Name of people in group: _______________________
Your name: ____________________________________
Nutrient Testing Lab
EVALUATION
1. Complete a proper chart ready for the very start of the lab period (on time, title, ruler, proper headings)
(2
marks)
2. Accurate observations recorded in chart.
(3 marks)
3. Lab skills including proper use of equipment, following instructions correctly, safety (such as wearing goggles),
and clean up.
(2 marks)
4. Answers to analysis questions to be randomly selected by teacher.
(TBD)
In this activity, you will use laboratory tests to identify monosaccharides, starches, proteins and lipids. You will then use these
tests to identify the nutrients present in an unknown sample.
Each test will use a different indicator reagent that will change color in the presence of the particular organic compound that
is being tested for.
As part of the experimental method, you must include control samples to insure the validity of your results. A control is a test
sample with a known result. If your control samples do not give you the expected result, then your experimental results are not
valid and you must reevaluate your experimental set-up (maybe your test chemicals are no good).
A negative control will result in no change in color. It will either contain no sample at all or it will contain a nonreactive
sample like water. For example, if you are testing for the presence of monosaccharides, the test chemical, called Benedict's
solution, will remain the original color blue when mixed with water.
A positive control will result in a color change indicating the presence of the compound you are testing for. For example, a 5%
glucose solution will react with Benedict's solution and change it from blue to rust (brown-red).
Reagent background information:
A. Benedict’s reagent is an indicator for monosaccharides and some disaccharides, such as maltose. The following table
summarizes the quantitative results obtained when a simple sugar reacts with Benedict’s reagent.
Colour of Benedict’s reagent
Approximate sugar concentration
Blue
nil
Light green
0.5%-1.0%
Green to yellow
1%-1.5%
orange
1.5%-2%
Red to red-brown
>2%
B. Lugol’s solution contains iodine and is an indicator for the presence of starch. Iodine turns blue-black in the presence
of starch.
C. Biuret reagent detects proteins. The reagent reacts with the peptide bonds that hold amino acids together, producing
colour changes that range from blue (no protein) to pink (+) to violet (++) to purple(+++). The + sign indicates the
relative amounts of peptide bonds present.
Safety Precautions: Be careful when handling Biuret’s reagent and Benedict’s reagents as
they are toxic. Notify the teacher if a spill occurs. Flush with lots of water if any of these
gets on your skin. Lugol’s iodine will stain clothing. You must wear goggles during this
lab.
Materials: Each lab bench has the appropriate materials to complete each of the four parts.
graduated cylinder
test tubes
test tube rack
400 mL beaker
gelatin solution
glucose solution
starch solution
hot plate
Benedict’s solution
Biuret reagent
Lugol’s iodine
Procedure.
1. Thfe observation table should have a title, and clearly identified sub-headings. You will use the three indicators
described above to determine what is in the unknown. Your chart should include observations for the indicator in the
unknown, the positive control and the negative control.
Part A - Benedict’s solution
2. Prepare a water bath by heating approximately 250 mL of water in a 400 mL beaker on a hot plate. Heat the water to
about 80C. Do not let the water boil strongly. You can tell that the water temperature is about right when you start
to see bubbles form on the bottom of the glass.
3. Make sure you identify what is in each of the test tubes. A simple method for doing this is placing a blank sheet of
paper under the test tube rack and writing on the sheet. A more difficult way would be to write on each test tube a
letter using the wax pencils and keeping a legend.
4. Pour 3 mL of tap water in a test tube, and 3 mL of the glucose solution in another test tube. Add 10 drops of
Benedict’s solution to each test tube. Place both test tubes in the hot water bath and wait five minutes. Carefully
remove the test tubes from the water bath and record the colour in the test tubes.
Part B - Lugol’s iodine
5. Pour 3 mL of tap water in a test tube, and 3 mL of the starch solution in another test tube. Add three drops of Lugol’s
iodine to each test tube and record the colour in each test tube.
Part C - Biuret’s reagent
6. Pour 1 mL of tap water in a test tube, and 1 mL of the gelatin solution in another test tube. Add twenty drops of
Biuret’s reagent to each test tube. Tap the test tubes gently with your fingers to mix the solutions. Observe and
record any colour change.
Part D – Unknown sample!
7. Obtain about 8 mL of the unknown solution you have been assigned. Repeat the tests to identify which nutrient(s) are
present in the unknown solution. Record your observations .
8. Empty contents of the test tubes in the sink, under running water. Wash the test tubes and graduated cylinders in
soapy water, with a test tube brush.
A
+
–
–
–
E
–
–
+
+
G
–
–
–
–
M
–
+
–
–
Lipids
Proteins
Test results – are these nutrients present?
Starch
Lipids
Test
Monosaccharides
Protein
Test
Sudan Red
test
Lugol
test
Benedict
test
Food
sample
Carbohydrate Tests
Biuret
test
Analysis
1. What reagent is commonly used to test for the presence of monosaccharides?
2. What reagent is commonly used to test for the presence of starch?
3. What reagent is commonly used to test for the presence of protein?
4. What was the purpose of testing water with each indicator? Use proper scientific terminology in your
description.
5. What is one possible source of error in this laboratory?
6. Why is it important to know what compounds are present in food? When answering this question think of
someone who may be threatened by eating something they did not know they were eating.
7. Which nutrient(s) were present in your unknown? Explain your results.
8. Define and give an example of a positive experimental control.
9. Define and give an example of a negative experimental control.
10. Look at the results for samples A, E, G, and M (below). Complete the table. Which sample is most likely
to be rice? Justify your answer.
11. Why is it important to understand the nutritional content of food? Where can you find this information for a
product you are buying?
12. In general, a balanced diet contains relatively even amounts of carbohydrates, proteins, and lipids. Too much
sugar (monosaccharides and other simple sugars) is unhealthy. Fruits and vegetables are important sources of
vitamins and minerals. Do you consider your diet balanced and healthy? Why or why not?