Introduction

In this lab, you’ll explore how yeast ferments sugar into carbon dioxide and ethanol—the same process used in making biofuels. In the U.S., ethanol is mainly made from corn, while in Brazil it is made from sugarcane. Scientists are also researching advanced biofuels made from plant materials such as grasses. Today, you will set up a simple fermentation experiment using yeast and one common feedstock (sugar or corn syrup). By observing the carbon dioxide released as the yeast ferments, you’ll see how this biological process works and connect it to real-world ethanol production.

Materials 

• 1 packet of dry yeast
  Note: One packet of dry yeast is equivalent to 2 tsp.
• Feedstocks: 2 tbsp of sugar OR 2 tbsp of corn syrup
• 1 cup of warm water: ~40°C or 100°F (1 cup per trial)
• 1 500 mL clear plastic bottle or glass beaker
• 1 balloon
• Funnel
• Stopwatch or timer
• Flexible measuring tape or string and ruler
• Safety goggles and gloves
• Graph paper or digital graphing tool
• Flinn Scientific Kit (optional): Complete lab kit available here: Biofuel Enzyme Kit

Hypothesis

How large do you think the balloon’s diameter will get during the experiment? Make a prediction (in centimeters) and explain your reasoning based on what you know about yeast and sugar fermentation.

Procedure

1. Put on safety gear (goggles/gloves).
2. Add 1 cup of warm water to the bottle/beaker (use funnel if needed).
3. Dissolve the feedstock in the water.
4. Add 2 tsp of yeast and swirl gently. 
5. Immediately place the balloon over the bottle/beaker opening.
6. Record the starting time and measure the balloon circumference every 5 minutes for 30 minutes total, recording your observations in the data table below.
7. To calculate the diameter of the balloon, use the formula: Diameter = Circumference/ℼ
   • Measure the circumference by wrapping the flexible tape measure around the widest part of the balloon to get its circumference. 
   • Or, use a piece of string to measure the circumference, and then lay the string along a ruler to find the exact measurement. 
8. After completing the experiment, discuss and reflect on the results by completing the data analysis questions.

Experiment Tips

• Temperature matters: Yeast is sluggish if water is too cool; too hot (>45 °C / 113 °F) can kill yeast. Aim for ~40 °C / 100 °F.
• Balloon fit: Air leaks sink your signal. Check the seal; make sure it’s secure.
• Kickstart: If activity is slow at 5–10 min, gently swirl (don’t shake) to resuspend yeast.
• Consistency: Measure circumference at eye level and avoid stretching the balloon during measurement.

Data Table

Graphing

On graph paper or using an online graphing tool, plot time (x-axis) vs. balloon diameter (y-axis). Label your graph clearly and don’t forget to include a title!

Analysis Questions

1. Describe the overall shape of your graph. Did the balloon inflate steadily, or did it slow down over time?

2. Calculate the average rate of diameter increase for the first 15 minutes. Compare it to the second 15 minutes. What does this tell you about how fermentation changes over time? 

3. Approximate the balloon as a sphere. Use your final diameter to calculate radius (½ diameter) and then volume (4/3 x π x r³). How much CO2 gas do you estimate was produced? 

4. Was your data consistent? What possible errors could have affected your measurements? 

5. Was your hypothesis about the balloon’s diameter supported by your data? Explain.

6. Suggest one change to the experiment and predict how it might affect results. 

7. In this experiment, yeast produced CO2 as it fermented sugar into ethanol. How is this process similar to how ethanol is produced from corn or sugarcane on an industrial scale? 

8. Our lab used a small amount of feedstock. What challenges might arise when scaling this process up to supply fuel for millions of cars? 

9. Farmers can grow crops like corn or sugarcane for food or for fuel. What are some benefits of using crops for biofuel? What are some drawbacks? 

10. Scientists are researching cellulosic ethanol made from grasses or agricultural waste instead of food crops. Why might this be a better option? What challenges might make it harder to use? 

Assessment Rubric