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Single-Celled Science: Yeasty Beasties


Key Concepts
Biology, microorganisms, microscopic, metabolism, carbon dioxide
Teisha Rowland, PhD, Science Buddies


Have you ever looked at a package of dry yeast and found it hard to believe that it contains organisms that are alive? Add the right ingredients and presto, the mixture becomes a bubbly, oozing, mess of life! But what are the conditions that are needed for this to happen? What does that yeast need to become active and thrive? Try this science activity to find out for yourself!  

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


Yeasts are tiny, microscopic organisms – or microorganisms – that are a type of fungus. This means that they are more closely related to a mushroom than to plants, animals and bacteria (which are also microorganisms). These little critters might sound strange and different, but people have actually been using them for thousands of years to make bread rise. How does this work? It has to do with the metabolism of the yeasts, or, in other words, what they eat and what they turn that food into.

Yeasts must get their food from their surrounding environment to grow and reproduce, or make more yeast. What do they eat? Yeasts feed on sugars and starches, which are in bread dough. They turn their food into energy and release carbon dioxide gas as a result. This process is known as fermentation. The carbon dioxide gas made during fermentation is what makes a slice of bread so soft and spongy. The pockets of gas are produced by yeasts when the dough is allowed to rise before baking. Fermentation is also used to make beer, wine and champagne. 


  • Three plastic two-liter bottles
  • Measuring tablespoon
  • White table sugar
  • Salt, baking soda, or vinegar
  • Permanent marker (optional)
  • Measuring cups
  • Warm tap water
  • One medium-sized pot or bowl, at least two quarts in size
  • Six packets of dry yeast or an equivalent amount from a jar
  • Three standard-sized latex balloons
  • Clock or timer


  1. Rinse each bottle thoroughly with water and remove any labels.
  2. Add two tablespoons (tbsp.) of sugar to two of the three bottles. How do you think the sugar will affect the activity of the yeast?
  3. To one of the bottles that you added sugar to, also add two tbsp. of salt, baking soda, or vinegar. How do you think adding salt, baking soda, or vinegar will affect the activity of the yeast? 
  4. Throughout the experiment, keep track of what you added to each bottle. If needed, you can label the bottles with a permanent marker.


  1. Fill the medium-sized pot or bowl with at least eight cups of very warm tap water. Adjust the temperature of the hot water coming from the tap until it is almost too hot to hold your hands under. Use this temperature water to fill the pot. 
  2. Using the warm water from the pot, fill each bottle with about 2 ½ cups (or about one-third full). Put the lid back on to each bottle and shake them each thoroughly to dissolve all of the ingredients. 
  3. To each bottle, add two packets of dry yeast (or an equivalent amount from a jar). Put the lid back on to each bottle and shake each one gently to mix in the yeast.
  4. Remove each lid and stretch a balloon completely over the opening of the bottle (over all of the ridges). Why do you think it is important to form a tight seal with the balloon on the bottle’s opening?
  5. Leave the bottles to rest in a warm location for 45 minutes. Keep the balloons out of direct sunlight. How do the balloons change over time?
  6. After 45 minutes, examine the bottles and the balloons. Which balloons have become inflated? How big are they compared to each other? Do you notice any differences in the contents of the bottles?
  7. In which environment did the yeast make the most carbon dioxide? What does this tell you about the conditions needed for yeast fermentation to take place?  

Extra: You could quantify your results from this activity by using a water displacement test. To do this, you could fill a large pot completely full with water, place it in a larger tray, pan, or pot, and then submerge the balloon in the water. You can measure how much water overflowed from the pot into the tray to determine how much water the balloon displaced, and consequently the volume of the carbon dioxide gas inside the balloon. If you quantify your results, exactly how different are the sizes of the balloons?

Extra: Another environmental condition that can affect the activity of yeast and the process of fermentation is temperature. You could explore this by preparing several bottles using the same conditions, and then placing each bottle in a different place with a different temperature. After 45 minutes, how do the balloons vary in size? 

Extra: You could try this activity again but next time just focus on how using different types and sources of sugars affect the carbon dioxide production. How do the sugars from different juices or other sources affect how much carbon dioxide is produced

Observations and Results

Did the balloon on the bottle with only yeast and water remain un-inflated? Did the balloon on the bottle with only sugar added inflate the most? 

When yeasts eat sugar and turn it into energy, they also produce carbon dioxide. This process is known as fermentation. In this activity, the balloons on the bottles should have captured carbon dioxide produced by the yeasts during fermentation. In the bottle that contained yeasts but not sugar, the yeasts did not have the food that they needed (i.e., sugar) so the balloon should not have inflated. In the bottle that contained yeasts and sugar (but not salt, baking soda, or vinegar), the yeasts should have thrived and made a lot of carbon dioxide, clearly inflating the balloon. When salt, baking soda, or vinegar was added, the yeasts should have made less carbon dioxide, inflating the balloon less than when only sugar was used. This is because the addition of these substances changed the environment and made it less ideal for the yeasts. Specifically, adding salt increased the salinity of the environment, and adding baking soda or vinegar changed the pH of the environment, making it more basic or acidic, respectively, compared to the neutral environment provided by the plain water.

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  1. When you are done with this activity, dispose of the yeasts by composting them or (with permission) dumping them outside somewhere. Do not pour the yeasts down the drain without diluting them with water as they may damage pipes when they expand.

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