe
can have a point as small as a single atom. This probe moves back and forth across the surface feeling the
area to be imaged.
Slide 12 (How does an AFM work?): If you think about it, we really haven’t solved the problem yet. If
we can’t directly detect the tiny atomic changes on the surface, how does running a probe over the surface
help? How can we detect the tiny movements of the probe as it responds to the surface?
By shining a laser light on the top of the probe, we can detect the tiny movement of the probe because it
will change the angle the light shines off the probe on its way to hit the detector.
Slide 13 (Testing crystal size): In this activity, you will first look at the AFM images collected for the
chocolate samples. There are five images, one for each of the four 1 month storage conditions and one for
the fresh sample of chocolate. You will see peaks and valleys. Find a typical crystal and see how wide it
is. You can determine the size by using the scale along the bottom axis. Each division is 10 microns.
Slide 14 (Testing for color): Next, look at the chocolate samples. Record how much bloom you see. Is
the chocolate color rich and dark, or is it dull and white? Are there some white parts? How does it look?
Slide 15 (May not taste as good): Finally, taste each sample. Remember that bloomed chocolate is
nutritionally fine, it just looks different and may taste different. To taste it, take a very small nibble and
let it melt on your tongue. You’ve probably heard that we taste through our sense of smell. You want the
6
Univ
volatile compounds to vaporize and move into your nose where you can sense them and really taste the
chocolate. The other thing you want to pay attention to is how grainy the chocolate tastes. Each person is
different but many people can sense crystals of about 30 microns or so. See if the chocolate tastes grainy.
If it does, it probably has some larger crystals. After you have made these determinations, feel free to then
eat the rest of the piece. You will use the handouts to guide this activity.
References:
Beckett, S. T. The Science of Chocolate; The Royal Society of Chemistry: Cambridge, U.K., 2000; pp
10–11.
Tannenbaum G. “Chocolate: A Marvelous Natural Product of Chemistry.” Journal of Chemical
Education. Vol. 81, no. 8., August 2004. p 1131-1135.
Tannenbaum, G. Lessons in Chocolate; Flinn Scientific: Batavia, IL, 1993; p 4.
7
Univ
Investigating Chocolate
Please look at the AFM pictures and estimate the crystal size by the width of the crystal. When you
receive the chocolate samples, unwrap the fresh chocolate and look at the difference in color and gloss
between the samples. Then taste the chocolate (in small bites) and pay attention to any differences in
texture or flavor between the samples. Write down any differences you may notice in the table below.
Crystal Size (μm)
Appearance
Taste – Texture & Flavor
Fresh
-20º C
4º C
22º C
31º C
In the AFM pictures: Which sample had the smallest crystals? Which had the largest crystals?
Does bloom seem to correlate with crystal size?
Which sample had the strongest chocolate flavor? Which had the least chocolate flavor?
Could you taste the larger crystals (grainy texture)? Did this change the chocolate flavor at all?
About how big do you think the crystals need to be for you to taste them?
8 A-1
Univ
AFM scan ofLearning
0 week-old
chocolate
sample
Activity
Series
A-2
AFM scan of 4 week-old chocolate sample stored at 31º Celsius
1
Univ
A-3
AFM scan of 4 week-old chocolate sample stored at 4º Celsius
2
Univ
A-4
AFM scan of 4 week-old chocolate sample stored at 22º Celsius
3
Univ
A-5
AFM scan of 4 week-old chocolate sample stored at -20º Celsius
4
Univ
A-6
Learning Activity Series
Notes:
Established in 2003, the Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems (NanoCEMMS) is funded by the National Science Foundation. Partnering Institutions include the University of Illinois,
North Carolina Agriculture and Technical State University, Stanford University, University of Notre Dame,
University of California – Irvine, and Northwestern University. Researchers are developing a nanomanufacturing
system that will build ultrahigh-density, complex nanostructures. The Center’s research will ultimately result in a
new way of working and has the potential to create millions of jobs for American workers. Our nation’s school
children must be prepared to assume the new roles that will be the inevitable outcome of these emerging
technologies.
This learning module is one of a series that is designed to interest middle and high school students in pursuing
this new field. The Center also offers ongoing professional development for teachers through a continuous series
of workshops and institutes. To sign up for a workshop or to order more learning modules, visit our website at
http://www.nano-cemms.illinois.edu.
For more information, contact: Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems; University of Illinois
at Urbana-Champaign, 4400 Mechanical Engineering Laboratory, 105 South Mathews Avenue, MC-244, Urbana, IL 61801
Phone: 217.265.0093 Email: nano-cemms@illinois.edu Website: http://www.nano-cemms.illinois.edu
14