B4.1 Lab: Testing for Different Macronutrients
Supplies:
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Iodine
Benedict’s
solution
Ethanol
Biuret reagent
Dimple tiles
Test tubes,
racks, &
holders
Hot plate &
large beaker
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Glass stirring
rods
Spatulas
Pipettes
oil
Egg albumin
powder
Glucose
powder
Starch powder
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Milk (casein or
whey) powder
Bread
Potatoes
cream cheese
butter
beans
bananas
carrots
Background information:
Indicators are substances used to identify whether or not a particular chemical is present. There are
four indicators used to test for the presence of particular macromolecules found in food.
Iodine is an indicator for starch.
Negative result (starch is absent): orange/yellow color
Positive result (starch is present): blue/black color
Benedict’s solution is an indicator for reducing (simple) sugars, such as glucose.
Negative result (sugars are absent): blue color
Positive result (sugars are present): green, yellow, orange, brick red
Weak positive  strong positive
Biuret reagent is an indicator for protein.
Negative result (protein is absent): blue color
Positive result (protein is present): purple color
Ethanol may be used to test for lipids. It is a physical test called the “emulsion test” and is done by
shaking a mixture of water, ethanol, and the test substance.
Negative result (lipids/fats/oils are absent): liquid remains clear
Positive result (lipids/fats/oils are present): a cloudy emulsion forms
Before beginning this investigation, create a data table for parts 1 & 2.
Investigation:
Part 1: Testing known positives and negatives
Begin by performing the following tests on known substances to observe a positive and negative
result. The observations will be helpful when you do part 2 and test foods of unknown composition.
1. Set up a hot water bath that you will need for the Benedict’s test
2. Iodine test for the presence of starch:
a. Place a little starch powder in a depression on a spotting tile.
b. Place a few drops of distilled water on a different depression.
c. Add a few drops of iodine to each.
d. Compare and record your results.
3. Benedict’s test for the presence of glucose:
a. Place approximately 5mL of strong glucose solution into a labeled test tube.
b. Place approximately 5mL of distilled water into a second test tube labeled “water.”
c. Add about 10 – 20 drops of Benedict’s solution to each test tube.
d. Lower the test tube into a beaker of boiling water. Wait until the test tube contents boil
and leave it for two minutes.
e. Compare and record your results.
4. Biuret test for the presence of proteins:
a. Dissolve a little egg albumin powder in 5mL of water in a labeled test tube.
b. Place approximately 5mL of distilled water to a second test tube labeled “water.”
c. Add about 10 – 20 drops of Biuret reagent in each test tube.
d. Compare and record your results.
5. Ethanol test for lipids (oils & fats):
a. Place about 1mL of ethanol in a test tube. Add a few drops of oil and mix by shaking.
Add an equal amount of water and shake vigorously again.
b. Place about 1mL of ethanol in a second test tube. Add a few drops of water and mix by
shaking. Shake the test tube.
c. Compare and record your results.
Part 2: Testing foods of unknown composition
1. Choose a food from the options provided, divide it into four samples and perform one test on
each sample.
2. Crush solid samples in warm water before performing the Benedict’s (glucose) and Biuret
(protein) tests. This can be done in this method:
a. Cut the food sample into small pieces.
b. Place a few small pieces into a beaker with about 10 mL of warm water.
c. Using a metal spatula to stir and push the food against the walls of the beaker, crushing
the food pieces against the wall.
d. Strain out some of the liquid into two separate test tubes – one for Benedict’s test and
one for Biuret test.
3. Crush and strain solid samples in ethanol to extract fats and oils before doing the Ethanol
(lipids) test.
4. The Iodine (starch) test can be done directly on solid foods.
5. Determine which of the four macronutrients are present in each food.