• Question: i wasnt sure who to ask, but why exactly does yeast expand and rise when it is heated? thanks very much

    Asked by nmizdrak to Will, Suze on 13 Jun 2011.
    • Photo: William Eborall

      William Eborall answered on 13 Jun 2011:

      Yeast is a living animal and when it’s heated in the presence of a food source (so the dough of bread, or the sugar when making beer/wine) it grows and reproduces. As it lives and grows it releases as a gas called carbon dioxide (just like we do) which forms bubbles, which you can see in a slice of bread or the head of a pint of beer. These bubbles make the dough expand.

      Hope that helps!

    • Photo: Suze Kundu

      Suze Kundu answered on 13 Jun 2011:

      Aah, Will just beat me to it! Yes, it’s because the yeast is actually made up of lots of little spheres. Although these don’t look like animals, they are a type of fungus. That sounds disgusting, I know, but actually mushrooms are fungi too. As they grow and multiply, they use up (eat) food, such as the little amount of sugar that we put into break dough. As they are alive, they also breathe, just like we do. When we breathe in, we take in oxygen, and when we breathe out, we release carbon dioxide. It is bubbles of this that get trapped in the dough of bread, making the bread look like it is expanding. There is still the same amount of bread dough there, but it is filled with holes, a bit like a sponge.

      So really, the yeast are expanding as they multiply, but the visible increase that we notice when we are making bread is because of bubbles forming as the yeast breathe. I try not to think about it though. Imagine all of those little creatures breathing happily, until we put them all in a big, hot oven?! 🙁

Comments

  • Photo: James

    James commented on :

    There’s some interesting materials science in bread-making. The flour needs to develop long-chain “polymer” molecules that can stretch to hold the gas bubbles. If you don’t have these, then the bread doesn’t rise well. These molecules are developed (I think – need to check with a biologist – is there one here?) when the flour is mixed with water and kneaded. Some flours are better than others.