The yeasts are one very important group of fungi. The common yeast used in baking bread grows very fast. You can complete an experiment in two days! The basic idea in this method is to measure the amount of carbon dioxide (CO₂) released during the growth of yeast. The growth of the yeast stops when one of the nutrients required by the yeast is gone, or when the liquid gets too acid (low pH) and kills the yeast. If you want to try this experiment, follow the directions below.

You will need the following items:

• A teaspoon measure
• A permanent marker
• Active dry yeast (used in baking bread--do not use quick-rising varieties.) This yeast is available in jars if you are planning on doing a large experiment.
• Bottled soda pop or water in equal amounts. Different items contain different ounces per container. Shake each soda bottle and let the foam settle before opening, or open and allow to go flat overnight.
• Identical round, thin latex balloons--"water balloons" are slow to expand. Non-Mylar® "helium-quality" balloons give good results.

Directions for growing yeast

• Label each bottle with a number to keep track of what each one contains--control, treatment and contents, so that you can tell bottles containing the same solution (replicates) apart. Color is not a reliable means of identification--the caramel color used in cola is a carbohydrate and the yeast can eat it.
• Put a teaspoon of dried yeast in each bottle.
• Seal the bottles tightly and shake the bottle.
• Remove the lids and stretch a balloon over the mouth of each bottle. The balloon should fit very tightly so that the carbon dioxide does not leak into the air. Click for an Alternative Method
• Place each container in a warm area out of direct sunlight (top of refrigerator or clothes dryer) where they will not be disturbed.
• Record the diameters of the balloons, time since start of experiment, etc. for each bottle. One good method of measurement is to wrap a string around each bottle at its widest point, and then measure the length of the wrapped string against a yardstick. Record any other things you see happen. Did the color change? Did one balloon have a hole in it?
• Calculate the average diameter of the balloons in each treatment and the controls. The average is calculated by adding all the diameters of all the balloons in a treatment then dividing by the number of balloons in the treatment.
• Compare the results (average balloon diameters) of the experiment.
• A graph of the averages might help show your results.

Questions to help design experiments

• Is the average of the treatments larger than the average of the controls?
• Is the average of one treatment larger than the averages of the other treatments?
• Is carbonated water a better control than non-carbonated water in experiments with different kinds of soda pop?
• Is the amount of sugar used in a bottle related to the amount of carbon dioxide released into the balloon? Hint: graph sugar concentration versus average balloon size.

Use of trade names does not imply endorsement.

Last update: 16 Feb 99. This page was written by Robert Fogel and was edited by Patricia Rogers. We would like thank Kathy Ross for testing the procedure beforehand.