demonstrations:watching_yeast_make_bubbles

Watching Yeast Make Bubbles

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★☆☆ Minimal safety procedures required

Categories: Microbiology, Reproduction

Alternative titles: Observing Yeast Fermentation

Summary

This demonstration shows how bread yeast ferments sugar to release carbon dioxide, which can inflate a balloon, and allows students to observe live yeast cells under a microscope.

Procedure

  1. Place a packet of active dry yeast into a bowl.
  2. Add warm water and about two tablespoons of sugar, then stir to mix.
  3. Transfer the yeast mixture into a bottle.
  4. Stretch a balloon over the neck of the bottle and wait about 10 minutes. The balloon should begin to inflate as yeast releases carbon dioxide.
  5. To observe the yeast, place a drop of the mixture on a glass slide (diluting if necessary).
  6. Cover with a plastic coverslip and examine under a compound microscope at increasing magnifications.

Yeast Fermentation Under the Microscope - Sci- Inspi:


📄 Watching bread yeast make bubbles - MRC Laboratory of Molecular Biology: https://www2.mrc-lmb.cam.ac.uk/microscopes4schools/yeast.php

Variations

  • Try using different sugar sources (e.g., honey, fruit juice, or corn syrup) to see if fermentation varies.
  • Compare the rate of balloon inflation at different temperatures.
  • Observe yeast cells at intervals (e.g., 5 minutes, 30 minutes, 1 hour) to notice budding or growth.

Safety Precautions

  • Use warm water, not hot, to avoid killing the yeast.
  • Handle glass slides and coverslips carefully to prevent cuts.
  • Do not consume the yeast mixture.
  • Wash hands thoroughly after handling yeast and materials.

Questions to Consider

  • Why does the balloon inflate when yeast ferments sugar? (Because carbon dioxide gas is released during fermentation.)
  • What role does yeast play in bread-making? (It ferments sugars in flour, releasing carbon dioxide that makes dough rise.)
  • What happens to the ethanol produced in bread dough? (It evaporates during baking.)
  • How do yeast cells reproduce? (By budding in asexual reproduction, and by producing spores under stress.)
  • Why are yeasts useful in scientific research? (They are simple, fast-growing eukaryotic cells used to study fundamental cellular processes.)