(Note: This post drew extensively from articles in The Complete Joy of Homebrewing by Charlie Papazian, How to Brew by John Palmer, and a nice little web page by NASA. Please see their information for a more thorough understanding of yeast.)
Yeast is a single cell organism. Technically it is a fungus. There are many types of yeast, but here we will be discussing the most fantastic form of yeast other than baker’s yeast: brewer’s yeast.
Yeast is extremely important for beer – it fuels the fermentation that produces alcohol and much of the flavor. But yeast should not be thought of as simply an ingredient. It is a living thing, and as with all living things the environment it resides in affects what it does. This means the way you treat their world, the wort, will ultimately determine the type of beer you have.
Yeast comes in either dried or liquid form. Dried yeast is simply dehydrated, which increases its shelf life, and usually comes in a small packet. Liquid yeast usually comes in a can. There are more strains of yeast available in liquid form than dry, but the amount of viable yeast is higher in dry.
Yeast is available in two types: ale yeast and lager yeast. Ale yeast generally ferments on the top of the wort, while lager ferments at the bottom. One major difference in the types is optimum fermentation temperature: ale yeast works better in warmer temperatures, while lager prefers the cold.
This brings us to the wort environment. Yeast, like all organisms, requires certain environmental conditions in order to survive and propagate. They are oxygen, temperature, pH, and food.
Oxygen is a unique requirement for yeast. They can actually live without oxygen, but they thrive when the right amount is introduced. Oxygen powers the yeast’s initial life stage, called respiration, where it absorbs oxygen and sugars to store for energy. Too little oxygen in the system and the yeast will have a very slow or incomplete fermentation.
Temperatures that are too high or low cause the yeast to shut down or die, or allow other microbes to flourish until there is nothing left for the yeast.
pH is a pretty simple concept, and applies to all living creatures: you can’t live (comfortably at least) in an environment that is too acidic or basic. Yeast prefer a more acidic environment, and luckily beer wort is just about right for them.
Food is also pretty simple. Yeast requires sugars, fats, proteins, and minerals (just like us). Sugars and fats are a result of the malting process, while oils come from the hop flower. Elements come from the water.
All of these factors rely on the concept of moderation. Too much of one thing or not enough of another can cause a funky home for the tiny yeast. Just enough of everything and the yeast will flourish.
Finally, let’s discuss the life cycle. As mentioned above, yeast begin (for our purposes at least) their life when pitched into the wort. Initially, the yeast begins respiration by taking in free oxygen molecules, and also converts sugar to carbon dioxide and water. It is storing energy for the next stage. Yeast also begin to reproduce by budding (yeast are asexual, and reproduce by forming sister cells that pop off every 24 hours.)
Fermentation and more reproduction occur next. The yeast population grows rapidly, increasing three to five times the original amount until the optimum cell number is reached (50 million per milliliter). At this stage the yeast is still converting sugar to CO2 and water, but it begins to produce alcohol as well.
Finally, as food supplies dwindle, the yeast begin to prepare for dormancy once again. They drift down to the bottom of the wort to form sediment. As the yeast settles it produces a substance that reserves a little energy for the yeast in case it finds more food. This is how (beer) yeast strains are propagated over time.
There you have it. They are a wondrous little bunch. Just think – every time you drink a beer you are consuming millions of tiny little things that worked very hard for you.