Bio A 2013 - Biochemistry
Identifying Food Nutrients
Intro: You have just started a job as a food-quality analyst. Your task will be to develop a kit to test foods for
sugar, starch, protein and lipids. The test kit will be used by dietitians who will analyze foods to create
nutritional menus for hospital patients. Your first assignment is to demonstrate proficiency using standard tests
for nutrients in foods.
Background: Substances or compounds that supply your body with energy and the building blocks of macromolecules are called nutrients. The food you eat contains nutrients that are 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 carbohydrates provide your body with indigestible fiber, or roughage, which aids
digestion. Organic compounds called proteins are important for growth and repair. Proteins also act as
enzymes which aid in all the chemical reactions that happen in your body. Lipids, or fats, are organic
compounds that can supply as significantly more energy than either carbohydrates or proteins alone.
You can perform qualitative tests to identify the presence of organic compounds in food. Qualitative tests will
identify if a substance is present, but it will not tell you how much is present. To determine how much of a
substance is present we would need a quantitative test. These tests rely on chemicals called Indicators, that
react in a certain way when a particular substance is present. We used pH indicators in the previous unit that
changed color depending on whether something was an acid or a base. In this lab we’ll be using three
indicators: Benedict's solution turns white or orange in the presence of sugar, Lugol's iodine solution turns
black or deep blue in the presence of starch, and Biuret solution turns purple in the presence of protein. To test
for lipids we will take advantage of a property that we should all be familiar with: Oil and Water don’t mix!
A standard a substance that is known to give a positive result for a particular indicator. For instance, a standard
for the iodine tests would be a solution that we know has starch in it. Unknown substances can be tested and
compared with the standard for positive identification of the substance (standards will be available at the front
desk, please do not move them. As a good scientist you should also have a negative control: a substance that
we know will have a negative result for all tests.
DIRECTIONS:
1) First Identify a name for the entire lab, and a purpose for the lab
2) For each test be sure to fill in the required information (the first three questions) BEFORE doing
the next test. Additionally, fill in the colors of the test tubes (approximate) prior to adding
reagent, and AFTER adding reagent (and in the case of the Benedict’s test, after heating
Procedure:
1. Put on safety goggles and a lab apron (yes you must)
2. Select 5 clean test tubes. Label the tops of the test tubes "1" "2" "3" "4" or "5"
Bio A 2013 - Biochemistry
Lab Test 1. Benedict’s Test:
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To test tube 1, add 10 drops of unknown 1
To test tube 2, add 10 drops of unknown 2… you get the picture. Do this for all 5 unknowns.
Record color of tubes BEFORE adding reagent. Then record color of tubes after adding 5 drops of
Benedict's solution to each test tube. Bring your tubes to the hot water bath. Heat until color change is
apparent (about 30-40 seconds). THIS REQUIRES SUPERVISION, Mrs. Landa must be there. If no color
change occurs, allow one more minute.
- Record your observations in your lab notebook.
- You will not need the water bath for any other test
- Clean and dry your tubes as best as possible for the next test using detergent and test tube brushes.
Lab Test 2. Lugol’s Iodine Test: Select 5 clean test tubes. Label the the test tubes "1" "2" "3" "4" or "5"
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To test tube 1, add 10 drops of unknown 1
To test tube 2, add 10 drops of unknown 2… you get the picture. Do this for all 5 unknowns.
Add 1 drops of Lugol's iodine solution to each test tube.
- Record your observations in your table for lab test 2. Be sure to write the colors of ALL tubes.
- Clean and dry your tubes as best as possible for the next test using detergent and test tube brushes.
Lab Test 3. Biuret Test: Select 5 clean test tubes. Label the tops of the test tubes "1" "2" "3" "4" or "5"
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To test tube 1, add 10 drops of unknown 1
To test tube 2, add 10 drops of unknown 2… you get the picture. Do this for all 5 unknowns.
Add 4 drops of Biuret solution to each test tube.
- Record your observations in your table for lab test 3.
- Clean and dry your tubes as best as possible for the next test using detergent and test tube brushes.
Lab Test 4. Lipid Test: Select 5 clean test tubes. Label the tops of the test tubes "1" "2" "3" "4" or "5"
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To test tube 1, add 10 drops of unknown 1
To test tube 2, add 10 drops of unknown 2… you get the picture. Do this for all 5 unknowns
Add 10 drops of COLORED WATER to each unknown
Cover each tube with a small piece of parafilm and gently invert the tubes a few times to mix in the water.
Record your observations on a separate piece of paper for later. (think about what you are looking for here!)
Clean up:
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Dispose of your materials according to the directions from teacher.
Clean up your work area including your test tubes and wash your hands before leaving the lab.
You work station will be inspected before you can receive the question sheet for this lab.
Bio A 2013 - Biochemistry
Analysis Questions:
8. In this experiment we used pure substances, but these tests will also work with real food (with a little extra
preparation). Why might interpreting the test results of real food be a little more difficult than the pure
substances we tested today? (For example: ketchup has sugar, would a benedict’s test be clear?)
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9. Bob the scientist is at work yet again. He is trying to analyze a substance to determine what macromolecules
are present. He gets the following data.
Test:
Iodine
Colored water
Biuret
Benedicts
Results:
Test tube turns bluish black
Mixes well
Test tube turns purple
Test tube turns white
10. What can Bob conclude about the components of his substance?
11. Which of the following food labels represents a food that could be Bob’s mystery substance?