Sourdough

The oldest (and some people would say still the best) way of leavening a loaf is by using a sourdough starter.

Yeasts and bacteria are found in the air, soil, water . . . basically everywhere. It might or might not be a coincidence that the surfaces of cereal grains tend to teem with types that are suited to bread making. After grinding the grains, some of these critters remain in the flour and by creating the right conditions you can nurture them into a thriving culture.

Eventually there will be enough yeast cells in the culture giving off carbon dioxide to make dough rise. At the same time, lactic acid bacteria will be producing lactic and acetic acids.