Yeast is a type of unicellular fungus, and the species Saccharomyces cerevisiae is used to ferment sugars and produce the carbon dioxide needed to make bread rise. Related yeasts are used to ferment sugar into alcohol and to make soy sauce. Fungi are immensely important to humans. The most familiar fungus is the mushroom, but in fact, most land plants have fungi growing into and around their roots. These fungi allow the plant to take up water and nutrients more efficiently – agriculture as we know it would be impossible without mycorrhizal association. The first discovered antibiotic, penicillin, is produced naturally by a fungus.
As a living organism, yeast needs a certain environment to thrive (and make your bread rise). Inside the yeast packet, the living cells clumped together and encased in nutrients and dead cells. Rapid rise/Instant yeast is a special kind of dry yeast that uses smaller “clumps,” that dissolve quicker. You can use either kind of yeast, but remember that you may need to wait longer for traditional dry yeast to activate. In the dry state, the yeast metabolism is slowed but they can’t live this way forever, so be sure to check the expiration date on the packet. Once the yeast are rehydrated, they need nutrition and warmth to thrive. They do best around 100 F (that’s just a bit above body temperature) and shouldn’t be heated much above that, and definitely not above 120 F.
Once the yeast set to work producing carbon dioxide, the next important thing is to trap the gas and form “bubbles” in the bead. This is done with kneading! Kneading bread means pulling, folding, and pressing the dough repeatedly. Stretch the dough out, fold it over, and press it down, working flour in as you go. When the dough has enough flour, it will have a smooth surface, and will hold itself together as a ball. At this point keep kneading to mix the flour with the wet ingredients. You may find that as the flour is better distributed the dough starts to appear sticky again. If this happens, add more flour. The exact amount of flour needed varies based on the humidity, the moisture in the flour, and the quality of the other ingredients (for example, how big the eggs are). There is no exact way to predict how much flour a loaf needs, so it’s important to be able to identify the perfect texture. The dough ball should be smooth and bounce back when punched. The dough should not be sticking to your hands.
By mixing the flour and the wet ingredients, kneading expands certain proteins in the flour to produce gluten, which gives the bread structure and elasticity and is thus able to trap the carbon dioxide produced by the yeast. If a bread is too flaky or crumbly it probably was not kneaded long enough to produce the right amount of gluten. Most beginning bakers under knead their bread – don’t worry, this is easy to fix! You will know that you over-kneaded if the bread comes out tough and rubbery.
Punching down is a special form of kneading used when the bread has already risen at least once. Begin with a few hard punches to remove air and reduce the volume of the bread, then knead as usual.