The contamination risk depends on your sanitation practices. If you’re concerned about keeping a quantity of yeast that you can go back to, you may want to buy some smaller 1-2 dram vials and split up the slurry in multiple vessels. The smaller vials are usually safe to freeze with the yeast in a glycerine mix if you want to/can do that.
At this point, the viability of that slurry is so low that you’ll have hardly any cells left that are still alive and will be able to survive another series of growth phases.
If you’re going to do multiple steps (esp. with a stir plate), just buy one packet of fresh yeast and step up from there.
I’ve done thisseveral times with old yeast, not four months old, maybe two months old.
I use about a 1/4 cup in 2L of wort and a stir plate.
Never had any issues, that I could taste.
I’ve always dumped the unused portion of the yeast.
I wouldn’t worry about contamination. Do your beers go bad after 3 months?
You’re going to have fewer viable cells but I don’t believe Mr. Malty’s doom and gloom estimate. I just built up a starter from some 4 month old slurrry and it was active as any starter I’ve ever made. I estimated 10% viability and made a 2 step starter, and the cell count showed significantly more yeast than the Mr. Malty prediction, almost double.
I think this is doable but you’re definitely pushing the clock. You might have to pitch Monday or Tuesday.
Viability should be in the 30-60% range, so let’s say ~25 billion viable cells per 15 mL tablespoon. I’d put 1 tbsp in a relatively small amount of wort, say 250 mL, let it ferment out (hopefully within 24 hours), then smell and taste as a quick contamination check. If it seems OK step it up to 1 L, then 4-5 L.
You could start with a larger amount of slurry and do a smaller starter (or fewer steps), but this approach has the advantage of keeping the overall viability above 90-95%. A 2 L starter wouldn’t get you anywhere near 650 billion cells anyway.
Once it’s all said and done, what I usually do is pitch the yeast, then rinse out the vessel with some sterile tap water and store what’s left for future propagations.
wow. speak of the devil. was just looking up this thread to post what I did.
couldn’t quite do what sean recommended, though I think that is the way to go.
but…on the morning of the 24th, i made a 1L starter with 2-3TBL of slurry. let that stir for all of Wednesday and by Thursday night it was clearly done. Crashed it, which thankfully didn’t take long - small starter and being that it was 007 - tasted the wort - it was very, very fruity and had some strong esters but what to expect with english ale at 74-76df, right?
Then on Friday I made a 4.5L starter, pitched into that and let it go Friday through Saturday afternoon, at which time the krausen started falling and the swirling appeared clumpy, so i figured it might not be ideal, but I crashed it at that point. Brewed the black IPA on Sunday. at about 9pm last night, there was a HUGE layer on the bottom of the flask, although it was still partially cloudy, but i poured off the spent beer, aerated and pitched at 60df.
as of this morning, full krausen and thumping away.
Update #2: the krausen was huge on the black ipa last night - almost had a blowoff (i never do - 2 gal of headspace and i pitch cool) and it smelled nice, hoppy and slightly fruity. hopefully things are going well.
