Manang asked me about the difference between active dry yeast and instant yeast. This is a great question, so I thought I'd liberate my thoughts from the comments to a post.
Active Dry Yeast
This is a granular product that has been dried using high temperatures that kill up to 70% of the yeast cells. The dead cells collect around the live cells protecting them and giving this product a long shelf life. To free and activate the live cells, the product is usually dissolved in warm water (proofed) before use.
This is also a granular product, although it has been dried using lower temperatures than active dry yeast. Only about 30% of the yeast cells are destroyed in the process. Because it contains more viable yeast cells, it can be mixed directly with the dry ingredients in a recipe. No proofing required.
Bread Machine Yeast, Rapid Rise Yeast, and Quick Rise Yeast
These are types of instant yeast. Like instant yeast, they work fast and don't require proofing. Mix them right in with your flour.
Note: Notice the larger granule size of active dry yeast compared to a rapid-rising yeast. The live yeast cells in active dry are surrounded by dead cells. Warm water will dissolve the dead cells away and hydrate the live yeast inside.
Since instant yeast is more potent than active dry yeast, less is needed. If all I have on hand is active dry yeast, I usually double the amount called for in a recipe using instant yeast.
Having said all this, I'm rethinking the type of yeast I use in making bread1, especially slow-rising bread. I usually use instant yeast in my recipes because it's more potent and doesn't require proofing. I'm pretty frugal with the amount I use, trying to get away with the smallest amount that will grow slow but still produce adequate volume. Manang's question is causing me to consider trying active dry yeast in place of instant yeast (ounce for ounce) in bread making since the reduced potency will encourage an even slower growth ... giving any wild yeasts or bacteria a chance to take hold. Thank you for this idea, Manang!
1 The Baker's Catalog carries a few strains of slow-growing, dried, wild yeast starters. I've tried some of them and they do produce a more tangy, chewy, artisan-quality bread, as does creating your own starter from wild yeast spores that adhere to grape skins or that float around your kitchen. But I wanted to develop a (relatively) easy recipe that most people could follow using ingredients on hand. (Myself included!)