Thanks to Maggie Glezer for her very informative comments on active
dry and instant yeast in bread-bakers.v107.n002.3. As she says, with
the appropriate volume adjustments, the home baker can substitute
active dry for instant (and vice versa) with equal results.
This is a topic I have been researching for a long time. I would like
to add a few more facts.
INSTANT DRY YEAST: as noted, instant yeast should be added to dry
ingredients. Besides being dried at a lower temperature than active
dry yeast, this is actually a slightly different strain of yeast
created by "protoplast fusion", the scientific technique of combining
two separate yeast strains into a new, single strain. Basically, the
cell membrane for instant yeast is thinner than that for active dry.
This allows instant yeast to absorb water readily so it does not need
to be dissolved in water. In fact, dissolving instant yeast in water
(especially very cold water) can damage or kill some yeast cells, as
can direct contact with salt in the dough. This is why recipes often
recommend that instant yeast be mixed in the flour first, then salt
is mixed in and then the liquid is added.
ADDITIVES IN INSTANT DRY YEAST: I buy my dry yeast in one-pound
packages. I notice that the ingredients listed on the package
include, besides yeast, ascorbic acid (vitamin C) and an emulsifier
(though both additives are in small amounts). My packages of SAF
Instant Yeast and Fleischmann's Instant Yeast both list the following
ingredients: yeast, sorbitan monosterate (the emulsifier), and
ascorbic acid. In contrast, whenever I've purchased Active Dry yeast
(usually Fleischmann's) the package ingredients always just say yeast
(nothing else).
MORE ON ADDITIVES: Ascorbic acid helps create a stronger gluten and I
have also read that small amounts help yeast multiply more rapidly in
the initial stages. Besides being present in Instant Dry yeast, it is
frequently added to supermarket brands of Bread flour (but *not* to
All-purpose flour). I assume that the emulsifier is added to make the
instant yeast perform better in doughs that contain a large amount of
butter or other fats.
EXPIRATION DATE: the expiration date on dry yeast refers to the time
it can be stored, *unopened*, under the recommended storage
conditions (a "cool, dry place"). It does *not* refer to how long the
yeast will remain strong after it has been opened, since yeast
manufacturers are clueless about how you store your yeast after
you've opened it and won't make predictions. Manufacturers do
recommend that opened yeast be stored in the refrigerator with the
package well sealed to prevent moisture from entering the package.
And we all know that opened dry yeast, whether Instant or Active Dry,
can last for years with very little degradation when stored, well
sealed, in the freezer. (I store my opened yeast in the package with
the top clipped closed, in a zip lock bag in the freezer.