Q: I am tired of buying yeast all the time. is it OK to just repitch the trub from a previous batch?
— @dangerolsen Via Twitter

A: Re-pitching of yeast is a normal method used by brewers around the globe. Although the practice is simple, there are a few rules that may make the method less than ideal for the typical homebrewer. The first rule is that the yeast should be harvested shortly after fermentation is complete and used within a short time period. The most common method of yeast harvesting is by bottom cropping, because lagers are the dominant beer type in the world. Most commercial lager brewers these days bottom crop yeast from the bottom cone of cylindroconical fermenters and store it for short durations in a yeast brink (chilled and often agitated storage tank). Ale strains are sometimes bottom cropped and other times skimmed from the top of open fermenters, then stored in a similar fashion to lager yeasts prior to re-use.

There are a variety of ways to harvest yeast when homebrewing and the yeast can successfully be stored and re-used if you are careful. I suggest storing yeast in a flask and using cotton batting to close the top. Yeast stored in this type of container can be placed in a refrigerator without any problems for up to about 10 days before re-use. It is really preferable to keep this duration as short as possible since yeast viability and vitality decrease with time, especially as storage temperature increases. Ideally the storage temperature should be around 34 °F (1 °C). Harvesting yeast for re-use at home has one major drawback and that is the fact that most homebrewers do not brew frequently enough to re-use for very long. Some brewers share yeast and are able to keep a culture going from one batch to another with short storage durations in between. This can work very well if you have a group of friends who are good, clean brewers.