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3.2: Starch Protein Teacher's Preparation Notes

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  • In this activity, students learn about the scientific method by carrying out key components of a scientific investigation, including:

    • Developing experimental methods
    • Generating hypotheses
    • Designing and carrying out experiments to test these hypotheses
    • If appropriate, using experimental results to revise these hypotheses.

    The Student Handout provides information and questions to guide students in designing the first experiment, which evaluates two indicator solutions to see whether they can be used to test for starch and/or for protein. Then, students use the results from the first experiment and inductive reasoning to formulate hypotheses concerning which types of food contain starch and which types of food contain protein (some or all foods derived from animals or plants or both). Next, students use deductive reasoning based on their hypotheses to make predictions for a second experiment to test their hypotheses. Finally, students carry out the second experiment and use the results to evaluate their hypotheses and, if necessary, modify these hypotheses.


    In order to complete the experimental portion of Part 1 in 50 minutes, you may want to discuss the material on page 1 and the top of page 2 of the Student Handout in a pre-laboratory session. You will probably want a second 50-minute laboratory period for Part 2.


    Learning Goals

    Students Engage in Scientific Practices

    "Planning and carrying out investigations" – Students should be able to:

    • "Decide what data are to be gathered … and how measurements will be recorded."
    • "Decide how much data are needed to produce reliable measurements and consider any limitations on the precision of the data."
    • "Plan experimental… procedures, identifying… the need for controls."

    "Analyzing and interpreting data" – Students should be able to:

    • "Analyze data systematically, either to look for salient patterns or to test whether data are consistent with an initial hypothesis."
    • "Evaluate the strength of the conclusion that can be inferred from any data set…"


    Additional Learning Goals

    • Accurate, consistent methods and replication of experiments are needed to produce reliable experimental results.
    • Inductive reasoning can provide useful generalizations based on specific observations, but the results of inductive reasoning should be treated with caution, since additional specific observations may show exceptions to the generalization.
    • To test a hypothesis, scientists use deductive reasoning to predict specific experimental results expected on the basis of the hypothesis.
    • Experiments to test a hypothesis often produce results that stimulate scientists to modify their original hypothesis; then scientists perform additional experiments to test their modified hypothesis.
    • An indicator is a substance that changes color in the presence of a particular type of organic compound.
    • To evaluate the specificity of an indicator, it is important to include negative controls.
    • Food contains organic compounds made by other organisms such as plants and animals.
    • Starch is only found in foods derived from plants (since animals do not make starch).
    • Protein is found in some foods derived from animals and some foods derived from plants.



    • Indicator Solution 1 = Iodine-Potassium Iodide Solution (~12 mL per class; available from; if not in an opaque container, should be stored in the dark)
    • Indicator Solution 2 = Biuret reagent (~50 mL per class; available from; Biuret reagent should be fresh since old Biuret reagent is less sensitive as a protein indicator.)
    • Dropper bottles for the indicator solutions (bare minimum of one for each indicator solution; ideally, as many as the number of student groups in your largest class, so each pair of student groups can share a pair of dropper bottles; available from; search for plastic dropping bottles)
    • Containers for testing (28 if you have containers that will be washed and reused; you may want to use small disposable plastic or Styrofoam cups, which are especially useful if you do not have sinks available, but you would need 48 per class; white containers or transparent containers placed on a white background make it easier to see the color change in the indicator solutions)
    • Marker and masking tape for labeling these containers
    • Stirrers (1 or 2 per student group in your largest class; more if you do not want students to have to wash stirrers during the experiment)
    • Water (tap water should be fine; ~50 mL per class for Part 1/~10 mL per class for Part 2 + water for washing unless you have enough containers and stirrers so students do not need to wash them)
    • Gloves (minimum of 1 per student group per day)
    • Samples for Part 1 (You will probably want a little extra of each of these.)
      • Corn starch (~2 mL per class; can be found in the baking needs aisle)
      • Potato starch (~2 mL per class; can be found in the baking needs aisle)
      • Powdered egg whites (~2 mL per class; can be found in the baking needs aisle)
      • Sucrose = "table sugar" (~2 mL per class)
      • Unsweetened Gelatin (~1 mL per class)
      • Vegetable oil (~4 mL per class)
    • Samples for Part 2 (You will probably want a little extra of each of these.)
      • Beans (canned beans, we have had good success with white beans; ~8 beans per class)
      • Butter (~4 mL per class)
      • Jelly (~4 mL per class; you may want to check the label to make sure your jelly does not contain gelatin)
      • Bread (~4 mL per class; whole-grain breads preferred because they have somewhat more protein; many breads contain a little bit of milk or milk product (whey); since Part 2 of the activity analyzes the starch and protein content of foods derived from plants vs. animals, you probably want to have a sample of bread that does not include any milk, whey or eggs)
      • Yogurt (~4 mL per class; you should check the ingredients list to make sure you have a brand of yogurt (e.g. Dannon or Stonyfield) that does not contain starch)


    Suggestions for Implementation and Discussion

    Part 1: Which Indicator Solutions can be used to Test for Starch and for protein?


    Depending on your preference, you can have students measure the amount of sample and water precisely or just estimate the approximate amount (which works equally well).


    To evaluate whether each indicator solution is a good indicator for starch or for protein, students should look for color changes when the indicator is added to:

    • More than one type of starch
    • More than one type of protein
    • Negative controls, including water, sugar, and oil.

    The negative controls are important to establish that an indicator solution shows color change only for starch or only for protein. The expected results are that, in the presence of starch, iodine will change color from yellow-brown to blue-black, and, in the presence of protein, Biuret reagent will turn from blue to purple. Biuret reagent is a little less reliable than the iodine indicator; it is important to use fresh Biuret reagent and it would be good to double-check this test yourself ahead of time. You may want to show your students the color change for each of the indicator solutions before they begin their testing, so they will know what to look for.


    Question 4 can be used to introduce the advantages of replication of each test. You may want to introduce the concept of false positives (which could occur if there were contamination) and false negatives (which could result from insufficient amounts of sample and/or indicator solution). Also, if there is only a small color change, this may be interpreted as a positive response by some observers, but not others.


    To accomplish the goals of having negative controls and replicating results, the experimental design should have duplicate tests with each indicator for each sample for a total of 2 replicates x 2 indicator solutions x 7 samples = 28 tests. I suggest that you have your students work in groups of four, and have replicate tests done by different groups (so whatever experimental error one group might make won't affect the replicate test).


    If there are any differences between replicate tests, you should lead a class discussion of methodological factors that may have influenced the test results and then repeat the test with the optimum methodology to resolve the conflict. You can point out to your students that an important and necessary part of scientific research is to refine and standardize methods in order to get consistent and reliable results.


    In discussing the last part of questions 7 and 8, you will want to include the limitations of inductive reasoning (see e.g. entific_method.html), especially when only a limited number of samples have been tested. To be more certain of conclusions concerning whether either of these indicator solutions can be used to test for starch or for protein, it would be desirable to test each indicator solution on a wider variety of samples, including, for example, glucose and amino acids (the monomers of starch and protein, respectively), as well as additional types of starch and protein.


    Part 2: What Types of Food Contain Starch? What Types of Foods Contain Protein?

    In this part, students:

    • First, use the data from Part 1 to generate hypotheses about what types of food contain starch and protein,
    • Then, design and carry out experiments to test these hypotheses,
    • Interpret the results to see whether they support the hypotheses,
    • And, if the initial hypotheses were not supported or only partially supported, formulate newly revised hypotheses.

    You will probably want to point out to your students that this is how real scientists work as they develop a progressively better understanding of a research question.


    Student hypotheses in response to question 10 will probably vary. This provides the opportunity to mention that this type of disagreement also happens in "real science" when different scientists have different interpretations of the same evidence; typically, these disagreements are resolved by obtaining additional evidence. Some of the student hypotheses may provide the opportunity to discuss how people formulate hypotheses based on both the results of the current experiment and also prior knowledge; this can be a useful part of the scientific process and contribute to cumulative improvements in our understanding of scientific questions. All of the student hypotheses should be compatible with the results from Part 1 which should show that:

    • Some, but not all, foods derived from plants contain starch.
    • At least some foods derived from animals do not contain starch.
    • At least some foods derived from animals contain protein.
    • At least some foods derived from plants do not contain protein.


    In response to question 12, students should describe the need to test each of the samples listed in question 11 with each of the indicator solutions, as well as the need to replicate each of these tests. Thus, you will want a total of 5 samples x 2 indicator solutions x 2 replicates = 20 tests.


    In interpreting the results of their tests, students should be aware that their tests may not be sensitive enough to detect small amounts of starch or protein. The level of protein is high enough to be easily detected in concentrated sources of protein such as egg whites, beans or milk (where it provides nutrition for the growing bird embryo, plant seedling, or baby mammal, respectively). The color of the protein test varies somewhat for different foods; for example, the protein test typically shows a darker purple for bread than for the egg white or gelatin samples. You may want to discuss how scientific results are sometimes ambiguous, and scientists try to improve their methodology and repeat the experiment to clarify any ambiguity.


    Comparing the results of testing the samples in Part 1 vs. Part 2 for protein demonstrates the risk of generalizing from a limited set of observations. In Part 1 none of the foods derived from plants and all of the foods derived from animals have significant amounts of protein, but in Part 2 beans and bread have a significant amount of protein and butter has almost no protein. You may want to point out that generating the hypotheses in question 10 requires inductive reasoning (generalizing from specific examples to more general hypotheses, which has the risk of overgeneralizing, as demonstrated in this activity), whereas making the predictions in question 11 requires deductive reasoning (reasoning from general hypotheses to specific predictions, often used to test hypotheses).


    Only plants produce starch, but starch is not present in significant amounts in some foods derived from plants, because the food is derived from a part of the plant that has little or no starch and/or because preparation of the food product has removed starch that was initially present. You will probably want to talk about the energy storage function of starch for plants, and you may want to talk about the energy storage function of glycogen in animals and fat in animals and seeds (where the greater amount of energy per unit of weight in fat is useful for mobility).


    Professional nutritional analysis provides the following values for starch and protein content for the food samples in this activity (% by weight;  – = missing data; data from





    Corn starch



    Potato starch

    Vegetable oil (corn oil)



    Dried egg whites


    Gelatin, unsweetened



    Whole wheat bread


    White bread








    Low-fat vanilla yogurt#



    White beans (canned)


    Kidney beans (canned)





    * Foods that contain more water tend to have a lower percentage of starch and protein.

    # These figures apply to brands like Dannon and Stonyfield which do not add starch; read the ingredients list on the label to purchase a type of yogurt which does not have starch added.


    Additional Resources for Teaching about the Scientific Method

    A wealth of resources for teaching and understanding the scientific process are provided in

    "Understanding Science – How science really works", available at


    "Using the Scientific Method" (available at is a simulation of a simple experiment with questions to guide the students in describing and analyzing their simulated experiment; the simulation can be carried out quickly, so students can spend most of their time on learning about important issues in experimental design and interpretation, including the need to have all variables except the experimental variable the same for the control and experimental groups in order to test the effect of the experimental variable.


    "The Strange Case of Beri beri" (available at, click on "The Scientific Method in Action".


    "Battling Bad Science" is an entertaining talk about the errors and deceptions behind misleading nutritional or medical advice, available at cre_battling_bad_science.html (the first 7.5 minutes are the most relevant).


    Additional Hands-on Activities in Which Students Design Experiments and Interpret the Results:

    "Moldy Jell-O", available at

    Students design experiments to determine how substrate and environmental conditions influence the growth of common molds. Students carry out their experiments, analyze and interpret their evidence, and prepare a report.


    "Enzymes Help Us Digest Food", available at

    Experiments with the enzyme lactase and discussion questions help students to learn about enzyme function, enzyme specificity and the molecular basis of lactose intolerance. Students also learn about the scientific method by interpreting evidence to test hypotheses and designing the second and third experiments to answer specific scientific questions about lactase.


    "Investigating Osmosis", available at

    Students make predictions about the effects of osmosis and design an experiment to test these predictions.


    "Regulation of Human Heart Rate", available at

    Students learn how to measure heart rate accurately. Then students design and carry out an experiment to test the effects of an activity or stimulus on heart rate, analyze and interpret the data, and present their experiments in a poster session.


    Discussion Activities for Learning about the Process of Science

    "Carbohydrate Consumption, Athletic Performance and Health – Using Science Process Skills to Understand the Evidence", available at

    This discussion/worksheet activity is designed to develop students' understanding of the scientific process by having them design an experiment to test a hypothesis, compare their experimental design with the design of a research study that tested the same hypothesis, evaluate research evidence concerning two hypothesized effects of carbohydrate consumption, evaluate the pros and cons of experimental vs. observational research studies, and finally use what they have learned to revise a standard diagram of the scientific method to make it more accurate, complete and realistic.

    "Vitamins and Health – Why Experts Disagree", available at

    In this discussion/worksheet activity, research concerning the health effects of vitamin E is used as a case study to help students understand why different research studies may find seemingly opposite results. Students learn useful approaches for evaluating and synthesizing conflicting research results, with a major focus on understanding the strengths and weaknesses of different types of studies (laboratory experiments, observational studies, and clinical trials). Students also learn that the results of any single study should be interpreted with caution since results of similar studies vary (due to random variation and differences in specific study characteristics).


    Related Activity

    "Who took Jerell's iPod? – An organic compound mystery" (available at uses some of the same experimental procedures but focuses on student learning about different types of organic compounds and engages students in testing for these organic compounds in the context of solving a mystery.