Most brewery fermentations are carried out with re-used yeast, but the question of how to store and maintain properly frustrates even the most skilled brewers. It is actually not as difficult as some believe, and there are techniques that brewers can use to significantly expand the life of their yeast.
The fact that we can take a by-product of beer production, yeast, save it and reuse it in successive fermentations is quite unique. We can do this because yeast is still alive and healthy after most beer fermentations. The low alcohol levels in beer prevent the yeast from dying off, as it does in wine production. The problem for most brewers then is not whether to reuse yeast, but how to store it and keep it healthy for future brewing sessions.
Yeast is a living organism and is most happy and healthy when feeding on wort sugars. When fermentation is complete, they flocculate to the bottom of the fermentor, then go into a resting state. Yeast under beer is fairly stable, and most brewers agree that this is the best place to store yeast. Note, two crucial factors are temperature and time.
The yeast cake at the bottom of a conical fermentor may rise in temperature. Yeast is an excellent insulator, and heat can build up in the middle of the slurry, 10-15°F (-12--9°C) above the beer temperature, typically for very flocculent strains. When yeast heats up, its lifespan plummets. If the cone is not chilled, effects are even more significant. For this reason, brewers try to remove yeast slurry shortly after fermentation is complete, and the beer is chilled. Once yeast is removed, you ideally want to use the yeast immediately. This allows little time for yeast to deteriorate and die. However, this is not always possible, due to brew schedules.
The most common way to store yeast for future use is to put it into 5-gallon, stainless steel soda kegs. These work well, and the lid can be modified to your desire. These kegs have the many small parts and gaskets that can harbor bacteria, which need to be properly sanitized. Also, carbon dioxide can build up quickly in yeast slurry, and if kept under pressure, will cross the cell walls and kill yeast cells. Pressures over 35 PSI can be toxic to yeast, and soda kegs are rated over 100 PSI. So if you use these kegs, shake and vent pressure on a regular basis, at least once per day.
Other vessels can be used for yeast storage. Brewers often shun plastic, because it scratches easy and scratches can harbor bacteria and wild yeast, however, this can actually be a good choice. Use a high grade (and food grade) plastic (polyethylene, polypropylene), buckets exclusively for yeast storage. The advantage of plastic is that yeast slurry is visible, so you can evaluate the condition and quantity of yeast by sight. For example, if you pull off yeast slurry and it is very runny, without counting cells under a microscope, you will be unsure of how much yeast to use in the next batch. By using a plastic bucket to store yeast, you can see how much yeast settles out, and pitch accordingly. Plastic buckets also need to be vented occasionally.