Yeasts work best when they are in a temperature range of 85 to 115 degrees F, depending upon the variety. Rapid rise yeasts prefer the higher temperatures.
There is a simple procedure called "Proofing" which is a way to test whether yeast is good.*
In the unopened package, Dried Yeast (<8% moisture) generally has a shelf life of up to two years at room temperature. Compressed yeast is only partly dried (about 70% moisture), and has a much shorter shelf life (several weeks).
Since Yeast is damaged by water
and moisture, it MUST be kept DRY.
use Yeast in a small glass jar in refrigerator
is best kept in a freezer
To use yeast from the refrigerator
jar: open the jar, pour out the required amount of yeast, cover the jar and return it to the refrigerator as quickly as possible. Avoid dipping measuring spoons into the yeast; they may carry moisture.
To refill the refrigerator jar: remove the freezer
jar and allow it to come to room temperature before opening. Notice that moisture will condense on the outside of the container. (If the jar were opened when it was frozen, moisture would condense into the yeast.) Dry the jar thoroughly. Open it when at room temperature and pour some Yeast into the refrigerator jar. Tightly close both containers and return them to their storage
* To proof Yeast: Stir 1 tsp sugar into 1/4 cup warm water
(110 to 115 degrees F.). Sprinkle 1 tsp yeast on the surface stir. In five minutes, mixture should foam. In 10 minutes there should be a fairly thick layer of foam.
Yeast which only slowly becomes active can still be used, but you will have to use more of it. If it shows no activity at all, it's dead and should be thrown out.