I really hate to contradict other posters, but in the interests of
accuracy, I feel that I must. A number of posts replying to a question on
yeast stated that instant yeast and rapid rise yeast are the same. But
according to King Arthur Flour, they are different creatures. Quoted below
is their answer when asked the same question.
Q:
What is the difference between instant yeast, active dry yeast, and rapid
rise yeast?
A:
Active dry yeast is what most people are used to seeing in supermarkets,
commonly found in strips of three packets or in jars. Because of the way it
is dried, active dry yeast contains a large percentage of dead yeast cells
surrounding the live yeast cells. For that reason, active dry yeast must be
hydrated -- immersed in warm water -- for at least a few minutes to release
the live yeast cells from the dead ones. This process, referred to as
"proofing the yeast," is essential for active dry yeast
Bycontrast, instant yeast -- often called "instant dissolving yeast" -- is
manufactured so that there are far fewer dead yeast cells. As a result,
instant yeast becomes active the "instant" it contacts moisture. Rather
than proofing it prior to use, simply mix instant yeast with all the other
dry ingredients. Then, when liquid is added, the yeast will activate right
away. Also, because there are fewer dead yeast cells in instant yeast, you
need less instant yeast than active dry yeast to achieve the same effect.
Some people like to proof their instant yeast anyway to assure themselves
that the yeast is still fresh. Doing this causes no harm. Instant yeast and
active dry yeast are created from different strains of yeast cells. Instant
yeast does make the bread rise a bit faster than active dry. Instant yeast
starts fast and works moderately. Active dry starts more slowly, works
moderately fast, and eventually (after several hours) catches up to instant
yeast. Both quit at about the same time.
Unlike both active dry yeast and instant yeast, rapid rise yeast is a whole
different strain of yeast altogether. If active dry and instant yeast were
marathon runners, rapid rise yeast would be a sprinter. It gives bread a
speedy initial rise, but dies out quickly. Avoid rapid rise yeast when you
want to develop flavor in the bread through long rising times.