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Yeast Bubbles

Is it living or non-living? Yeast breathes new life into an age-old question

Australian Curriculum links

  • Science > Biological Sciences > Year 2 > ACSSU030
  • Science > Biological Sciences > Year 3 > ACSSU044
  • Science > Biological Sciences > Year 6 > ACSSU094

You’ll need

  • A packet of Baker’s yeast
  • Sugar
  • Salt
  • Warm water
  • Snap-lock bag
  • OR small glass jars, balloons and rubber bands

Try this

Part A

  1. Add one tablespoon of sugar and one teaspoon of yeast powder to each of your containers
  2. Fill containers so they are half full of water
  3. Seal the bags after squeezing out the air or cover the jars with a balloons and seal with rubber bands
  4. Observe what happens to the yeast
  5. Leave the yeast mixture in a warm area and observe every 15 minutes. You might need to release some of the gas in the snap-lock bags.
  6. If you have used a balloon, measure the diameter of the balloon at its widest point.

Further investigation

You can repeat the experiment, or see what happens if you make the following changes. Is there a difference in the amount of gas, if any, produced?

  • Add one teaspoon of salt to one container
  • Do not add sugar to one container
  • Do not add yeast to one container
  • Put some yeast in the freezer for 1 hour before adding to a container
  • Put some yeast in the microwave and cook for 10 seconds before adding to a container
  • Use ice water with the yeast and sugar
  • Use boiling water with the yeast and sugar
  • Use warm water

Some other questions you might like to investigate, include:

  • What is the minimum amount of yeast, sugar and water needed to fill one snap-lock bag with gas?
  • Try using different materials such as honey, fruit juice or vinegar. What happens to your mixture?
  • What happens if you add more yeast to the mixture? What about sugar?
  • Is there a difference if you keep the mixture warm or cool?
  • What happens if you take some of the yeast solution and add it to another container that contains sugar and water?

What’s happening?

When you open your yeast packet, the yeast doesn’t look like it is alive. Looks can be deceiving. Yeast is a fungus, which includes plants like mushrooms. When it breathes, it produces carbon dioxide, which is in the bubbles you can see. Yeast is just like us! It doesn’t like the temperature too hot or too cold, and it likes to have lots of the right food to grow well.

Real life links

Humans have used yeast for raising bread since Ancient Egypt. Yeast creates bubbles of carbon dioxide to ‘raise’ (make it expand in size) the bread.

Sourdough is a dough containing yeast. A starter sourdough is 'fed' (given) a mixture of flour and water, once mixed, remove a small piece. Bake the larger piece of dough and keep the small piece as fed for future generations. Some starter doughs have been in use for over 100 years.

Brewer’s yeast is a special yeast used to brew beer. Scientists found some beer bottles from a 220-year-old shipwreck off Sydney. They then used the yeast from this beer to brew a fresh beer.