I’ve had a jar of yeast that I saved from the yeast cake of a previous brew about 3 months ago. It is probably on the order of 8 ounces of slurry. How do I revive it if I want to use it this weekend to brew?
Take about a tsp worth and step it up in a starter.
Kai
I’ve got a similar amount of yeast cake saved in a mason jar. Couldn’t I just take a few tsp and make a regular 2 quart starter? Or is it pretty necessary to step it up?
what size steps do you recommend 
I treat the slurry a little like a smack pack or tube
A tablespoon or two of the slurry and a 2 liter starter.
I’ll call that a 2 liter starter and go from there.
+1
The worst case scenario is that it will be infected, but (knock on wood) I haven’t had it happen to me yet.
If one has a good sanitary regimen, the chance of an infection is unlikely.
You won’t be able to fix that w/o some more advanced microbiological work (using plates).
Yes, a 2 l starter is a good start and should be sufficient if it is an ale. A lager you may want to step up to 4 l. Give it a taste once it started and finished. This will allow you to detect gross contamination. You can also taste the liquid that the yeast is sitting in. It should taste like stale beer.
Kai
I don’t actually go directly into a full-volume starter with yeast that old. I would put about 25 mL (~2 tbsp) of slurry into a 1 L wake-up starter first. That will get you into the 50-100 billion cell range, and then you can treat it like a smack pack and go into the full volume.
You mean once the starter is ready to be pitched, taste the starter beer and if it tastes like stale beer it’s fine? Is there any other way you can tell by looking/smelling the slurry whether it’s contaminated or not?
Yes, if it tastes sour or barnyard funky, you know it has bacterial contamination.
But just because it doesn’t taste sour or funky, doesn’t mean its not contaminated.
The best way to tell would be to “plate” the slurry on agar and look for bacterial colonies.
I haven’t heard of another low effort way of checking other than tasting. I used to think pH can be used but many of my starters have a pH that is lower than beer pH and they were fine.
Tasting is the best you can do and you should do it often. This will teach you how it is supposed to taste and it is a cheap insurance against significant contamination.
More advanced techniques involve using microscopes or selective growth media. The latter is not a real time thing since you need to let the bugs grow wile suppressing the yeast. Microscopes can work. I have seen occasional rods (bacteria) between my yeast but in some cases they were already present in the boiled starter wort that I used and therefore dead. Bacteria don’t respond to methylene blue stain the same way yeast does and it cannot be used to tell if they are dead or alive. I’m trying to keep an eye on that now rather than freaking out about it since it wasn’t even aware of them until I had a microscope. The beers have been fine and been w/o infection.
Kai
All of your senses are the first line of defense. Examining the wort by sight then aroma and finally tasting are a quick and easy way to determine if there’s an infection present in the beer, but often times the infection won’t rear it’s ugly head until it is too late. Practicing sound sanitization protocol and using common sense will enable one to make healthy beer.
I echo the use of starters to evaluate what it is U R stepping up PLUS a big starter goes a long way in covering inadequate practices