Is Yeast Alive? A Fun Experiment For Your Kids

There are lots of ways to make science fun, and designing experiments to do at home is one of the best ways to bring science into everyday life, explaining how the things around us work.

Read on for a quick and easy experiment to try with yeast, which is particularly great if you have been baking or making bread as a family during lockdown! It is yeast that makes the bread dough rise by converting the sugar found in flour into carbon dioxide (CO2) and ethanol (alcohol), through anaerobic respiration. This is the process of 'breathing' without the need for oxygen. The below experiments set out to answer the question 'is yeast alive?', as well as exploring what the perfect amount of sugar is to feed the yeast and how much carbon dioxide is produced.

What You'll Need:

Active dry yeast or baker's yeast

Sugar

Food colouring

Warm water (around 45° C)

Several bowls and spoons

Balloon

Plastic bottle

Method:

1.Measure out a small amount of sugar into different bowls, then add some food colouring into each bowl to create some different coloured sugars. It will be easier to identify which experiment is which later on, and it will help to make the experiment look more appealing to kids.

2.Set up  three or four different bowls, each with the same amount of dry yeast and warm water (follow the instructions on the back of the packet regarding quantities).

3.Using the coloured sugars, add different amounts into each yeast and liquid mixture - remember to make a note of how much you used in each bowl.

4.Watch to see what happens to these different mixes, looking for signs of growth or change.

Results:

You should see little bubbles start to rise, or even a foam forming on the top of the mixture, which is caused by carbon dioxide being released by the yeast. Bubbles are a sign that the yeast is alive, and that it is performing anaerobic respiration.

You should notice that the mixtures with too little and too much sugar do not grow, bubble or foam properly. This is because the yeast needs the perfect amount in order to produce carbon dioxide. Photograph the results, or write them down in a table so that you can identify what ratio of yeast to sugar and liquid produces the best bubbles, and perhaps share and/or discuss the results with friends or siblings.

A slightly different set up for the experiment will help visualise that the yeast is 'breathing' .

Method:

1.Using the same ingredients and set up as the first experiment, place the ingredients into a plastic bottle instead of a bowl.

2.Place a balloon over the neck of the bottle.

Results:

After a few minutes, you should start to see the balloon begin to inflate as it fills up with the gas produced by the yeast. Wait to see how large the balloon gets before it stops growing. As a follow-up experiment, why not set up several bottles, and fill each with a different amount of the three ingredients?  You can then see which balloon grows the biggest. This way you can work out the optimum quantities of each ingredient - though, as with all experiments, only test one variable at a time.

Further Exploration

If the yeast doesn't react as expected, it may be because it has died. Overheating yeast can kill it, meaning that it won't bubble and react when added to water.

Out-of-date or old yeast also might not work. Alive yeast will bubble, and so if you are unsure how old your yeast is, it can be a good idea to do a small test to check before starting any experiments. Simply mix with warm water and see if it bubbles, if it doesn't - you cannot use it.  This is useful to know for your experiments, as well as for any baking and making dough.