Yeast and making bread

We have baked bread quite a few times, and the boy looks forward to watching it rise.  However, I only recently remembered that we used to grow bread dough as a demo at Cambridge Hands-On Science's annual Crash, Bang, Squelch event when I watched the Maddie's Do You Know episode about eggs and bread with the boy.

I wanted to make a big batch of bread and breadsticks this week, so I tried to bring out a bit of science whilst my little helper and I made them.  He was very enthusiastic as he loves 'helping' in the kitchen and he'd clearly been paying attention to Maddie's visit to a bakery.

I usually just throw in easy bake yeast when we are making bread, but this time I used normal dried yeast.  Maddie mentioned yeast as an ingredient that makes bread rise, and mentioned that it likes to be warm, but didn't explain any further.  We tipped some into a large jug, and had a look at it (and he said it smelled funny). 

I explained that yeast is alive, but in the tin it is dried out and waiting to grow again.  He was interested to find out that we have to add water and give it some food so it can grow; he's familiar with needing water and food himself so he seemed to readily accept this idea.  The dried yeast is in little balls and he initially thought that each ball was a yeast.  I tried to gently explain that each one contains a very large number of yeast - much bigger than 100 which is the furthest he's counted - but that they are too small to see with your eyes. 

He didn't quite seem to believe me, so I resorted to YouTube and found a video of yeast cells dividing.  We watched it and I explained that you need a 'special camera' that makes them look bigger so you can see them; he's familiar with Maddie Moate's array of special cameras, and she's used a microscope camera before so I think he got the basic idea although I doubt he has any sense of just how small they are.

We had a discussion about how the yeast doesn't just grow bigger and bigger like his baby brother, but that it divides i.e. it gets to a certain size and then a bud appears and a 'baby' yeast grows on the bigger one.  The 'baby' eventually splits off and then does the same thing again itself (this is true for Saccharomyces cerevisiae which is the yeast you use in baking, but not for all types of yeast).

Dried yeast, with added water and then after it had grown for a while

Back in the kitchen, he added some lukewarm water to the yeast and gave them a stir.  He enjoyed feeding them with half a teaspoon of sugar.  He seemed quite fond of 'his' yeast, and had a little talk to them and came back to check on them repeatedly.  He also brought a collection of his soft toys to admire them too and explained to them that his yeast were growing! 

We saw that there were bubbles appearing at the top and I explained that these were carbon dioxide, the same gas that his body makes when it's growing or playing (again, thanks to Maddie's Do You Know, this wasn't a new idea to him), and that this is why the yeast is useful in making bread.  

Bread dough before and after rising

After a good head of bubbles had developed on our jug of yeast, we added it to some flour, with a little olive oil and a pinch of salt then gave it a good stir and a knead.  I used to talk about the gluten molecules in the flour when doing this with slightly older children, but I thought it was probably too much to take in for one day.  Instead, we just talked about how the yeast were being mixed in and would continue to grow in the bread and make the bubbles of carbon dioxide that make the bread rise. 

We left the bread for about 20 minutes (on a hot day!) and went back to check on it.  It had risen as he predicted it would.  The bread rose further in the oven and produced a nice little loaf and some chunky breadsticks.  The big one sampled the bread with some of his home-made blackberry jam from the other day and his brother had a chomp on some breadsticks.  Both seemed impressed and wanted more!

We've since watched the episode of Maddie's Do You Know twice more at his request...