How do bakers make a small lump of dough swell up and turn it into spongy bread?

The key ingredient used for making bread is yeast, a small plant in the fungus family, which is a living organism. It is microscopic in size and one gram of fresh yeast contains about 10 billion living yeast cells. There are in fact, many different species of yeast, but the one in which we are most interested is the one used for bread-making. Each of the tiny plants in a lump of yeast is actually a single cell, which is made of protoplasm, a nucleus, vacuole and a cell wall.
When yeast cells are provided with food they grow and multiply very rapidly. They do this by a process called budding. On the side of a mature cell a small bulge appears. (See diagram). The nucleus moves towards this ‘bud’ and divides into two. Half of the nucleus moves into the bud while the other half remains in the parent cell. The bud then breaks off to form a ‘daughter’ cell. Not all species of yeast reproduce by budding though. Some reproduce by fission, which leads to two daughter cells of the same size.

Yeast loves to east sugar and flour in bread. During this process, fermentation takes place, converting the sugar into alcohol and carbon dioxide. The alcohol burns off in the oven, but small bubbles of carbon dioxide are trapped inside the dough, creating innumerable small cavities which make a dough rise. Eventually, the dough doubles in size and what you get then is soft and spongy bread.

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