Short(ish) version: Leave some beer behind in the fermenter after racking, and swirl up the slurry and save some in a sanitized container. Simply store the yeast under the beer it made. Store very loosely covered (with foil, or a lid not tightened down – it will probably still be producing some gas!) at, ideally, 34°-37°F. Do not rinse the yeast with water or store in water; old sources may have advised doing so, but these practices are now known to be quite harmful to yeast and to invite infection. Storage life is strain dependent and dependent on storage conditions. Most strains can be repitched directly for up to two weeks, some even longer; if in doubt of the vitality of the culture, you can make a new starter from it to be sure. (Note that your harvested yeast, even if you keep it a while, is usually still much fresher than the new stuff you buy at the LHBS.) For longer storage, you can periodically add wort on top of the slurry to feed it, or just plan to propagate it back up when needed. If the yeast looks odd, smells odd, or the supernatant beer develops off flavors or aromas, you probably want to avoid repitching the yeast. (If you search this forum for this and related topics, your head will soon be spinning and you will be desperately trying to claw your way out of the rabbit hole…)
[EDIT to correct erroneous substitution of “beer” for “yeast” in 2nd sentence. No real change.]
Robert has it covered - what I do is re-pitch within 2 weeks, typically. Half a cake for lagers and a third for ales. The most I have done is 25 generations with an American Pilsner yeast, but I presently have 2206 at the 10th generation burbling away in a Vienna Lager.
Yeah, Robert has it covered. Personally I don’t stress out about repitching out to roughly 6 weeks since the yeast isn’t under any pressure. Beyond that I’ll harvest about a tablespoon and build up a new pitch - 120 billion cells/liter is about what a stirred starter yields me.
That’s a great point about pressure. Most of the stats you see about viability are based on commercial conditions. But homebrew fermenters and storage conditions avoid the hydrostatic pressure and buildup of heat in a large biomass that are inevitable there. Score another advantage for homebrewers.
I’m day 4 of fermentation with a yeast I saved on 3/20/18. I keep near freezing in a mason jar with a pickle pipe lid. The attenuation right now is at 78%, which is upper range for that yeast strain.
Drinking the 23rd brew from a vial of WLP644. I make a big starter and on brew day save 1/3 quart to a mason jar with the lid tight and in a ziplock bag to the bottom of the fridge. I haven’t noticed any degradation…I trust my starter much more than the fermentor to be free of any unwanted bio…