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Yeast fermentation at different pH levels

Success Goal

Demonstrate how environmental pH levels can affect the growth rate of yeast. Students can explore pH scales and how this relates to the process of fermentation, which is the process in which microorganisms (like yeast) break down sugars into energy sources for its own use. Changes in pH can ultimately affect the growth ability of yeast. Baker’s yeast thrives in a pH range of 4 to 6, and growth can be inhibited in pH levels outside this range.


  • Three available Pioreactors
  • Dry baker's yeast
  • YPD broth mix, or any other media mix
  • Vinegar
  • Ammonia
  • A pH meter or pH strips (optional)


  1. Prepare a sterile stock of 50g YPD / 1L distilled water. Aim to make 20ml × the number of Pioreactors you are using, as each Pioreactor will require about 15mL of media.
  2. In volumes of 15 mL, add vinegar to one vial, ammonia to a second vial, and the YPD media to a third vial.
  3. Optional: Use pH strips or a pH meter to measure the pH of each vial.
  4. Inoculate each vial with a very small amount of baker's yeast using best practices to avoid other contamination.
    • A yeast stock solution can be made by diluting a small amount of yeast in 15mL of YPD stock media, then a drop of this stock solution can be added to your vials.
  5. Wipe the vials and place them in the Pioreactors.
  6. Visit pioreactor.local and start a new experiment.
  7. On the left menu, select the Pioreactors page. Add any additional Pioreactors that you would like to use (more information here).
  8. Select Calibrate for each Pioreactor and under the Blanks tab, click Start.
  9. Select Manage all Pioreactors, and start Stirring activity, Temperature automation activity (set to an optimal temperature; ex. 30°C) and OD reading activity.
  10. Confirm that everything looks normal (ex: receiving optical density signal).
  11. Back on the Pioreactors page, select Manage all Pioreactors and start Growth rate. It will take a minute for results to begin showing up.
  12. Optional: you can change the names of the Pioreactor in the UI to display the media or pH.
  13. Students can watch growth progress on the Overview page.