Active dry yeast was introduced during World War II to ensure that American soldiers had fresh bread to eat while fighting. Until then, commercial yeast was only available as compressed yeast cakes that required refrigeration. Because dry yeast could be stored at room temperature in an airtight container, military bakeries could keep yeast for longer. Active dry yeast's simplicity and convenience soon made its way into home kitchens.
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Compressed Cake Yeast
Compressed yeast cakes are solid and moist, similar to a thick cream cheese, and are kept in refrigerated cases, usually near the dairy aisle in the grocery store. They contain active yeast cultures -- tiny organisms that feed on the sugar from starches -- that are activated by exposure to warm water. Yeast cakes have the disadvantage of a shelf life of only two weeks and must be used before the expiration date to ensure a good rise in baked goods. Yeast cakes also need to be kept refrigerated. Active dry yeast does not require refrigeration until the original sealed container has been opened.
Active Dry Yeast
Active dry yeast looks like tiny grains of sand and contains dormant yeast cultures. Adding warm water and starch activates the granules. One advantage of active dry yeast is it can be stored in the cupboard -- although it will last longer in the fridge or freezer. It is sold in individual packages or in glass jars. Active dry yeast is easy to measure, activates easily and stores well. It produces light, airy bread similar to that produced by compressed yeast cakes. Active dry yeast can be substituted for compressed cake yeast, but the amount needed differs. You need half as much dry yeast as compressed yeast to produce the same results. Active dry yeast can be stored for up to a year in the refrigerator.
Instant Dry Yeast
Instant dry yeast is similar to active dry yeast, but it does not need to be dissolved in a warm liquid to activate the yeast. An advantage of instant dry yeast is the ability to add it to the dry ingredients in your recipe, avoiding the fuss of mixing liquids and dry ingredients separately. Instant dry yeast is also known as bread machine yeast. You can substitute instant dry yeast for active dry yeast using the same measurements and store it in the fridge for up to a year.
Rapid-Rising Dry Yeast
Rapid-rising dry yeast is specially formulated with the addition of enzymes to make bread rise quickly. One advantage of baking with rapid-rising dry yeast is that it eliminates the need to rise bread the second time. After the dough has been kneaded, it can be formed into loaves and allowed to rise in the pans for baking. Rapid-rising dry yeast can be stored for up to a year in the fridge and saves time in baking, but alters the flavor and texture of breads. A disadvantage of rapid-rising dry yeast is its effect on the flavor and texture of baked goods. Bread made from rapid-rising dry yeast resembles commercial breads, with a tight crumb and lacks the characteristic flavor of homemade bread. Like other dry yeasts, rapid-rising dry yeast can be stored in the refrigerator for up to a year, a clear advantage over compressed yeast cakes.