Labels on food products describe the contents of the food to provide the consumer with valuable information. The label lists important facts about the ingredients, calories, vitamins, minerals, fats, proteins, and carbohydrates in the food.

How does anyone figure out how much protein, how many carbohydrates, how much fat, etc. food contains? Chemists and nutritionists use specific chemicals to test and identify nutrients in foods and determine the levels of each.

Testing a Food Sample

In this activity, you will test an unknown food sample for the presence of carbohydrates, proteins, and lipids.

  1. In the Gizmotm, drag test tube A from the Food samples to the food sample holder. First, you will test this sample for carbohydrates, which are the main energy source for humans. Carbohydrates are found in foods such as potatoes, pasta, bread, and fruit. The Gizmo has two tests for carbohydrates.
    1. In the Carbohydrate test area, under Benedict test, click Test. Observe what happens. Did you get a positive result (solution turns orange-red) or a negative result (no color change)?
    2. A positive result from the Benedict test indicates that a monosaccharide, such as fruit sugar, is present. What can you conclude about sample A?
    3. Under Lugol test, click Test. Observe what happens. Did you get a positive result (solution changes to a deep purple), or negative (solution is orange like the Lugol solution that was mixed in)?
    4. A positive result from the Lugol test indicates the presence of a polysaccharide, or a starch. Does sample A contain a polysaccharide?
    5. If no color change occurs in either test, then a disaccharide, like table sugar may be present. Could there be a disaccharide in this sample?
    6. Make a table like the one shown below and record your results for food sample A. Leave plenty of room for results from all 19 food samples. (Note: A full version of this table is given at the bottom of this Exploration Guide. Print it out if you like.)


    data table
  2. Next you will test Food sample A for proteins. Proteins are organic molecules that are essential for the structure and function of living cells. Good sources of protein are meats, fish, dairy products, and peanuts.
    1. In the Protein test area, under Biuret test, click Test. Observe what happens. Did you get a positive result (solution changes color to purple), or a negative result (no color change)?
    2. A positive result indicates that protein is present. Do you think sample A contains protein?
    3. Record this result in your table.
  3. Finally you will test Food sample A for lipids. Lipids are organic molecules in the form of fats (solid) or oils (liquid) found in both animals and plants. Foods that are sources of fats are beef, whole milk, and butter. Foods that contain oils include peanut butter and cooking oils.
    1. In the Lipids test area, under Sudan red test, click Test. Observe what happens. Did you get a positive result (solution is red with bubbles), or a negative result (solution is red with no bubbles)?
    2. A positive result indicates that lipids are present. What can you conclude about sample A?
    3. Record this result in your table.

Analyzing More Food Samples

In this activity, you will test several unknown food samples for the presence of carbohydrates, proteins and lipids.

  1. Click Reset (reset button). Test Food samples E, G and M for carbohydrates, proteins, and lipids. Record your results in your table.
    1. Based on your test results, is Food sample E most likely to be apple juice, steak, bread or butter? Explain why, based on your test results.
    2. Is Food sample G most likely to be table sugar, pasta, vegetable oil, or peanut butter? Justify your answer.
    3. Is Food sample M most likely chicken, rice, orange juice or butter? Explain why.
  2. Click Reset. Test each remaining food sample. Record your findings in your table as you do.
  3. Below is the full data table. Use this to record all your findings from this Gizmo. (The results for food sample A are already filled in for you.)

    data table