LAB: Food Nutrients

BIOLOGY LAB
NUTRIENT IDENTIFICATION
IN FOODS

INTRODUCTION:

Nutrients such as carbohydrates (sugars and starch) and proteins can be detected by the use of an indicator, which is a chemical that produces a characteristic color when a particular substance is present. Lipids can be detected by their ability to make paper translucent. In this lab, you will test foods for various nutrients, and compare your tests to standards, which are the results of tests that show a positive response for a known substance.

SAFETY: Always wear safety goggles when working with chemicals.

MATERIALS:

Safety goggles
Wax pencil
9 test tubes
Test-tube rack
Test-tube holder
Distilled water
Wooden splint or spatula
Hot-water bath
Solutions for preparing standards

2 mL of 15% glucose solution

2 mL of 1% starch solution

2 mL of gelatin solution

2 mL of vegetable oil
Indicators

Dropper bottle of Benedict’s solution

Dropper bottle of Lugol’s solution

Dropper bottle of biuret solution

5 inch square of brown paper
Food samples for the four tests

PREPARATION OF SOLUTIONS:

15% glucose solution - Add 15 g of glucose (dextrose) to 85 mL of distilled water. Stir until dissolved.

1% starch solution - Add 1 g soluble starch to a small amount of cold water. Dilute to 100 mL with boiling distilled water, and stir until dissolved. Cool before using. Do not store solution, always prepare fresh.

Gelatin solution - Dissolve one package of gelatin in 1000 mL of distilled water.

Food samples may need to be thinned slightly with distilled water.

The three test solutions can be purchased from a science supply company.

PROCEDURE:

1. Prepare a DATA TABLE and QUESTION SECTION, for each food sample you will be testing.

2. Using a wax pencil, label nine test tubes as follows:

sugar-positive; sugar-negative; sugar-sample;

starch-positive; starch-negative; starch-sample;

protein-positive; protein-negative; protein-sample.

Group the tubes in a test-tube rack according to the substance being tested for. Add 2 mL of distilled water to each test tube labeled negative.

3. Pour 2 mL of glucose solution into the sugar-positive test tube. To test for sugar, add a dropperful of Benedict’s solution to both the positive and negative test tubes.

4. Place the test tubes in a hot-water bath using a test-tube holder. Heat for about 5 minutes. Using a test-tube holder, set the tubes in the rack to cool. Record the color and appearance of each tube in the proper place on the DATA TABLE.

5. Pour 2 mL of starch solution into the starch-positive test tube. To test for starch, add a dropperful of Lugol’s solution to both the positive and negative test tubes. Record the color and appearance of each tube in the proper place on the DATA TABLE.

6. Pour 2 mL of gelatin solution into the protein-positive test tube. To test for protein, add a dropperful of biuret solution to both the positive and negative test tubes. Record the color and appearance of each tube in the proper place on the DATA TABLE.

7. Place a square of brown paper on a dry area of your lab table. Smear a drop or two of vegetable oil on the paper, and label the area positive test below the spot. Place a few drops of water in another area away from the oil spot. Label the area negative test below the water spot. Let the spots dry, and record your observations in the DATA TABLE.

8. Obtain enough food sample to test for each of the four nutrients. Using a wooden splint or spatula, place a small amount of food in each of the three test tubes labeled sample. Use the proper indicator, and the same procedure you used for the standards, to test for the nutrient marked on each test tube. Smear the fourth sample on an unused area of the brown paper. Compare your results with the standards you prepared earlier. Record your results in your DATA TABLE.

DATA TABLE:

Nutrient Indicator Positive test
(standard) Negative test Food sample
___________

sugar Benedict’s
solution

starch Lugol’s
iodine

protein biuret
solution

lipid appearance
on brown
paper

DATA TABLE:
Nutrient	Indicator	Positive test (standard)	Negative test	Food sample ___________
sugar	Benedict’s solution
starch	Lugol’s iodine
protein	biuret solution
lipid	appearance on brown paper

QUESTIONS:

Which test tubes contained standards? __________________

What indicator detects

sugar? __________________

starch? __________________

protein? __________________

What color resulted when

sugar was present? __________________

starch was present? __________________

protein was present? __________________

How did the test for lipids differ from the other tests?

____________________________________________________________

What nutrients were present in the food sample/

____________________________________________________________

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