Yeast is a living microscopic organism which converts sugar or starch into alcohol and carbon dioxide, which is why beer brewers, wine makers and bread bakers like it. Baker's yeast is what we use most often for leavening when cooking. Baker's yeast is either active dry yeast (where the yeast is alive but inactive due to lack of moisture) or compressed fresh yeast (where the yeast is alive and extremely perishable as a result). Brewer's yeast is a non-leavening yeast used in brewing beer and can be eaten as a food supplement for its healthful properties (as you would wheat germ), unlike baker's yeast which is used for leavening. Brewer's years has a bitter hops flavor. Nutritional yeast is similar to brewer's yeast, but not as bitter because it is grown on molasses. You should not use a live yeast (i.e. baking yeast) as a food supplement because it continues to grow in the intestine and uses up vitamin B instead of replenishing it.
Active dry yeast comes in envelopes, jars or bulk and can be regular or quick rising. Quick rising will half the time needed for rising to occur. Because you are purchasing live organisms, please note the expiration date on the package when using (fresh yeast will only last 1 week).
Always store in a cool, dry place, preferably the refrigerator (a must for fresh yeast), but bring to room temperature before using. Fresh yeast is extremely perishable and should be used within 1-2 weeks or date on package.
Yeast needs both a warm environment and food to grow. The process is often called "proofing the yeast." Yeast should be dissolved in warm water (100-110 degrees F), but not hot water or it will die. Sugar is usually added to "feed" and grow the yeast. You should see activity within 5 minutes, bubbling and expansion during yeast activation. If you see no activity, your yeast is too old, the water was too warm or too cold. Time to start over.
1 (1/4 oz) package dry active yeast = 2 1/4 teaspoons = 1 (1 oz) cake compressed fresh yeast
See Yeast Substitute Recipe 51462