Baking leavened bread requires the baker to combine leveling agents, such as yeast or baking powder, and salt with flour to make the dough expand into a loaf of bread. While bakers can purchase self-rising flour, that flour and regular flour doesn't include yeast. Flour includes yeast when someone mixes the two ingredients together, such as in bread making.
There are more than 600 different species of yeast. It is a living organism belonging to a group of fungi and multiplies rapidly, especially when added to ingredients containing sugar. Bakers use yeast as a leavening agent for bread, for when added with the right combination of ingredients, it expands, causing the dough to rise. Yeast can be a byproduct of beer and wine making, and early bakers made beer bread, not understanding the chemical reaction of the yeast in the beer.
Yeast in Air
Yeast occurs naturally in the air. Before the manufacture of commercial yeast, bakers left bread dough uncovered to rise, whereby the yeast in the air would land on the bread dough, resulting in a fermenting process, thus causing the bread to rise. While the flour didn't initially contain yeast, after exposed to the open air, yeast attached itself to the flour in the dough.
Commercial manufacturers of yeast offer it in compressed cakes, which require refrigeration, and in a dry form, which has a longer shelf life and does not require refrigeration.
Four is a powder-like substance made from crushed grain, such as wheat, rice or rye. The most common is wheat flour, a cooking ingredient bakers use to make bread, cakes and as a thickening agent. Wet wheat flour forms a sticky substance called gluten. Gluten combined with yeast helps the dough rise. Manufacturers offer different types of flour, each producing varying degrees of gluten, such as bread flour, cake flour and all-purpose flour. Bread flour forms stronger gluten, to work more effectively when used with yeast to make yeast-leavened bread.
When purchasing flour at the grocery store, it is possible to purchase self-rising flour, which is flour already mixed with a leveling agent. Instead of yeast, self-rising flour contains baking powder and salt. Since yeast is a live organism, if packaged in flour and left on the shelf of the supermarket, it would eventually die.