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
- Place a packet of active dry yeast into a bowl.
- Add warm water and about two tablespoons of sugar, then stir to mix.
- Transfer the yeast mixture into a bottle.
- 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.
- To observe the yeast, place a drop of the mixture on a glass slide (diluting if necessary).
- Cover with a plastic coverslip and examine under a compound microscope at increasing magnifications.
Links
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.)