Identification of Proteins and Lipids in Food Samples

Objective: To analyze food samples for the presence of lipids, starch, monosaccharides, and proteins.

Background:    Substances or compounds that supply your body with energy and the building blocks of macro-molecules are called nutrients.  The food you eat contains nutrition important to your body.   Sugars and starches make up a group of organic compounds called Carbohydrates, which are important in supplying your body with energy.  Some starches provide your body with indigestible fiber, or roughage, which aids digestion.  Organic compounds called proteins are important for growth and repair. Lipids are organic compounds that can supply as much as four times the amount of energy as carbohydrates or proteins.

You can perform qualitative tests to identify the presence of organic compounds in food using Indicators, chemical substances that react in a certain way when a particular substance is present.  Benedict's solution is used to identify the presence of reducing sugars, such as glucose. Lugo's iodine solution is used to identify the presence of starch. Biuret solution is used to identify the presence of protein. Sudan III is used to identify the presence of lipids. A standard is a positive test for a known substance.  Unknown substances can be tested and compared with the standard for positive identification of the substance.

Materials:

• 5 test tubes
• biuret solution
• glass marking pencil
• known lipid,protein and carbohydrate solutions
• unknowns for protein,lipid, carbohydrate test
• paper bag

Procedure:

Lipid Test

1. Place 1 drop of oil on a piece of paper bag. Allow to sit for 5 minutes

2. Place 1 drop of water on a paper bag. Allow to sit for 10 minutes

3. Observe the paper bag for any changes. The oil is your positive control and the water is your negative control.

4. Draw 4 circles on your paper bag and label as follows:

• A. Potato Chip
• B. Bread
• C. Cheese
• D. Cookie

5. Place a piece of each sample on the paper bag and allow to sit for 5 minutes. Remove samples and analyze bag for presence of lipids. Record data below.

Table 1-2 Test for Lipids

Protein Test

1. Label two test tubes as follows:

• A. Protein Solution - Positive
• B. Water - Negative

2. Place 20 drops of protein solution in test tube A

3. Place 20 drops of water in test tube B.

4. Add 5 drops of Biuret solution to each test tube and record the results.

5. Label four test tubes with the names of your unknown solutions.

6. Add 20 drops of each unknown to the correct test tube

7. Add 5 drops of biuret solution to each test tube.

8. Observe and record your results.

Table 1-2 - Results of Biuret Test for Proteins

Benedicts test-Monosaccharides

1.      Perform the Benedict's test, select three clean test tubes.  With a wax pencil, label the tops of the test tubes "1" "2" "3"

·         To test tube 1, add 40 drops of glucose

·         To test tube 2, add 40 drops of starch

·         To test tube 3, add 40 drops of water

2.      Add 10 drops of Benedict's solution to each test tube. Heat the test tubes in a hot water bath with a temperature rage of 40-50 degrees celsius for five minutes

3. Record Observations. Color change from blue to any color indicates the presence of monosaccharides

Lugols Test- Starch

     1. To perform the Lugol's iodine test, select three clean test tubes. With a wax pencil, label the tops of the test tubes "1" "2" "3"

·         To test tube 1, add 40 drops of glucose

·         To test tube 2, add 40 drops of starch

·         To test tube 3, add 40 drops of water

    2. Add 2 drops of Lugol's iodine solution to each test tube.

3.      Record Observations. Color change from amber to blue-black indicates the presence of starch

4.      Repeat iodine test on unknown solutions. Record your observations

 Authors:
Shannon Tice  and Barbara Lillien,