Chemical Identification of Foodstuffs
UNKNOWN
If a friend walks up to you and starts talking, do you need to see this person in order to identify them?
Long before scientists could see molecules, they knew they existed. When a pathologist is asked to
determine if someone was poisoned, can the pathologist see the poison? Of course not, the pathologist
uses chemistry to solve the problem.
INTRODUCTION
Purpose: Use chemical indicators to test for the presence of a simple carbohydrate, complex
carbohydrates, or protein in an unknown solution. Special note: your unknown could contain a simple
sugar, starch, protein, all of them, just two of them, or none of them.
Background Information: Define foodstuff and describe properties of each foodstuff being tested. This
means you need to research background information and write at least 3 sentences on simple (sugar) and
complex (starch) carbohydrates, as well as proteins.
Hypothesis: Does your unknown (u/k) contain a sugar, starch or protein? Hint: compare it to the known
solutions at your lab table.
Variables:
o Controlo Manipulatedo RespondingMaterials:
Every station should have:
 250 or 400 ml beaker
 4 test-tubes
 Test-tube rack
 10 ml graduated cylinder
 2 plastic pipettes
Every lab table should have:
 Hot plate
 Benedict's solution (*stains clothing)
 Iodine solution (*stains clothing)
 Biuret's reagent (*corrosive)
 Monosaccharide (corn syrup)
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Polysaccharide (starch)
Gelatin (protein)
Test-tube brush
Tap or distilled water dropper
Sharpie pen
PROCEDURE (GIVE A SUMMARY FOR EACH OF THE 4 PARTS)
Part 1: (Do this at home before you turn in your lab)
1. Dissolve a teaspoon of table sugar in glass of water.
2. Answer questions a and b in the data section of your lab write-up.
Part 2: Test for Simple Sugar
3. Fill your beaker about 1/2 full of tap water and place it on the hot plate. Plug in hot plate and turn it on to
the highest setting. Allow the water to boil while you continue with the lab.
4. Clean and dry all your test tubes to ensure no left over contaminants from the previous period.
5. Mark one test tube unknown and ask your instructor to fill the tube 1/4 full with your assigned unknown.
Mark the tubes on the glass portion, near the top.
6. Record which unknown you are given at the top of your write-up under your names. If you don't identify
the unknown at the top right corner of your final lab report then you will lose half the points on the
lab - no exceptions! Place unknown tube in your test tube rack furthest away from the other three test
tubes.
7. The remaining 3 test tubes should be marked as test tube #1, test tube #2, and test tube #3. Mark the
tubes on the glass portion, near the top.
8. Place ten drops of water into test tube #1, ten drops of corn syrup (simple sugar) solution into test tube #2,
and ten drops of your assigned unknown in test tube #3. **Be sure to always use a separate pipette for
your unknown. Answer question c in your data analysis section.
9. Using a graduated cylinder, add 3 ml of Benedict’s solution to all three tubes.
10. Carefully place all three test tubes in the boiling water bath for 3-5 minutes. When the color stops
changing in test tube 2 then you can turn off the heat.
11. Wait for test tubes to cool down and use the test tube holder to carefully remove the test tubes.
12. Observe the 3 test tubes and answer questions d-h.
13. Let the water in the beaker cool down and carefully pour the hot water down the drain.
Part 3: Test for Starch
14. Clean test tubes #1, 2 and 3 from Part 2.
15. Add ten drops of water to test tube one, ten drops of starch solution to test tube two and 10 drops of
unknown to test tube three. (make sure you gently swirl the known containers before taking your
sample)
16. Add three drops of Iodine solution to all three tubes.
17. Record the color results in your data table and answer questions i-m.
Part 4: Test for Protein
18. Clean test tubes #1, 2 and 3 from Part 3.
19. Add ten drops of water to test tube #1, ten drops of gelatin (protein) solution to test tube#2, and ten drops
of your unknown to test tube #3.
20. Add ten drops of Biuret’s Reagent to all three tubes (Caution: Biuret’s Reagent is corrosive, if you spill
any reagent on your skin or eyes, wash it off with plenty of water--and tell your instructor!)
21. Hold all three test tubes against a piece of white paper and record the resultant colors in your data table
and answer questions n-q.
22. Clean test tubes #1, 2 and 3 from Part 4 and leave them upside down in your test tube rack.
23. Leave your station and lab table set-ups as it was when you arrived or as indicated by your instructor.
*points will be deducted during lab for incorrect procedural steps performed by students. This includes
failing to wear or correctly wear safety goggles, fail to tie hair back, boil other indicators, contaminate
your unknown, forget your unknown, etc.
DATA COLLECTION
Fill in the color that each respective test tube looked like at the end of each testing procedure. You must
recopy this filled in table for your final report.
Test Result Colors
Tube #
1
2
3
Simple Sugar
Starch
Protein
DATA ANALYSIS
a. Does the sugar solution look any different than plain water?
b. If I gave you another beaker of clear liquid, how could you tell (without tasting it) if it contained any
sugar?
c. Why do you always use a separate pipette for your unknown?
d. What was the initial color of all three solutions just before you heated them?
e. What is the final color of test tube #2?
f.
What is the function/purpose of test tube #1?
g. What is the color of a positive test for simple sugar? (Positive test = the color that shows you sugar is
present)
h. Compare your unknown in test tube #3 to test tubes 1 and 2, to determine whether or not your unknown
contains any simple sugars.
i.
What is the color of test tube #1?
j.
What is the color of test tube #2?
k. What is the function/purpose of test tube #2?
l.
What is the color of the negative test for starch? (Negative test = the color that shows you starch is not
present)
m. Compare your unknown in test tube #3 to test tubes 1 and 2, to determine whether or not your unknown
contains any starch.
n. What is the color in test tube #1?
o. What is the color in test tube #2?
p. Compare your unknown in test tube #3 to test tubes 1 and 2, to determine whether or not your unknown
contains any protein.
q. Think of another way this procedural testing can be used in the real world. Give an example.
CONCLUSION
Address Hypothesis
What was your unknown letter and based on your test results (refer to them), what did your unknown
contain? Did your results support or reject your hypothesis? BE SPECIFIC!
If you don't refer to the specific colors you will lose points. You will automatically lose points if you get 1 out of
2, or 2 out of 3 correct for your unknown.
Error Analysis
Modify the Procedure/Suggestions