Testing for Compounds in Foods
Name:
Purpose:
The compounds your body needs for energy and buiding materials are carbohydrates, proteins, fats,
vitamins, and minerals. These compounds are present in the plants and animals you use as food. You will
observe the tests for specific compounds and then use those tests to determine the compounds in ordinary
foods.
Procedure:
Testing for Carbohydrates
There exists two simple laboratory tests for the two main types of carbohydrates: Simple sugars
(monosaccharides and disaccharides) and starches (polysaccharides).
1. Simple sugars are tested with Benedicts’s Test:
Place 1 dropper full of the food solution to be tested and add 1 dropper full of Benedict’s solution.
Heat the tube gently in a water bath for 3 minutes. If simple sugar is present, there will be a color
change from blue through green to yellow/orange/red. Actually, an orange precipitate is formed.
2. Starches are tested with the Iodine test:
Add 3-5 drops of Iodine to a dropper full of the substance. Do not heat. A color change from
orange/brown to blue/black shows the presence of starch.
Testing for Proteins – Biuret Test
Add 1 dropper full of Biuret solution to 1 dropper full of the substance. Do not heat. Results may not be
immediate and could take 5 minutes. If there is protein, there will be a color change from blue to mauve.
Testing for Fats and Oils – Sudan IV test
Add 5 drops of Sudan IV stain to one half dropper of the food substance. Do not heat or shake! Look
for a deep red color at the interface of the food substance with the dye. You may need to hold the tube
over a piece of white paper.
Data:
Food
tested
Benedict’s
test
Iodine
test
Biuret
test
Sudan IV
test
Observations
This food
contains…
Questions:
1. Why are carbohydrates subdivided into simple sugars and starches?
2. What are the advantages and disadvantages of these two carbohydrates?
3. What is the origin of most of the saturated fats in the diet?
4. Why is sodium listed on nutritional labels (what is the cause for concern)?
5. What is the recommended amount that an average person should consume daily from each of these
food categories (sugars, starches, proteins, fats)?
6. How is the average diet in the US different from #5? (find statistics, etc.)
7. What problems occur when too much of each of these compounds are eaten? (discuss examples)
8. Protein molecules contain C, H, O, and _____________.
9. Use the following to answer which substance will give a positive test:
Starch. Tap water, Sugar, lipids/fats, proteins
a. Benedict’s test:
b. Iodine test:
c. Biuret test:
d. Emulsion test:
10. What is the difference in terms of energy use by the body and storage in the body for lipids and
carbohydrates?