Objectives: To determine the effect of substrate on yeast fermentation rate

Background: Products of carbohydrates fermentation include carbon dioxide and ethanol. The rate of fermentation will be affected by how readily yeast is able to utilize substrates for energy. In this lab, you will observe differences in fermentation rates based on the type of substrate used.


  • Vernier biodigester
  • Vernier gas sensor
  • Vernier carbon dioxide sensor
  • Yeast substrates (sucrose, glucose, corn starch, pinto bean starch, powdered milk)
  • Warm water
  • Stirring rod


  1. Collect 400 ml of warm water (37oC)  from an incubator
  2. Pour in biodigester
  3. Weigh and add 6 g of assigned substrate to the warm water and mix with a stirring rod to dissolve or disperse thoroughly
  4. Weigh and add 1.5 grams of yeast and disperse with a stirring rod
  5. Cover biodigester with lid and insert carbon dioxide and ethanol probe to monitor the production of carbon dioxide and ethanol. Note: Probes should be about 1cm from above the liquid. It should NOT be immersed in the liquid.
  6. Start collecting data on the lab quest by pressing the green arrow button on the bottom left of the screen
  7. Allow data collection for 45 minutes (2700 s)
  8. After completion of data collection, transfer to your notebooks, CO2 and ethanol production in ppm at 5 minutes (300 s) intervals
  9. Make a table of your results and share your data with the other groups
  10. You will report on all the data, not just your own
  11. Include in your report (1) a table showing substrates and rate of CO2 and ethanol production (ppm/second), and (2) a graph showing growth curves for CO2 and ethanol. Example of growth curve for carbondioxide is below (concentration in ppm versus time in seconds). Note: The curves shown do not necessarily reflect what you will observe in the experiment. Create the graphs using the actual data you collect. Construct a similar graph for ethanol production.

Lab Questions

  1. Explain briefly how ethanol for beer or fuel is made commercially
  2. Draw the chemical structure for each of the substrates used (Note: The basic starch structure will be the same for corn and pinto beans)
  3. Which substrate(s) was used most efficiently by yeast? Which substrate(s) was not utilized by yeast? Compare rates and explain your data. How does the structure of the carbohydrate affect how efficiently yeast is able to utilize the substrate for ethanol production?

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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