How to Measure Yeast Growth

Yeast grows through a process called fermentation, which happens when the yeast has access to food, like sugar. When the yeast grows, it produces a gas called carbon dioxide, or CO₂, which you can use measure how yeast grows over time. This experiment uses bottles and balloons to capture the CO₂ from fermenting yeast and helps you to visualize how yeast uses sugar to grow.

Part 1

Part 1 of 2:

Filling the Bottles

1
Pour 1 cup (240 mL) of warm water each into 2 clean, empty bottles. Heat up the water in a microwave or on the stove until it’s between 105–115 °F (41–46 °C). Check the temperature with a kitchen thermometer. Use a funnel to ensure that none of the water spills when you pour it into the bottle.^{[1]XResearch source}
- Make sure the bottles are larger than 20 fluid ounces (590 mL) so they can hold the liquid and gas during the experiment without causing the balloon to explode.
2
Label 1 bottle as control and the other as experimental. Write “E” on one of the bottles for experimental, and “C” on the other for control. Be sure to use a marker that won’t rub or wash off of the bottle if it gets wet.^{[2]XResearch source}
- If you don’t have a marker, you can tape a piece of paper marked “E” and “C” to each of the bottles to ensure that you remember which one is this.
3
Pour 1 teaspoon (4 g) of granulated sugar into each bottle. Add the sugar to both bottles, since you’re going to compare how much CO₂ is produced with yeast and without yeast. The control and experimental bottles should have exactly the same ingredients aside from the yeast.^{[3]XResearch source}
- The sugar acts as the “food” for the yeast to consume while it produces the gas in the bottle.
4
Add 1 teaspoon (3.5 g) of active dry yeast to the experimental bottle. Next, pour the yeast into only the bottle labeled “E.” This will start the reaction of the yeast and sugar, so try to work quickly after you add the yeast.^{[4]XResearch source}
- Don’t add the yeast to the control bottle since you want to be able to compare the growth of the balloon from the bottle with yeast to the bottle without. This ensures that the yeast is the ingredient producing the CO₂.
5
Combine ingredients until the sugar and yeast dissolve. Hold the bottle in your hand and move the bottle in small circles until there are no solid particles remaining in the liquid. The liquid will turn to a light brown color due to the yeast, and some bubbles may start forming at the top of the water.^{[5]XResearch source}
- If you still have the caps to the bottles, you can screw the caps onto the bottles and invert them a few times to mix the ingredients.
- Make sure to dissolve the sugar in the control bottle as well.
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Part 2

Part 2 of 2:

Tracking Growth with Balloons

1
Stretch a balloon over the mouth of each bottle. After the ingredients are combined, remove any bottle caps and quickly attach a balloon to the mouth of each bottle. Make sure there is no air leaking out of the balloon from the mouth of the bottle.^{[6]XResearch source}
- Since the balloon isn’t yet inflated, it will probably fold over to one side of the balloon.
2
Place the bottles in a sunny spot to encourage fermentation. Yeast works more efficiently in warmer temperatures, so move the bottles next to a window. If the sun isn’t out, you can place the bottles next to a heat source like a vent or a radiator.^{[7]XResearch source}
- If you can’t find a warm place for the bottles, you can leave them at room temperature and the yeast will still work since the water is warm.
3
Measure the circumferences of the balloons every 10 minutes. Wrap a piece of flexible measuring tape around the widest part of each balloon, and write down the measurement. Make sure to note how many minutes have passed since the balloon was placed on the bottle. Continue taking measurements for 1 hour to gather at least 6 measurements.^{[8]XResearch source}
- The control balloon will likely not expand. If you’re having trouble taking the measurement, have someone hold up the balloon for you, or measure the widest part of the flat balloon and multiply by 2 to get the circumference.
4
Discuss what happened to make the balloons expand and the yeast grow. Research how yeast reacts with sugar to create carbon dioxide. Then, you can make a graph of your data to show the rate of CO₂ production over time. Think about what would happen to the yeast and how it would grow if you added more sugar or used colder water.^{[9]XResearch source}
- If you’re curious about finding the answer to these questions, you can repeat the experiment using cold water, additional sugar, more yeast, less yeast, or even artificial sweetener instead of sugar.
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Question
What measurements should I be looking for?
Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.
Bess Ruff, MA
Environmental Scientist
Expert Answer
To get a sense of how much CO2 the yeast produces compared to the control, you are measuring the circumference of the balloon attached to each bottle. You can conduct these measurements in inches or centimeters, just make sure you measure both balloons in the same units.
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Question
What is your control in the first method?
Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.
Bess Ruff, MA
Environmental Scientist
Expert Answer
Your control for this experiment is the bottle with only warm water and sugar.
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