The Life and Science of Percy Julian
Activities and Readings
10A (Lab Activity)

Changing the Starch in “Yams” to Sugar

Introduction: Medicine from Yams

Have you ever eaten yams? More likely than not, if you live in the United States, you have eaten sweet potatoes, which are not yams though they look similar. Sweet potatoes are often called yams in the United States, even though the two root vegetables are not related.

Some real yams contain poisonous substances that must be removed before eating them, primarily by soaking the root portion of the yams in water for several days. Sweet potatoes do not need to be soaked; they can be eaten after cooking. Interestingly, certain yams have a toxic chemical, called diosgenin, that can be changed through chemical reactions into useful substances. They include the medicine cortisone (used to treat joint pain as in arthritis, allergic reactions like bee stings and rashes, and general body inflammation) and several hormones, including estrogen, progesterone, and testosterone. The hormones mentioned are in a category called steroids. Perhaps you have heard of steroids as substances used improperly to increase a person’s athletic ability. (See the activity The Many Faces of Steroids.)

Starting in the late 1930s, several chemists became interested in trying to use diosgenin from the Mexican yam as a starting chemical, changing it into different steroids. First was testosterone, then progesterone. Then, in the late 1940s, came cortisone. Just for the record, 66 pounds of fresh Mexican yams (from two to three yam roots or tubers—they are BIG!) can produce one pound of the toxin diosgenin, from which two ounces of cortisone can be produced. By the 1960s, these steroid medicines could be produced from Mexican yams and sold to the public at affordable prices.

In this lab activity, instead of carrying out chemical reactions to change chemicals found in yams into steroids, as Percy Julian did, we’ll do something a little easier, and with a more common chemical found in foods—starch. And instead of yams, we’ll use sweet potatoes. Yams and sweet potatoes aren’t the same, but they do both contain starch.


To introduce chemical reactions involving common chemical groups, such as starches, found in food; to illustrate how these chemical groups can be changed to new substances with the assistance of a special chemical substance called an enzyme that is found in human saliva; to illustrate the importance of using a control, a concept that will be explained in class by your teacher.


  1. Wear safety goggles at all times.
  2. Be aware of hot surfaces; use test tube holders to remove test tubes from hot water bath.
  3. Do not get iodine solution into your mouth.
  4. Wash hands after completing this laboratory activity.

Background Information

Starch is a way that plants store energy and is an important food for people. It is found in the seeds and in the tuber (a root structure) of some plants, including yams, potatoes, and sweet potatoes. They are considered to be “energy foods” because the starch in them is chemically changed by our bodies to sugars for energy. This lab exercise will use sweet potato to illustrate the conversion of one chemical substance (starch) to another (sugar).

Iodine is used to test for the presence of starch. Something called “Benedict’s test solution” used to test for the presence of sugar. Your teacher will provide you with both of these, as well as the starch solution.


Day 1

  1. Obtain five test tubes (16mm x 150 mm). Label the tubes #1–5. Mark each tube at 1 cm and 2 cm from the bottom of the test tube (you will need to use either a special wax crayon or a permanent marker). It is easiest to stand the test tube upright on a table with a metric ruler next to it.
  2. Chewing on a clean rubber band, add your saliva to test tubes #1–4 to the 1 cm mark. For test tube #5, add distilled water to the 1 cm mark. Test tube #5 will be your control (your teacher will explain the concept of a control).
  3. Obtain a solution of starch from your teacher and add it to the 2 cm mark in test tube #1. Add 3 drops of iodine solution. Note any changes in the test tube and record your observations on the Data Sheet 1.
  4. Add starch solution to test tube #2 to the 2 cm mark. Add enough Benedict’s test solution to double the volume in the test tube to 4 cm. Place the test tube in boiling water for 5 minutes. Observe any changes and record your observations on the Data Sheet 1.
  5. To test tubes #3, #4 and #5, add starch solution to the 2 cm mark. Allow the test tubes to stand for 30 minutes.
  6. After 30 minutes, conduct the iodine test in #3, and the Benedict’s test in tubes #4 and #5. Record you observations on the Data Sheet 1.

Day 2

Obtain five new, clean test tubes and repeat steps 1–6 above using a starch solution from sweet potato provided by your teacher. Record your observations and results on Data Sheet 2.

Homework: Observations and Results

For both the starch solution reactions and the sweet potato reactions, answer the following questions in writing. Click here for a printable version of this homework assignment.

  1. What chemical changes, if any, took place in test tube #3? What evidence do you have to support your answer?
  2. What chemical change, if any, took place in test tube #4? What evidence do you have to support your answer?
  3. What chemical change, if any, took place in test tube #5? What evidence do you have to support your answer? What results would you expect in this test tube? Explain your answers.
  4. What is the relation between starch and sugar? Explain using your laboratory test results.

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