Daily Science Factlet – watching yeast breathe

Big thanks to the Naked Scientists’ Kitchen Science website for providing me with the rough amounts to use.

Yeast are the tiny, single-celled relatives of mushrooms that we rely on to make bread and alcohol. When they gobble up sugars in flour or in fruit juice, in an environment with not much oxygen, they produce alcohol and carbon dioxide gas.

Glucose –> Ethanol + Carbon Dioxide

This gas is what makes dough rise and puffs up a loaf of bread, and is also what gets trapped in bottles of champagne, so that they fizz up when you pop the cork.

What you will need:

1 empty drinks bottle (500ml)

1/2 a sachet of dried yeast (about a teaspoon)

3 or 4 teaspoons of sugar

Warm water

1 balloon (blown up and let down a couple of times to stretch it)

What to do:

Pour the yeast and sugar into the bottle. Add warm water until the bottle is around half full (mine was a bit more than that). Screw on the lid and shake it around a bit to dissolve the sugar and disperse the yeast.  Take off the lid and stretch the balloon over the bottle opening. Leave it somewhere warm for about 20 minutes-half an hour.

You’ll see the balloon start to inflate and a thick layer of bubbly scum on the top of the water in the bottle. This is the yeast respiring! You can vary the experiment by comparing variables like the temperature of the water, the sugar source used etc.

And that’s it really. The video below is a speeded up version of the reason this whole experiment is worthwhile. I’ve removed the sound, so you can’t hear me giggling like a loon…


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