There are two different categories of yeast: wild and commercial. Not only is commercial yeast less temperamental to use, it is accessible and easy to use. Wild yeast needs to be created and maintained, and it provides a sour taste that may not be desirable in all types breads.
There are many different types of yeast, but the commercial one used by bakers is called Saccharomyces cerevisae. The San Francisco Baking Institute explains the Latin word as:
“Sacchar means sugar loving or feeding, myces means mold, and cerevisae is a word once used for beer.”
Yeast is a single-celled, living organism that thrives off of sugars present in the flour. When the yeast consumes these sugars, it begins a process called, fermentation. Although yeast is pretty easy to work with, it thrives in warm (75-78F) environments. When yeast is in cool (around 40F or below) environments, it retards, or slows, the process down significantly. On the other hand, in too hot of an environment (above 130F), the yeast dies.
There are three common commercial yeasts: active dry, instant, and fresh baker’s. Active dry and instant yeast have the longest shelf live, while fresh yeast has a shelf life of only about 2 weeks. Each type of yeast has its own advantages and disadvantages, but ultimately, it is the baker’s decision to choose which to use.
Active dry yeast is probably the most well-known yeast available to the baker and the consumer. Active dry yeast needs to be rehydrated before it can be used. The yeast is dissolved in warm water (around 100F), sometimes with a little sugar added to stimulate activation. Active dry yeast provides a yeastier flavor, relaxes the dough a little more, and creates a product with a longer shelf life.
Instant yeast has recently become the most popular type of yeast. Unlike active dry yeast which requires activation, instant yeast can be added directly to the dough. Not only does this reduce a step, but it also reduces the risk for human error in measuring or killing the yeast. Instant yeast is more concentrated than both active dry and fresh yeast. This means that less yeast is required if the recipe calls for different types of yeast.
Recently in the bakery, we’ve began to use a special type of instant yeast called “omsmopotent” yeast. This yeast works best with breads heavily enriched with sugar or acidity. We use this in the croissant and brioche dough because the sugar inhibits a quick rise.
Fresh yeast has the shortest shelf life of these three types of yeast. Fresh yeast comes in small cubes, which are made of around 70% water. It is tan in color, and crumbles when touched. Fresh yeast is difficult to purchase, and it’s hard to insure its freshness when bought in a grocery store. Most bakeries choose not to use it because it has a short shelf life and it more labor intensive to use than instant yeast.
Here are the conversions for substitutions:
100% Fresh Yeast= 40 to 50% Active Dry Yeast= 33% Instant Yeast