What is yeast? We know that bakers use it to make the dough 'rise'; without it, our bread would be like flat, hard cakes. In the days when people made their own bread, they would go to a brewer and get a jug of brewer's yeast. It was fluid and yellow. Nowadays, yeast is made commercially on a large scale. The yeast you buy at your market, the yellow lumps done up in paper, has been compressed for convenient handling.
Commercial yeast is a by-product of the whisky distillers. If you are a yeast producer your by-product will be methylated spirits.
Yeast is a plant, according to the biologists, and is capable of reproducing itself. A piece of yeast consists of minute cells, with walls composed of cellulose, and an interior of living matter called protoplasm. You can feed it with a solution of sugar to make it grow, or it can be 'killed' by 'starvation' or heat. The ancients did not use yeast as we know it today; they prepared a leaven or 'barm' (which has the same action) from ground millet kneaded with 'must' out of wine-tubs. Wheat bran was also used, kneaded with a three-days-old must, dried in the sun, then made into little cakes. When required for making bread, the cakes were soaked in water, then boiled with the finest flour, after which the whole was mixed in with the meal. Another old method for making barm was to prepare cakes of barley meal and water; these were baked on a hot hearth, or else in an earthen dish upon hot ashes and left until they turned reddish-brown. Afterwards, the cakes were kept shut up in a vessel until they turned quite sour. When wanted for leaven, they were first steeped in water. Eight ounces of this was enough to make a quantity of bread of about 14 lb. or 6.3kg to rise.
The primary function of yeast is to supply carbon dioxide gas which inflates the dough during proof and the early stages of baking (oven spring).
Carbon dioxide cannot form a gas bubble on its own it requires a 'nucleating site' (i.e. somewhere it can gather to form a bubble). In fizzy drinks microscopic projections on the side of the bottle provide those sites which is why when you release the pressure as you open the bottle you see 'streams' of gas running from the sides. In bread dough the nucleating sites are provided by the nitrogen gas bubbles trapped in the dough during mixing. The oxygen from the air having been used up by the yeast.
During proof stages the carbon dioxide goes into solution until the solution is saturated and then any more which is generated makes its way into the nitrogen gas bubbles which grow in size and the dough expands. The more yeast and the warmer the temperature the faster the expansion - we get oven spring because the maximum gassing rate occurs at 40-45C.
In bulk fermentation stages we also get dough expansion from carbon dioxide generation but most of that is lost when the dough is knocked back and divided, so the yeast has to start over again.
Yeast also contributes to dough maturity/development. Though its role is minor compared to in improvers no-time doughs, it is more significant in bulk fermentation where the enzymes, especially the proteolytic ones (they modify the gluten proteins), play a significant role.
To sum up, then, the dough is aerated by the action of the yeast. The little cells we mentioned ferment the dough, and produce tiny bubbles of gas inside it. As a result, the dough gets fatter and bigger, and rises, of course. Thus when the dough is baked, you have a 'bold' loaf, light and airy; when you cut it you can see all the tiny holes formed by the gas, so that it looks like a sponge.