when did they figure out that yeast made bread rise
The most common source of leavening in antiquity was to retain a piece of dough (with sugar and water in) from the previous day to utilize as a form of sourdough starter. Pliny the Elder reported that the Gauls and Iberians used the foam skimmed from beer to produce "a lighter kind of bread than other peoples." Parts of the ancient world that drank wine instead of beer used a paste composed of grape must and flour that was allowed to begin fermenting, or wheat bran steeped in wine, as a source for yeast.