Yeast that you buy in the grocery store comes in little packets, jars or in cakes. The small packets of active dry yeast are extremely convenient. Each little packet contains about 2 teaspoons of dry yeast.

The cost per gram of yeast by the jar or cake may be less than the packets, but if you aren't baking regularly the yeast will likely expire before you can use it all up. In either case pay attention to the expiration dates listed on the yeast container. Throw out any yeast that is beyond its expiration date as it will not be able to be activated, nor will it perform the magic of rising bread.

Yeast is a dry, powdery substance that is actually a living organism, a one-celled organism. A unique characteristic of yeast is that it can survive for long periods of time in a sort of limbo or dormant state. Yeast cells will remain in this stasis until the right conditions are provided. Yeast requires a warm and moist environment in order to grow and multiply. It also requires some type of sugar as a food source.

Yeast survives on sugars as it's food source. Depending on the kinds of sugars present and the environment yeast will produce carbon dioxide or alcohol as a result of the growing process. Carbon dioxide is a gas and that is what makes the bubbles in bread dough. As the bread bakes the gas is trapped by the dough forming small holes in the bread. So, we say that yeast rises bread. Yeast will consume the sugars in grape juice and produce alcohol, a process we take advantage of when we make wine. Yeast, which can live off of the sugars in barley mash, is also used to make beer.

Yeast can survive a large range of conditions. However, once the temperature gets too high or too low, the yeast will die. High temperatures that we use to bake bread will kill the yeast. Yeast won't survive temperatures higher than 140 degrees F. Since we bake bread at higher temperatures, most of the rising of the bread takes place in the beginning of the cycle.

Once dry yeast has gotten wet it becomes activated. After becoming activated the yeast is no longer in its dormant state and will start to multiply and produce gas bubbles or alcohol. If the conditions are just right the yeast will continue to feed on the sugars present. It will grow and multiply, producing more gas or alcohol, until the food source runs out or until other environmental factors control its fate.