As the name implies this is the fastest-rising of all the various packaged yeasts. A version of instant yeast, it’s made via similar methods but the granules are even narrower and thinner…almost rod-like if you can see them. That means they absorb moisture and dissolve even faster, so they start working, reproducing and making CO2 almost immediately.
What is this yeast for? Extremely light and fluffy breads. In fact this sort of yeast is commonly used for bread machines to produce ultra-light, sandwich bread-type loaves. The trouble is that it doesn’t produce a very flavorful product.
Why? Because slower rises provide time for other, flavor-giving, non-yeast related processes to work. Slower rises give enzymes that are present in the flour time to turn on and start cutting starch molecules down into sugars we can taste. They allow lactic acid bacteria present in the flour, the water and the yeast culture itself to start making lactic acid and other flavor-enhancing compounds. If the yeast takes off at a run and finishes its job before any of the rest of these processes can occur, you lose a lot of what makes bread interesting to taste.
Of course the natural bread crowd would say you lose most of what’s interesting in bread by using packaged yeast in the first place. That’s true to some extent, but properly used, packaged yeast can deliver a surprising amount of flavor given time to work. But when the rise is as explosive as it is with rapid-rise yeast, I guess you have to wonder what you’re really getting out of it.