What is the difference between active dry yeast and instant yeast (also known as bread machine yeast)?
Instant yeast is a little more potent than active dry yeast and can be mixed in with your dry ingredients directly. I generally find it easier to work with. Active dry yeast works just as well as instant yeast, but requires being activated in a little bit of warm water before being added to the rest of the ingredients. Failure to properly activate it will result in your loaf not rising adequately.
Can I substitute active dry yeast for instant yeast in my recipe (or vice versa)?
Yes. If you are substituting active dry yeast for instant yeast in a recipe, read the instructions on the package to figure out how to activate the yeast before adding it to the recipe and reduce the amount of water you add later in the recipe by the amount of water you proof the yeast in (i.e., if you activate the yeast in a half a cup of water, add a half a cup of water or milk less later so that you end up with the same total amount of liquid in the recipe). You may also want to add about 20 percent more yeast to the recipe than what is called for, although using less yeast and letting it rise more slowly will result in a more flavorful loaf. If you are substituting instant yeast for active dry yeast, you can reduce the amount of yeast you use in your recipe by approximately 20 percent. Be sure not to forget to increase the amount of water you add to the dry ingredients by the amount that you would proof the active dry yeast in, so that you end up with the same total amount of liquid in the recipe.
What about fresh yeast?
Professional bakers often use fresh yeast. If you encounter a recipe that uses fresh yeast, divide the weight by 3 to calculate the proper amount of instant yeast to use.
Many recipes in my baking books call for using a starter. I don't have the time/energy/patience to sustain a starter. Can I substitute yeast instead?
Absolutely. And vice-versa: you can turn a yeasted bread into a naturally leavened bread by omitting the yeast and including a starter. The flavor will be different, obviously, but in my experience it still can turn out quite good. You may even find you prefer your modified version to the original recipe. I don't know of an exact formula to calculate how much to substitute. I just assume that I'm going to need to add a little more flour and water (how much of each depends on whether it was a wet or dry leaven I am replacing) and enough yeast for a comparable size batch of yeasted bread. I usually figure around 1 or 2 teaspoons per loaf. Also be aware that yeast tends to move quicker than starters do, so expect to cut the rise time down by something like one half (or else reduce the yeast even more).