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Picture of items required for the experiment: balloons, jug, measuring spoon, timer, yeast, sugar, string, cellotape, notepad and pen, scissors, bottles, funnel
Figure 1: Experiment equipment

Home practical: yeast fermentation

In this Step, you’re going to try to blow up a balloon by harnessing the power of yeast, using everyday items you can buy in a supermarket. This home practical is optional, so you can just read the instructions if you’re unable to, or would prefer not to, carry out the practical yourself.

Yeast is a single celled (unicellular) fungus and some species (eg Saccharomyces cerevisiae) have been used in food production for centuries. The food industry exploits yeast’s ability to ferment sugars to make energy, which produces ethanol and carbon dioxide gas (CO2) as by-products. These CO2 bubbles are what makes bread rise and champagne fizz. When yeast is mixed with sugar and water in a bottle, the production of CO2 can be monitored by capturing it in a balloon and measuring how much the balloon inflates over a period of time. The faster the rate of fermentation, the faster the balloon will inflate.

If you would like to test your scientific skills, you can design your own experiment by following the instructions in this PDF. It’s a good idea to read through all of these instructions at least once before starting your experiment.

Five bottles lined up in a row and labelled A-E. Each bottle has a balloon over the opening and each is inflated to a different degree.

Figure 2: Inflated balloons from fermentation process

If you don’t have time to perform an actual experiment, why not try setting up a single plastic bottle with 200 ml warm water, 1 tbsp fast action yeast and 2 tbsp of sugar. Tightly seal the opening of the bottle with a balloon and watch it inflate. Take a photo and post it on the Padlet wall in the next Step – but make sure to take the balloon off the bottle before it explodes!

What factors do you think will affect the rate of fermentation? Share your ideas in the comments area, below.


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Small and Mighty: Introduction to Microbiology

University of Reading

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