The Life and Times of Percy Julian

For Teachers

Activity 10A (Lab Activity)
Changing the Starch in “Yams” to Sugar

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Lesson Overview This lab activity provides students with the experience of using their own enzymes to convert starch to sugar (a disaccharide called maltose), then testing to see if the conversion has taken place.
Learning Objectives Students will learn how to set up an experiment that includes a control, apply lab data to test the concept that starch can be changed (converted) to a sugar, and to determine if that conversion has taken place.
Lesson Concepts

This activity includes chemical reactions with an enzyme (amylase), establishing standards for detecting the existence of reactants or product(s)—starch and sugar (maltose)—and for utilizing a control.

Materials Required
  • Lugol solution (if preparing, 10 g KI in 100 ml water, add 5 g iodine crystals; transfer to dropping bottles)
  • Benedict’s solution in dropping bottles (best to buy from chemical supplier)
  • Test tubes (regular size, 16mm x 155 mm); 10 per group
  • Wax pencil or permanent marker
  • 10 cm metric ruler
  • Distilled water
  • Starch solution (If preparing, easiest to buy can of spray starch and add to distilled water in beaker; alternative is 10 g of soluble starch in a minimum of distilled water, 100 ml; after wetting and mixing, add another 400 ml water and heat, stirring frequently. If well dissolved, solution should be nearly transparent.)
  • Sweet potato starch solution (see procedure notes)
  • Clean rubber bands for chewing (can be disinfected with rubbing alcohol, then allowed to air dry)
Skills Required Students should be able follow printed instructions and respect safety guidelines while working.
Time Required

This exercise will require two 40-45 minute periods, the student’s time to complete the homework assignment, and class time to discuss the homework assignment.

Student Ability Level and Grouping Students of all abilities can work in groups of pairs through a maximum of 4 students.
Pre-lab Students should be instructed in the purpose of using reference standards (starch and sugar tests) and a control. Also, students should be instructed in the safe removal of test tubes from a water bath.
Procedure Notes
  1. See above for the preparation of the iodine, Benedict’s and starch solutions. All of these solutions can be purchased from a commercial school science supplier.
  2. To make a starch solution from sweet potatoes, purchase the tubers (do not use canned sweet potatoes as they most likely have sugar and corn syrup added). For one or two tubers, cut into small pieces and place in a blender or food processor with about 50 ml of water. Produce a pulp. If there is no liquid, add additional water in 50 ml increments. Let the liquid stand for 5 minutes, then filter through cheesecloth to produce a pulp-free solution.
  3. Do not let students use gum to produce saliva as it will contain sugars.
Post-lab It is important that the class come together to compare results and to make sure that students understand what was going on in the test tubes. Ask them why they were asked to wait 30 minutes before testing for starch and sugar. Also, what does a negative test for starch in test tube #3 mean? Why would there be less or no starch in test tube #3 after 30 minutes? If test tube #5, the control (no saliva), shows a positive test for sugar, how would you explain this? (There may be some starch that breaks down with out an enzyme.)

Students should respond in writing to the questions posed in the lab exercise as homework, and the class as a whole should discuss the responses of individual groups the following day.


  1. In test tube #3, the iodine test should show either a reduction in the amount of starch still remaining or no starch present (a negative test). Conversion of starch by an enzyme to sugar is affected by temperature (lower temperature means lower rate of conversion; body temperature is 37oC and room temperature may be about 22–25oC.
  2. The test for sugar made from starch should be positive and is related to test tube #3 in that the quantity starch was reduced or disappeared due to its conversion to sugar. Ask students if they have ever experienced a sweet taste if they leave a cracker “soak” in their mouth for a short time. Or have them try it at home.
  3. The results in the control tube, #5, should be negative for sugar. However, it is possible that a small amount of starch is converted to sugar without the presence of saliva.
  4. The lab results should help to illustrate the conversion of starch to sugar, that any sugar found in any of the test tubes that originally contained only starch had to come from the chemical change of the starch to sugar.
Student Resources for Further Study

This lab exercise relates to many different topics, including digestive enzymes (in the broader category of catalysts) and agriculture involving the production of tuber crops, including both yams and sweet potatoes. This lab is also related to the work of Percy Julian in the sense that he worked with yams from Guatemala and Mexico to extract the toxin called diosgenin. This chemical was then used as a base chemical to be converted to cortisone and several important hormones including estrogen, progesterone, and testosterone at a price that made these pharmaceuticals available to most people.


How Have Yams Changed Medicine?

What Is the Difference between a Sweet Potato and a Yam?

Mexican Yams—Cortisone, Steroids, and Oral Contraceptives


This activity meets the following National Science Education Standards (Grades 5-8) and Curriculum Standards for Social Studies.

National Science Education Standards (Grades 5-8)

Science as Inquiry

  • Abilities necessary to do scientific inquiry
  • Understandings about scientific inquiry

Physical Science

  • Properties and changes of properties in matter

Science and Technology

  • Understandings about science and technology

History and Nature of Science

  • Science as a human endeavor
  • History of science

Curriculum Standards for Social Studies

  • None.

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