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. 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.