So there are different types of yeast.There's commercial yeast and there's natural yeast.There's basically yeast all around us, and you can use a variety of different yeasts and leavening's for your bread dough.The most commonly known yeasts that everyone is familiar with is commercial bread yeast, at least that is in bread making, and so commercial bread yeast, there are basically three different types of yeast.
There's instant yeast, also called rapid rise yeast. There's active dry yeast and there's fresh yeast. When you're converting fresh yeast to instant yeast, the basic difference is that instant yeast is coated in dead yeast cells that need to be dissolved and then the active yeast is inside each granule. You don't necessarily need to dissolve the yeast in water for it to be active. It will dissolve in your dough over a period of time.
Fresh yeast basically has water in it. It's not dehydrated like instant yeast or active dry yeast is, so when you're converting between the two there's more water in fresh yeast, so there's more volume in fresh yeast. So you basically want to divide the fresh yeast by three, to get the instant yeast weight, if you're converting between the two. That's just a very brief explanation on what the differences are between all the different types of commercial yeast.
And then there's something called sourdough starter also called leaven or natural leaven, and what that is, is that's basically the natural yeast in your environment that have been trapped in a culture of flour and water, and those natural yeasts need to continue and to grow. So you must constantly feed them with flour and water in basically, a sourdough culture. So any time you see a bread that's calling for a sourdough starter, that itself is a yeast. That's a leavening.
The only difference is, is that there's already flour and water in that leavening, and so your formula your bread recipe needs to be adjusted to account for the fact that you're using the yeast in a different form. That's just a very brief explanation on the different types of yeast and leavening that you'll find when you're making bread at home.