Raising Agents: Biological (Fermentation)

How can fungus blow up a balloon?

What is a biological raising agent?

Yeast, a tiny single-celled microorganism, a type of fungus, is an example of a biological raising agent.  Yeast is used to make bread dough.

How do biological raising agents work?

Yeast feeds on the sugar contained with the dough, producing carbon dioxide and alcohol, in a process called fermentation.

During bread making, the dough is left in a warm place.  The warmth causes fermentation to take place.  However if the temperature is too high, for example during the cooking process the yeast is killed.

During fermentation, carbon dioxide is produced and trapped as tiny pockets of air within the dough.  This causes it to rise.  During baking the carbon dioxide expands and causes the bread to rise further.  The alcohol produced during fermentation evaporates during the bread baking process.


  1. Mix 1g yeast, 3g sugar, 300ml lukewarm water and place in a 500ml plastic bottle
  2. Put a lid on the bottle and shake the bottle gently
  3. Remove the lid and stretch a small balloon over the neck of the plastic bottle (it may help to stretch the balloon once or twice beforehand)
  4. Keep bottle warm near a radiator for 60-90 minutes
  5. After some time you should notice that the balloon starts to inflate and the yeast mixture turns frothy.
  6. If you leave the mixture for a few hours and then shake it, you should find that it froths up like a fizzy drink but it will not taste like one!
  7. What do you think will happen if you do the same experiment but keep the bottle in the fridge?


AQA video: Investigating how raising agents work

AQA transcript: Investigating how raising agents work