Page 1 – Classification of Carbohydrates
Classification of Carbohydrates
Group Leader: ____________________
Technician 1: ______________________
Recorder: ________________________
Technician 2: ______________________
Problem:
Which foods contain monosaccharides? Which foods contain polysaccharides?
Introduction
The MyPyramid logo is a familiar symbol of
healthy eating and good nutrition. As you can
see carbohydrates in the form of grains,
vegetables, and fruits make up over half of a
person’s recommended dietary intake.
Overall, the US Department of Agriculture
recommends that Americans consume all
types of organic macromolecules -carbohydrates, lipids, and proteins – but in
proportions that promote strong health.
Nutritional labels typically list the amount of
each of these macromolecules in different
foods. In this experiment, we will focus on
carbohydrates. We will use a qualitative
method to determine whether foods contain
monosaccharides or polysaccharides.
Image courtesy of U.S. Department of Agriculture
Background
Carbohydrates, proteins, and fats have different chemical structures and different chemical
properties. A variety of simple tests have been developed to identify these macromolecules based on
differences in their chemical properties.
Carbohydrates are the most abundant class of organic compounds found in plants. The basic
building blocks of carbohydrates are simple sugars, such as glucose (blood sugar) and fructose (fruit
sugar), and are called monosaccharides. Other more complex carbohydrates result when two or
more simple sugars combine. These compounds are called disaccharides or polysaccharides,
respectively. Examples of disaccharides include sucrose (table sugar), lactose (milk sugar), and
maltose (malt sugar). The most common polysaccharide is starch, which is composed of thousands
of glucose units joined together.
Simple carbohydrates can be identified using Benedict’s test. The Benedict’s test reagent consists of
copper (II) complex ions in aqueous (water) solution. The copper ions give this solution a bright blue
color. In the presence of all monosaccharides and most disaccharides (sucrose is an exception – there’s
always one!) Benedict’s solution will give a positive test result – the blue color of the copper(II)
Page 2 – Classification of Carbohydrates
complex ions disappears and an orange-red precipitate (solid) called copper(I) oxide appears.
Depending on the amount of sugar present in the foods, the final color of a positive Benedict’s test
result may range from green to yellow to red. A negative Benedict’s test result will show no change
– the solution remains bright blue and no precipitate is observed. Polysaccharides do not react with
Benedict’s solution.
The most common polysaccharides are starch and cellulose. Starch serves as an energy storage
molecule in plants, where it typically clumps into visible grains. The most familiar sources of dietary
starch are potatoes, beans (legumes), and cereal grains (corn, wheat, barley). Reaction with iodine
(the iodine test) is used to identify the presence of starch in foods. A positive iodine test result is
observed when the brownish color of the iodine solution changes to a dark blue color in the presence
of starch. Cellulose serves a structural role in plants (cell walls) and is not digested in the human
stomach. It is often listed on nutritional labels as dietary fiber. Fiber may not react with iodine test.
In this laboratory, you will identify the difference between monosaccharides and polysaccharides
in a variety of foods. The classification tests are Benedict’s test for “simpler” sugars
(monosaccharides) and the iodine test for more complex carbohydrates (polysaccharides). In order
to distinguish between positive and negative test results, each test will be run first on known or
reference compounds. The tests will then be run on a variety of food samples to identify the
carbohydrates found in these different foods.
Objectives:
In this experiment, you will…

use qualitative tests to determine the presence of monosaccharides and/or polysaccharides in
known reference compounds.

use qualitative test results from known reference compounds to determine the presence of
monosaccharides and/or polysaccharides in common foods.
Pre-Lab Questions – Answer on a separate sheet of paper.
1. What groups of foods on the USDA MyPyramid logo contain carbohydrates?
2. What are the functions of carbohydrates in living organisms?
3. Read the Procedure section and the accompanying Safety Precautions. What hazards are
associated with performing the Benedict’s test and the iodine test?
4. You test a carbohydrate with the Benedict’s test reagent to see if it contains
monosaccharides. How will you know whether this carbohydrate contains
monosaccharides?
5. Read the Nutrition Facts label on any packaged food item.
(a) What information is provided concerning the types of carbohydrates that are present in
foods?
(b) Starch is not listed on the label. Use the information in the Background section to guess
(infer) how much starch is present in the food.
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Hypothesis
You will test milk, apple juice, and potato flakes for the presence of monosaccharides and
polysaccharides. Use the chart below to make a prediction about which carbohydrate(s) can be
found in each of these foods.
Carbohydrate
Type
Milk
Apple
Juice
Potato
Flakes
Monosaccharides
Polysaccharides
Materials
Reference and Test Solutions (Part A)
Benedict’s test solution, 20 mL
Glucose solution, 1%, 4 mL
Iodine test solution, 5 mL
Starch solution, 1%, 4 mL
Distilled water
Food Samples (Part B)
Apple juice, 4 mL
Milk, whole, 4 mL
Potato flakes, instant, 0.5g
Other desired samples
Equipment and Glassware
Graduated cylinder, 10-mL
Beaker, 250-mL
Hot plate
Plastic pipettes
Stirring rod
Test tubes, 10 medium
Test tube clamp
Test tube rack
Wash bottle
Wax pencil (for labeling)
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Safety Precautions
Benedict’s solution contains copper (II) sulfate, sodium citrate, and sodium carbonate; it is moderately
toxic by ingestion and is a skin and body tissue irritant. Iodine solution contains iodine and potassium
iodide and is an eye and skin irritant; it will stain skin and clothing. Avoid contact of all chemicals with
eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. All food-grade
items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do
not taste or ingest any materials in the laboratory and do not remove any remaining food items after they
have been used in the lab. Wash hands thoroughly with soap and water before leaving the laboratory.
Procedure
Part A. Reference Tests
1. Prepare a boiling water bath: Fill a 250-mL beaker about one-half full with tap water.
Heat the water to a gentle boil on a hot plate (medium setting). This will most likely be
done for you by your teacher.
2. Label one test tube glucose. Label a second test tube starch. Add about 1 mL (20 drops)
of each of these solutions to the appropriate test tube.
3. Add 2-3 drops of iodine solution to each test tube. Record the color and appearance of
each test mixture in Data Table A.
4. Dispose of the test tube contents into the sink.
5. Wash the test tubes and rinse them once with distilled water.
6. Re-label two new test tubes as you did in Step 2. Add about 1 mL (20 drops) of each
solution to the appropriately labeled test tubes according to Step 2.
7. Using a graduated cylinder, add 2 mL of Benedict’s solution to each test tube and place
the test tubes in the boiling water bath.
8. After 2-3 minutes, remove the test tubes from the hot water bath using a test tube clamp.
Record the color and appearance of each test mixture in Data Table A.
9. Dispose of the test tube contents as directed by the instructor. Wash the test tubes and
rinse them with distilled water.
10. Determine which color represents a positive test for each type of carbohydrate.
Substances that contain polysaccharides will change iodine solution from brown to blue
or black. Substances that contain monosaccharides will change from light blue to a
yellow, orange, or red color when heated with Benedict’s solution.
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Part B. Testing Food Samples
1. Label five test tubes 1-5.
2. Add approximately 1 ml (20 drops) of each food sample to each test tube according to
the following chart.
Test Tube
Food Tested
1
Milk
2
Apple Juice
3
Potato Flakes
4
Unknown #1
5
Unknown #2
3. Add 2-3 drops of iodine solution to each test tube. Record the color and appearance of
each test mixture in Data Table B. Note whether each classification test result is positive
(contains that compound) or negative (does not contain that compound). You may need
to refer to Data Table A and the Background section to confirm your results.
4. Dispose of the test tube contents as directed by the instructor. Wash the test tubes and
rinse them with distilled water.
5. Repeat Steps 1-2.
6. Using a graduated cylinder, add 2 mL of Benedict’s solution to each test tube and place
the test tubes in the boiling water bath.
7. After 2-3 minutes, remove the test tubes from the hot water bath using a test tube clamp.
Record the color and appearance of each test mixture in Data Table B.
8. Dispose of the test tube contents as directed by the instructor. Wash the test tubes and
rinse them with distilled water.
9. Note whether each classification test result is positive (contains that compound) or
negative (does not contain that compound). You may need to refer to Data Table A and
the Background section to confirm your results.
10. Clean and organize your lab station so that you can leave it as you found it.
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Name: ______________________________
Class/Lab Period: _____________________
Food Testing Lab
Data Tables
Part A. Reference Tests
Classification
Test
Reference Compound
Glucose
Starch
(Monosaccharide)
(Polysaccharide)
Iodine Test
Benedict’s Test
Part B. Testing Food Samples
Classification
Test
Food Samples
Milk
Iodine Test
Benedict’s
Test
Apple
Juice
Potato
Flakes
Unknown #1
Unknown #2
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Analysis
1. Using your results from the classification tests in Part A, describe the difference between a
“positive” test result and a “negative” test result for each reference compound.
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2. For each food tested in Part B, list the type of carbohydrate that was present in each.
Explain how you know.
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3. Many vegetables will test negative for both carbohydrate types investigated in this
laboratory. What type of carbohydrate might be present in these vegetables? Why
might this type of carbohydrate test negative?
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4. Are there any errors or inconsistencies in your data?
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How might you explain them? Did your group do anything during the lab that
could explain the inconsistent data?
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Conclusion
Do you agree or disagree with your hypothesis? Why or why not?
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References
Cesa, Irene (2003) ChemTopic Labs: Chemistry of Food. Flinn Scientific Inc., Batavia, IL.
pp. 1-14.
Food Guide Pyramid (1996) U.S. Department of Agriculture and the U.S. Department of
Health and Human Services (accessed on October 19, 2004 from <www.lose-bodyweight.com/ food-guide-pyramid.html>