Yeast can be saved and reused because it is still living and healthy following the majority of beer fermentations. The low level of alcohol in beer keeps the yeast alive. The problem is how to store the yeast while keeping it healthy for future beer brewing.
Yeast is healthiest when it is feeding on wort sugars. After the completion of fermentation, the yeast cells will flocculate to the base of the fermenter. Once there, they go into a state of resting. When yeast is under beer, it is fairly stable. The majority of brewers agree that this is the most ideal place to store your yeast. However, two critical factors you need to consider are time and temperature.
A yeast cake at the base of the conical fermenter can have a rise in temperature. This happens because yeast is an amazing insulator. The temperature can rise as much as 10° to 15° F above the temperature of the beer. This occurs quite frequently in extremely flocculent strains. Because of this, brewers will generally try to remove the yeast slurry quickly after the fermentation is completed and the beer has chilled.
The most frequent way brewers store their yeast is inside a five-gallon, stainless steel soda keg. These kegs work rather well and allow for modification of the lid. However, the drawback to using this kind of keg is the gaskets and numerous small parts tend to harbor bacteria. These kegs do not have the ability to vent pressure either, unless modified in some way.
Because carbon dioxide builds up rapidly in yeast slurry sometimes, if kept under this pressure it will cross the yeast cell walls and kill the yeast. Pressures above 35 psi can be deadly to yeast. The soda kegs are rated higher than 100 psi. For this reason, should you use these kegs, be sure to shake and then vent the pressure regularly. This should be done once daily in the least.
Brewers will usually shun plastic due to its ability to easily become scratched. These scratches have the potential to harbor bacteria as well as wild yeast. However, food-grade and high-grade plastic (polypropylene and polyethylene), can be used just make sure you use the buckets solely for the storage of your yeast. One of the main advantages of using the plastic is that the yeast slurry can be seen. This allows you to evaluate the condition and the quantity of the yeast by sight.
Plastic buckets also require ventilation on occasion. You do need to be careful about contact with air so use a screw on lid.
Two weeks seems to be the maximum amount of time that yeast can be used without encountering any problems.
While the yeast cells are being stored, they eat their reserves of glycogen. The deprivation of glycogen that occurs over time will weaken their cell wall making them more susceptible to rupturing. Cold temperatures slow this process, but freezing yeast is not recommended. Ice Crystals will also cause the cells to rupture. Yeast should be stored somewhere between 33° to 38° F.
Yeast cells release their contents when they rupture. Bacteria sometimes feed off the nitrogen that has been released by the cells. This allows the bacteria to multiply quickly. For this reason, the yeast slurry must be as free of contamination as possible when it is stored. The colder temperatures will also help to slow the growth of the bacteria.
Keeping your yeast stored between 33° to 38° F and beneath beer for no longer than two weeks should help ensure you have healthy yeast for your next batch of brew!