Humans have been eating raised breads for 6,000 years, but it wasn’t until the investigations of Louis Pasteur 150 years ago that we began to understand the nature of the leavening process. The key is the gas-producing metabolism of a particular class of fungus, the yeasts. The word “yeast,” however, is as old as the language, and first meant the froth or sediment of a fermenting liquid that could be used to leaven bread.
