Is my yeast cake still good?

[stosh]

I brewed a kolsch 5 weeks ago and had left the cake in the bucket with a little liquid on top with the plans of brewing another kolsch and just tossing it in on top of the cake. The cover has been on the bucket since then and it smells and looks ok.

Can I still use the cake for my next kolsch or is it no good since it's been sitting at room temp for 5 weeks?

 

[Yooper]

Room temperature for 5 weeks is pushing it. In a fridge it would likely be possible. I would get fresh yeast I'm afraid.
Next time, even if you think you're bring again in 5 days, just pour the yeast cake into a couple of sanitized mason jars and stick them in the fridge, just in case. I've used 6 month old yeast easily that way.

 

[9Kegs]

Next time, even if you think you're bring again in 5 days, just pour the yeast cake into a couple of sanitized mason jars and stick them in the fridge, just in case. I've used 6 month old yeast easily that way.[/QUOTE]

Up to 6 months is impressive. Time to buy some more jars!

 

[IslandLizard]

Even if 80% of the yeast has died off, the whole yeast cake may have enough viable cells left. But I'd be more concerned about infections gotten in, and taken a foothold. Such as wild yeasts, bacteria, like acetobacter, due to air/oxygen in the huge headspace.

 

[day_trippr]

SO want to post a Dirty Harry "Do You Feel Lucky?" meme but they all have "Punk" in 'em and don't want to come off as harsh
Anyway...given the total cost and time invested in a typical batch, risk and its avoidance cost should be measured.
fwiw, I don't repitch, instead preferring to "ranch" overbuilt starters...

Cheers!

 

[Vale71]

Even if 80% of the yeast has died off, the whole yeast cake may have enough viable cells left. But I'd be more concerned about infections gotten in, and taken a foothold. Such as wild yeasts, bacteria, like acetobacter, due to air/oxygen in the huge headspace.

Live cells sure, vitality on the other hand will be very questionable at best. It's not enough to have live cells to get a good fermentation, their actual vitality will still have a great impact on fermentation and beer quality. The rule of thumb in commercial operations is that if you have less than 95% of live cells their vitality is most likely too low for the yeast to be usable. Contamination on the other hand is not a big issue if all the beer has been removed as any contaminants will not have anything to eat.

In any case I'd toss it and start with fresh yeast, it's just not worth the risk IMHO.

 

[hopjuice_71]

Live cells sure, viability on the other hand will be very questionable at best. It's not enough to have live cells to get a good fermentation, their actual viability will still have a great impact on fermentation and beer quality. The rule of thumb in commercial operations is that if you have less than 95% of live cells their viability is most likely too low for the yeast to be usable. Contamination on the other hand is not a big issue if all the beer has been removed as any contaminants will not have anything to eat.

In any case I'd toss it and start with fresh yeast, it's just not worth the risk IMHO.

Is this really what the commercial practice is? The accepted definition of viability in academic micro/biochemical research is simply the percentage of live/dead cells in a population. Vitality is a different matter and it does not necessarily correlate with viability. Vitality is difficult to quantify. It would be surprising if brewing adopted an entirely different definition for viability.

All things considered, the OP would definitely be rolling dice using yeast under those conditions.

 

[Vale71]

You're right, I wrote viability where I actually meant vitality.

As for quantifying vitality, there are ways to measure it but they are too complex and costly to be used in commercial operations (intra-cellular PH measurement). More practical tests based on fermentation of a standard wort in controlled conditions have been developed.
Otherwise one has to use a rule-of-thumb to estimate vitality based on viability, the rationale being that if so many cells died due to storage conditions the survivors will still have suffered great stress although they haven't died yet so that their vitality will be suboptimal. The more cells die off the worse off the survivors will be. One then draws a line and any yeast stock that crosses it gets discarded. Viability will have to be measured of course but there are ways to quickly and automatically measure that, they are expensive but beyond a certain size of operation they are definitely accessible. If measuring viability is too costly then best practices are adopted so that yeast is not stored for too long and always under optimal conditions so as to minimize risk. If even that is not possible (nano-squared operations) there is always dry yeast...

 

[monkeymath]

Well, you don't need a huge starting population to use it in your next beer. Just start a couple of days (maybe a week) ahead with a small starter (maybe 100ml) and a spoonful of yeast, then step it up from there. (I mean, you can even culture up yeast from bottle dregs this way, and you most certainly still have way more living cells than that.)

 

[stosh]

SO want to post a Dirty Harry "Do You Feel Lucky?" meme but they all have "Punk" in 'em and don't want to come off as harsh
Anyway...given the total cost and time invested in a typical batch, risk and its avoidance cost should be measured.
fwiw, I don't repitch, instead preferring to "ranch" overbuilt starters...

Cheers!

Lol...Don't leave me hangin' day trippr post it!

Thanks for the feedback everyone. I was not feeling lucky so I ended up taking the advice you all gave and dumped it.

I'll just overbuild my next starter so I have some on the shelf for the next few brews.

RDWHAHB