Yeast is a single-celled microbe belonging to the fungus family. There are around 1,500 known species, but the one used in baking is Saccharomyces cerevisiae. Its key property is the production of carbon dioxide (CO2) when it feeds on sugar molecules (a process known as fermentation). The CO2 gets trapped, creating a soft, spongy texture.
There are billions of yeast cells in a single sachet of yeast. The sugar they feed on comes from the bread’s flour, which is mainly starch: long chains of glucose molecules. These chains are too large for the yeast to metabolise, but bread flour also contains ‘amylase’ enzymes (these occur naturally, but are also often added to flours to boost levels), which, when you add water, chemically chop the starch chains into individual glucose molecules. The yeast cells eat this glucose, which they convert into energy, releasing water and the all-important CO2 as a by-product.
Yeast grows and multiplies fastest between 30°C and 35ºC, which is why your dough rises best in the airing cupboard or on a warm windowsill. Towards the end of the proving, the yeast starts to run out of oxygen, so it can’t fully metabolise the sugar. Instead, it switches to an anaerobic process that doesn’t need oxygen, and produces alcohol (ethanol) instead of water as the by-product.
You can often smell the slightly fruity/sour products of this process in raw bread dough. When you bake your loaf, the heat of the oven evaporates all the alcohol, and also kills off the yeast cells, which have now done their job.