Does anyone know exactly how yeast works? I bought a large package of yeast (that was all they sold), and it was very hard. Then, when exposed to air, it became all soft and expanded. Is it still good once it has been exposed to air, or do I need to buy separate small packets? If it is still good, how long does it keep? Please let me know. Thanks!
Hi, I went to about.com to find an answer to your question:
Here is what yeast is and how it works:
Yeast is made up of a single-celled organism, Saccharomyces cerevisiae, which multiplies rapidly when fed sugar in a moist environment. One pound of yeast contains 3,200 billion yeast cells! Yeast also thrives on starch, which it converts to glucose, a simple sugar. This process ferments the sugar, which converts to alcohol and carbon dioxide. As we've mentioned earlier, the carbon dioxide expands the baked good to produce the light, fluffy texture.
The ideal temperature for yeast growth is 100 to 115 degrees F., but for leavening purposes, the ideal temperature is 80-95 degrees F. If the yeast grows too quickly, it will produce large bubble pockets in the end product. Yeast begins to die at 120 degrees F. So, it's important to let your yeast dough rise in a spot where the temperature is regulated. One-half an ounce of yeast will raise 4 cups of flour in about 1-1/2 to 2 hours under ideal conditions.
You should also proof your yeast to be sure it's viable before using in a recipe. To check it, mix a bit into 1/4 cup of lukewarm water with 1/4 teaspoon sugar. It should begin to bubble and ferment within about 5-10 minutes. If not, the yeast is dead and should be discarded.
Salt inhibits the growth of yeast. Never mix yeast into salted water. Since most tap water goes through a filtering process which utilizes salt as a refining/cleaning agent, many cooks use only distilled water for baking. Howe'ver, if you are baking during the hot summer season and find your dough rising too much, the addition of a little extra salt can control that runaway yeast growth.
Most yeast is sold in single-use packets or bulk bags known as dry active yeast. Compressed yeast is not as widely available, but can be used to the ratio of one standard cake of compressed yeast to one scan't tablespoon of dry yeast. If the dry yeast is stored in airtight packaging, in a cool dry place, it's not necessary to refrigerate it. Yeast should always be at room temperature to begin a recipe. Standard single-use packets contain about 2-1/2 teaspoons (1/4 ounce) of yeast granules.
There's much more information here.
Two more things to note: if you're using fresh yeast, it must always be refrigerated.
Also, nutritional yeast is a different type of yeast than the yeast you bake with. Baking yeast looks like tiny white balls, and nutritional yeast looks like yellow fish food. Nutritional yeast doesn't leaven dough, but it does impart a cheesey sort of flavour.
I store my baking yeast in an tightly closed jar in the fridge, and my nutritional yeast in a tupperware jar in the pantry.
I hope this helps!