Experiment 2

Now, let's see what happens if we change things around a bit.

  1. Reproduce experiment 1, but this time replace the 1/2 cup sugar with a 1/2 cup (.12 L) white flour.
  2. Mix the flour thoroughly into the water so there are no lumps. (This is most easily done by mixing the dry flour with a small amount of water to create a paste, then adding a little more water, and so on until all the water has been added.)
  3. Seal the flour-water-yeast mixture in a plastic bag as you did in Experiment 1 and come back in an hour or two.

What you will notice is that this mixture produces carbon dioxide, but somewhat more slowly. (If you run Experiments 1 and 2 simultaneously, you will be able to see the different rates more easily.). Where did the sugar for the yeast to eat come from in Experiment 2? We didn't put any sugar at all in this bag, right? It turns out that, in the mixture of flour and yeast, there are enzymes that turn the starch in the flour into maltose, another sugar. The yeast uses this sugar in the same way it uses the glucose in white sugar. It takes time for the enzymes to convert starch to maltose, and that's what causes the delay. However, the yeast is able to produce some carbon dioxide, and that's how you know the enzymes are working. In a loaf of bread, it is this flour-to-maltose reaction that actually drives the expansion of the bread for the most part -- the small amount of sugar you mix into the bread dough is used up by the yeast fairly quickly.