How to tell if Yeast is dead?

[Beervezas]

Pardon the newbie question. I have searched the forums but havent found a definitive answer. One of my Wyeast packs got lost in shipping then arrived about 10 days late. I tossed it in the fridge anyways to see if it was still usable. Today is brew day so yesterday I popped the bubble inside and waited a few hours to see if it would swell. It seemed to swell a little but not much at all. So I made a starter and put it on the stir plate yesterday(about 20 hours ago). Today not much seems to have happened... well to my untrained eyes nothing has happened.
How can I tell if the yeast is dead or not?
-Thanks!

 

[mongoose33]

Put an airlock on your flask and see if there's any bubbling? If you didn't use Fermcap when boiling the wort, you'd expect to see something approximating krausen on top of the starter, bubbles, foam, just a thin skim of something.

The definitive way--and you almost certainly don't have the equipment to do this--is to take a yeast sample, dilute it, stain it, and then examine it under a microscope. Live yeast cells don't take up the stain, while the dead cells are colored blue from the stain.

Here's a couple pics of what that looks like:

1. Sample of good yeast, just a few deaders in there; there's one good example of a stained cell in the lower center of the pic:



2. Sample of yeast that was microwaved in order to demonstrate a mostly-dead sample; there are a couple that are still viable, but most are dead:

 

[bracconiere]

if your worried about losing a batch of beer...you could try to make a loaf of bread with some...

 

[Hoppy2bmerry]

What @mongoose33 wrote is spot on. The fact that the pack did expand means some viable yeast are left, your started could be waking up in the next 4-12 hours. Keep your wort in a cool spot and wait for it. Or brew tomorrow if you can wait.

 

[SoCal-Doug]

In theory, if there was one live cell out of the billions of dead cells, eventually your batch will ferment and that one cell will create billions more.

In reality, there's probably millions of viable cells remaining. Use it.

In the future, do a starter to bring the cell count up before it's pitched.

 

[Beervezas]

Thanks for the replies. I understand it is difficult for a krausen to form when on a stir plate, correct? I put a bung and airlock on for the past two hours and while it is on the stirplate it is creating some bubbles. Off of the stirplate I did not see any activity. I guess I will pitch it and see what happens over the next 24 hours... if necessary I have a dry yeast I could prepare and pitch tomorrow or the next day.
-thanks

 

[mongoose33]

Thanks for the replies. I understand it is difficult for a krausen to form when on a stir plate, correct? I put a bung and airlock on for the past two hours and while it is on the stirplate it is creating some bubbles. Off of the stirplate I did not see any activity. I guess I will pitch it and see what happens over the next 24 hours... if necessary I have a dry yeast I could prepare and pitch tomorrow or the next day.
-thanks

You're not going to get a krausen like you would in a fermenter, but you should see something on the surface of the start, just a skim of white something or other. Once it's done, of course, nothing.

 

[grampamark]

It's not dead; it's just pinin' for the fjords.

 

[passedpawn]

Pardon the newbie question. I have searched the forums but havent found a definitive answer. One of my Wyeast packs got lost in shipping then arrived about 10 days late. I tossed it in the fridge anyways to see if it was still usable. Today is brew day so yesterday I popped the bubble inside and waited a few hours to see if it would swell. It seemed to swell a little but not much at all. So I made a starter and put it on the stir plate yesterday(about 20 hours ago). Today not much seems to have happened... well to my untrained eyes nothing has happened.
How can I tell if the yeast is dead or not?
-Thanks!

There's been a way of "proofing" yeast since the days of your forgotten ancestors. Put it in a bowl with a 1/4 cup of water and a teaspoon of sugar. Wait it for it to foam. Shouldn't take too long - an hour or two, max. Proofing yeast. Baker's method. But nobody does that for beer.

Packets of dry yeast never needs proofing anymore though, neither for bread or beer. It's got a long shelf life.

Liquid yeast is a different story. That Wyeast smack pack is a brilliant innovation. It's proofing in the pkg. If it doesn't puff up in a day, toss it and get another. I've used pkgs that didn't expand and I then ended up in an emergency. I experiment with a lot of things, but the yeast and related timing is not a place to cut corners or take risks.

 

[Lefou]

Pretty direct and simple description of "proofing". I like it.
My version consists of .5-.7l of weak wort collected pre-boil. After a pre-boil gravity check the sample gets diluted and the liquid yeast is added after sitting at room temps for a spell. I typically use White Labs packets and they swell as they warm. While the boil is ongoing the yeast starter is perking at room temps. By the time the wort gets cooled the starter is already foaming and ready to add.
Technically, you could call this an underpitch but most of my beers are ales 1.050 or slightly below, and I underpitch as a rule for my beers. It's pretty reliable and evidence of primary fermentation shows up in a few hours.

 

[IslandLizard]

When a yeast's vitality/viability is low due to adverse shipping conditions, storage, age, etc. it may take a week on a stir plate to get a decent and viable population again.

I always wait for the starter color to become noticeably lighter, which could take a few days, then stir for 24 hours longer. For a good pitchable quantity, cold crash, decant and step it up one more time after that. You may even save some out to make a starter from for a future brew.

 

[passedpawn]

When a yeast's vitality/viability is low due to adverse shipping conditions, storage, age, etc. it may take a week on a stir plate to get a decent and viable population again.

I always wait for the starter color to become noticeably lighter, which could take a few days, then stir for 24 hours longer. For a good pitchable quantity, cold crash, decant and step it up one more time after that. You may even save some out to make a starter from for a future brew.

I used to maintain a pretty sweet collection of frozen yeast samples. I'd "step-up" the small cell count as you described, finally arriving at pitching quantities after 2 or 3 iterations of growing/decanting. When it worked, it was great. But I did also end up with gushers (I bottled then). I got a microscope and cell counter and started looking at the actual cells, and found out that those stepped-up starters were also growing bacteria (rods). I abandoned the whole thing. IMO, you really need a true lab-grade sterile environment (glove boxes, etc) if you expect to step up starters like that without problems.

Kegging hides some of these latent infections (if you keep your kegs cold).

Anyway, that's just my experience. I suppose others might be better at it than I was.

 

[kh54s10]

I used to maintain a pretty sweet collection of frozen yeast samples. I'd "step-up" the small cell count as you described, finally arriving at pitching quantities after 2 or 3 iterations of growing/decanting. When it worked, it was great. But I did also end up with gushers (I bottled then). I got a microscope and cell counter and started looking at the actual cells, and found out that those stepped-up starters were also growing bacteria (rods). I abandoned the whole thing. IMO, you really need a true lab-grade sterile environment (glove boxes, etc) if you expect to step up starters like that without problems.

Kegging hides some of these latent infections (if you keep your kegs cold).

Anyway, that's just my experience. I suppose others might be better at it than I was.

This is interesting, I don't have the microscope to check. I have been freezing yeast for years. Certainly not lab sterile. I then got involved in fixing up my house for sale then a move and didn't use any of my frozen yeast for several years. All of my samples are at least 3 years old now... I did a couple in the last year, stepping up from just 5 ml yeast. I have not had gushers with those, but I have a couple of times with dry yeast in darker beers......