Bread makers, brewers, lovers of alcohol everywhere are indebted to the generally single-celled bit of fungus known as yeast. Because the purpose of culinary yeast is to eat sugars and release alcohol and carbon dioxide gas. The alcohol is generally used for drinking and various types of flavor extraction and enhancing. The carbon dioxide is for lift.
One of the cool things about yeast is that it's alive. So many of the good things related to cooking are, by the time they reach the kitchen, some combination of mechanical or chemical. The Maillard Reactions, as wonderful as they are, are just some protein recombination at favorable conditions. A soufflé rises or falls due to a combination of the strength of protein bonds and the expansion of gasses when temperature rises while pressure remains constant. All great stuff, but there's nothing quite like life.
Yeast, though. Yeast grows. Yeast multiplies. Yeast survives, if all is well. And that gives yeast a certain amount of magic above and beyond chemical bonds and pv = nrt.
There are a few different categories of yeast. Brewers have their own yeasts, designed to not taste funny in high-alcohol environments. Bakers use commercial yeasts and wild yeasts. You can learn a bit about wild yeasts from this question I answered for Donna at my Fine Cooking Blog. From a baker's perspective, most of your commercial yeast is going to be much like the rest. There are three main types that you might be likely to run into.
- Fresh Yeast - comes in a big block, and is really useful for people who make lots and lots of bread and other yeast-leavened products. It doesn't live long, but it's the most alive, so it's the quickest to breed and turn your water, flour, and salt into bread.
- Active Dry Yeast - this is shelf-stable yeast, so it's easy to store in a pantry. Active dry yeast has been put into suspended animation by drying the yeast and sealing them into airtight containers. Often your packets of yeast are this type. Although not completely necessary, it's wise to bloom the active dry yeast in warm water before mixing into the dough. The active dry yeast needs to wake up to work, and although it can be done after mixing in with the rest of the ingredients, it's not as sure to work.
- Instant Yeast - My yeast of choice. Stores nicely refrigerated, lasts a long time when properly stored, and doesn't need blooming. Just pour it in with the dry ingredients and, when the time comes, it'll start eating sugars and expelling gases and liquids in their effort to demonstrate what a viable life form they are.
No matter which kind you have, if your yeast is past its expiration date, go ahead and bloom it in some warm water, maybe with a little sugar mixed in just to keep the critters happy. If you see them making bubbles after a minute or two, you know the yeast is still alive and ready to help with the bread making. If not, it's probably time to get some new yeast. You want the water to be barely warm. Around 100°F would be great, but between 80 and 115°F should work just fine. 120°F will kill them, so aim low.
With active dry and instant yeast, contact with salt while dry won't hurt anything. When they get wet, you don't want the bulk of your yeast touching salt, so I will mix up my dry ingredients well before adding water. With fresh yeast, it's best to be a bit more careful with the salt.
Treat your yeast well, and the wonders of bread can be yours. Treat yeast poorly or with neglect, and the yeast will not help you in your quest for leavening and flavor. It doesn't take much love, but yeast does still need love.