Abstract

Objective

In this project you will measure the fermentation activity of yeast in different conditions.

Introduction

Yeasts are micro-organisms that are more closely related to a mushroom (they are a single-celled fungi) than to bacteria. Fungi are not green, do not have chlorophyll, and cannot undergo photosynthesis to make their own food. Instead, yeast must get their food from their surrounding environment. Yeasts use sugars (simple carbohydrates) and starches (complex carbohydrates) as food sources.

Yeasts break down these sugar molecules to make energy and release carbon dioxide gas (CO₂) as a result. The carbon dioxide gas is what makes a slice of bread so soft and spongy. The pockets of gas are produced by yeast when the dough is allowed to rise before baking. This is the process of fermentation. Fermentation is also used to make beer, wine, and champagne.

In this experiment you will grow yeast in different conditions to see which conditions will cause the yeast to be most active during fermentation. You will use balloons to trap the CO₂ gasses released by the yeast during fermentation. Then you will submerge the balloons under water and use water displacement to measure the amount of gas in each balloon. Which conditions will cause the formation of the most CO₂ gas and the most active fermentation?

Terms, Concepts, and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• yeast
• micro-organism
• fermentation
• carbon dioxide gas (CO₂)
• water displacement

• Are yeasts alive?
• How can the activity of yeast be measured?
• How much gas can yeasts produce?
• What are the best conditions for yeast to ferment?

Bibliography

• This project is based on a classic science demonstration, nicely described by the Exploratorium, a hands-on science museum in San Francisco:
  Exploratorium Staff, 2006. "The Science of Bread: Yeast Air Balloons Activity," The Exploratorium, San Francisco, CA. [accessed July 3, 2006]
  http://exploratorium.edu/cooking/bread/activity-yeast.html
• Here is an interesting twist on the project using different juices as a sugar source, submitted by Cara from Maryland to Zoom Science at PBS Kids:
  Zoom, 2005. "Yeast: Part II," Zoom Science, PBS Kids. [accessed July 3, 2006]
  http://pbskids.org/zoom/activities/sci/yeastpartii.html#results
• Wikipedia contributors, "Yeast," Wikipedia, The Free Encyclopedia [accessed July 3, 2006]
  http://en.wikipedia.org/w/index.php?title=Yeast&oldid=65628541

Materials and Equipment

• 5 packets of dry yeast
• 5 recycled plastic liter bottles
• white table sugar
• salt
• baking soda
• vinegar
• water
• small balloons
• string or clips for tying off balloons
• large, clear, metric liquid measuring cup
• permanent marker

Experimental Procedure

1. First, prepare and label the water bottles for your experiment. Rinse each bottle thoroughly with water and remove any labels. Label each bottle with the permanent marker, using this data table as a guide (all bottles will have yeast):

   Bottle	Condition	Balloon Color	Observations	Water Before (mL)	Water After (mL)	Water Displaced (mL)
   #1	no sugar
   #2	sugar
   #3	sugar + salt
   #4	sugar + baking soda
   #5	sugar + vinegar

2. Add 1 tbsp of sugar to bottles 2–4. Do not add sugar to bottle #1 because it is the negative control. Yeast needs sugar for the process of fermentation, so bottle #1 should stay inactive. Bottle #2 will only have sugar and will be a positive control.
3. Add 1 tbsp of salt to bottle #3. This will increase the salinity, or amount of salt, in the environment. How do you think increasing the salinity will affect fermentation?
4. Add 1 tbsp of baking soda to bottle #4. This will lower the pH of the environment, making the environment more basic. How do you think a basic environment will affect fermentation?
5. Add 1 tbsp of vinegar to bottle #5. This will increase the pH of the environment, making the environment more acidic. How do you think an acidic environment will affect fermentation?
6. Fill each bottle with 300 mL (about 1/3 full) of warm water, replace the lid, and shake thoroughly to dissolve all of the ingredients. Then add a packet of dry yeast to each bottle.
7. Replace the lid and shake gently to mix in the yeast.
8. Remove the lid and stretch a balloon over the opening of each bottle. Choose a different color balloon for each condition and write the color in your data table.
9. Leave the bottles to rest in a warm location for 1 hour.
10. After 1 hour, look at the balloons. Are they the same or different? Write your observations in the data table.
11. Tie a length of string tightly around the base of each balloon to seal the balloon so that any gas inside cannot escape. When the balloon is tightly sealed, remove it from the bottle top, being VERY CAREFUL not to loosen the string or allow any air to escape from the balloon.
12. Now you are ready to measure the amount of gas inside each balloon with a water displacement test. Fill the large measuring cup with enough water to fully submerge the balloon. To be sure that the container is large enough for the balloons, and that you do not fill it with too much water, do a trial run with a balloon you blow up yourself. Practice dunking the balloon without spilling any water out of the measuring tank and without dunking your hand in, too. Then, when you feel ready, dunk each experimental balloon into the water, each time writing the water level before and after into the data table.
13. Subtract the water level before from the water level after to get the volume of gas produced by the yeast.
14. Graph your results for each environmental condition. Under which conditions did the yeast produce the most CO₂ gas? What does that say about the amount of fermentation and activity of the yeast?

Variations

• Another environmental condition that can affect the activity of yeast and the process of fermentation is temperature. Prepare several bottles using the ideal conditions you describe above. Now place each bottle in a different place with a different temperature. After one hour, measure the amount of gas produced and compare.
• In this experiment, you added the same amount of sugar to each bottle because it is what the yeast uses to make CO₂ gas during fermentation. If you add more sugar, will you always get more CO₂ gas? How much more sugar can you add? Is there a limit?
• More advanced students may want to try these Science Buddies projects using yeast:
  • Rise to the Occasion: Investigating Requirements for Yeast Fermentation
  • Yeast Reproduction in Sugar Substitutes
  • An Aerobic Exercise: Yeast Metabolism with and without Aeration

• For more science project ideas in this area of science, see Microbiology Project Ideas.

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2006-08-04 20:00:00