How long does yeast keep?

I recently got a mason jar of yeast from a local brewery... I used half of it for a current brew (worked great, tastes awesome).
It's been 4 weeks since I got the yeast and still have half the jar left. Is it still good? How long would it last for? I've just been keeping it sealed in my fridge in hopes that I could use it for another brew. Any help is much appreciated. Thanks!

It should still be good. You might make a starter to 'proof' it, and then put a little of the starter back in the fridge for another brew day.

Yeah, it's still good. Like well said, make a starter to ensure it is ready to ferment. If you keep it in a sanitized container and the lid is closed tightly, it will easily last 6 months or more. I've used yeast that was a year old already, but did make a few starters to get it ready. It was a bit tired in the beginning, but fermented out fine.

If you're stepping up a starter, then the age of a yeast isn't really an issue.

Bobby M did a test on year old stored yeast here; https://www.homebrewtalk.com/f13/testing-limits-yeast-viability-126707/

And my LHBS cells outdated tubes and packs of yeast dirt cheap 2-3 dollars each and I usually grab a couple tubes of belgian or other interesting yeast when I am there and shove it in my fridge. and I have never had a problem with one of those tubes.

I usually make a starter but I once pitched a year old tube of Belgian High Gravity yeast directly into a 2.5 gallon batch of a Belgian Dark Strong, and after about 4 days it took off beautifully.

With any stored, old yeast you just need first to apply the "sniff test" if it smell bad, especially if it smells like week old gorilla poop in a diaper left on the side of the road in the heat of summer.

Then make a starter, and if it takes off you are fine. The purpose of a starter is to reproduce any viable cells in a batch of yeast....that;s how we can grow a starter form the dregs in a bottle of beer incrementally...and that beer may be months old.

Even if you have a few still living cells, you can grow them....That's how we can harvest a huge starter (incrementally) from the dregs in a bottle of some commercial beers. You take those few living cells and grow them into more.

If yeast can be grown from a tiny amount that has been encased in amber for 45 million years, 45 million year old yeast ferments amber ale we really don't need to sweat too much about how old a yeast is, if it's properly stored.

we just need to think in terms of making starters. Viability isn't really an issue if you are reproducing a lot of healthy cells. Which is what you are doing when you make a starter.....

Really even with "old yeast" if there is a few cells, they will reproduce.

The viability of the yeast depends entirely on the storage conditions. If some yeast was just dumped into a sanitized jar, I'd worry about bactera after more than a few weeks. Containers like White Labs vials can be used far after the expiration date mainly because they work very, very hard at keeping the vial as low in bacteria as humanly possible; autolysis is really the only negative byproduct of extended storage.

As far as your jar of yeast, I'd say make a starter with it asap. That way you'll have it sitting in some low abv beer that'll work as a more sanitary environment.

0110x011 said:

The viability of the yeast depends entirely on the storage conditions. If some yeast was just dumped into a sanitized jar, I'd worry about bactera after more than a few weeks. Containers like White Labs vials can be used far after the expiration date mainly because they work very, very hard at keeping the vial as low in bacteria as humanly possible; autolysis is really the only negative byproduct of extended storage.

As far as your jar of yeast, I'd say make a starter with it asap. That way you'll have it sitting in some low abv beer that'll work as a more sanitary environment.

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Many of us store our yeast in sanitized mason jars all the time, and have used it after years with no issues.