Yeast Buster

Setting the Scene

Yeast are one-celled microorganisms related to fungi. The yeast you find dried and packaged in the supermarket (called Saccharomyces cerevisiae) is used for making bread. "Feed” it some water and a little sugar, and it springs into action! It uses the sugar as food, and it begins multiplying itself. In this process, which is called fermentation, the yeast gives off two valuable waste products -- alcohol and carbon dioxide. The alcohol is essential to the production of beer and wine. The carbon dioxide is used to make breads and cakes rise. (The alcohol evaporates during baking.)

Human skin is host to a type of yeast (called Pityrosporum ovale) that normally doesn’t bother us. However, an outgrowth of this particular yeast causes the skin on the scalp to react, producing dandruff. One treatment is often found in dandruff shampoos, particularly those that contain the antifungal agent ketoconasole. By inhibiting the growth of yeast, the dandruff is reduced.

Does anti-dandruff shampoo inhibit fermentation of baker’s yeast? Do this experiment to find out.

Stage Direction

This procedure lends itself to a nice hands-on investigation. You can do this as a class demonstration, or students can do it in cooperative groups. The results take several hours, so you want to set it up first thing in the morning.

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PLOT

Act I

1. Empty about half of the water from each of the three bottles; leave an equal amount of water in each bottle. (Save the bottle caps.)
2. Use the funnel to add 1 tablespoon of sugar to each bottle. Put the caps on the bottles and shake them so the sugar dissolves.
3. Label one bottle CONTROL.
4. Put 2 teaspoons of baby shampoo into the second sugar-water bottle and label that one BABY SHAMPOO.
5. Put 2 teaspoons of anti-dandruff shampoo into the third bottle and label it ANTI-DANDRUFF SHAMPOO.
6. Remove the caps and add 1 teaspoon of dried baker’s yeast to each bottle. Put the caps back on the bottles and invert them slowly back and forth to thoroughly mix the contents without causing foam to form from the shampoos.
7. Remove the bottle caps and quickly stretch the mouth of a balloon over the top of each bottle. Make sure that the neck of each balloon completely covers the screw channels at the top of the bottle.
8. Note the time.

Act II
Watch your experiment over the next 2 to 3 hours. Note the time when each balloon pops into an upright position. After that, you can measure the carbon dioxide production by wrapping a string around the largest part of each balloon. The length of the string will give a measure of the circumference of the balloon and, therefore, a rough measure of how much gas has been produced.

Smell the balloons. Can you smell any alcohol? If you can, that tells you that the alcohol molecules are small enough to pass through the rubber of the balloons. Can you smell anything through the bottle?

Act III
Can students discover any other yeast-buster substances? Like anti-dandruff shampoos, some antifungal preparations for feet and antiseptics for cleaning bathrooms are yeast busters too.

Behind the Scenes

The balloon that popped up first was your control. The baby shampoo was a test to see if shampoo alone inhibited fermentation. How good was the anti-dandruff shampoo at inhibiting yeast action?

The End

Use this experiment to stimulate a conversation about advertising.

Article By Vicki Cobb
Education World®
Copyright © 2007 Education World

03/22/2007