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You are here: Home --> Forum Home --> Brewing Forum --> Brewing Discussion --> Contaminated Yeast?

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flapjackcarl
Houston, TX
Posts


I typically save yeast from starters for future use. After I make a starter I'll pour a pint into a sanitized mason jar and pop it in the fridge.

Yesterday I was doing a bit of yeast inventory to see what I had on hand, and I realized that all three yeast cultures seemed to be under pressure (slightly bulged lid). The only explanation I could come up with for this is that there's contamination and some nasty is eating residual sugars and off gassing co2. But it seems unlikely that this would be present in all my yeast (I know that the yeast I pitched was good as the beer produced showed no signs of infection). Any alternative theories on what could cause this?




Posted 34 days ago.

homebrewdad
Charter Member
Birmingham, AL
2480 Posts


Incomplete starter fermentation jumps to mind - especially if you are harvesting at high krausen (or close to it).



Posted 34 days ago.

flapjackcarl
Houston, TX
2480 Posts


Good point. Historically I've harvested well past high krausen (generally after yeast have begun to flocculate, so around 3 days after pitching). It's possible that there could be a small amount of fermentation left to complete. 

I'm hesitant to believe that all of my yeast magically became contaminated between. The starter and the storage vessel (and I don't belive the starter was contaminated as in all cases it produced beer without infection).




Posted 34 days ago.

homebrewdad
Charter Member
Birmingham, AL
2480 Posts


I'd agree, it seems unlikely that everything would be contaminated.



Posted 34 days ago.

mchrispen
Bastrop, TX
485 Posts


Yeah if you are doing vitality and/or the Shaken Not Stirred method - there was likely sugar left. I do find that US-05 seems to continue to ferment in the 40's as well. I have to go through and occasionally off-gas my little collection. Not an issue if you are just using the Whitelabs vials - they seem to hold the pressure really well.




Posted 34 days ago.

testingapril
Charter Member
Atlanta, GA
595 Posts


Yeast can be active slowly at fridge temps. I've had that happen before.

If you want to clean them up you can try Chlorine Dioxide, but this will only kill bacteria and not Brett/Dekkera/Yeast if it's contaminated with those things. Basically the idea is that you decant the spent beer off the top of the yeast down to about 200mL then stir the yeast with 2 tablets of potable aqua. www.amazon.com/Potable-Aqua-Chlorine-...

If you can't decant that far, it's just 1 tablet per 100mL. Stir for 20 mins, then pitch into a new starter or you can actually store the yeast as-is, but I'm skeptical of how long it can be stored that way. I think I'd just spin up a new starter of it.




Posted 34 days ago.

KidMoxie
Charter Member
San Elijo Hills, CA
405 Posts


Could just be some residual CO2 coming out of solution or the last bits if active yeast finishing a snack. Happens to most of my saved starters, I usually keep the lid loose for a few days before finally tightening it down. 



Posted 34 days ago.

flapjackcarl
Houston, TX
405 Posts


Matt: I spin mine on a stirplate for 48 or more hours (I'll be changing this process after the feedback during that white labs AMA).

I also don't buy the residual co2 argument as solubility should increase at lower temperatures. 

I think based on the fact that it occurred in all my samples I'm going to blame it on residual fermentation as that seems the most likely culprit (although surprising as I've certainly left yeast spinning for ~4days).





Posted 34 days ago.

vinpaysdoc
Charter Member
High Point, NC
321 Posts


Agree with what's been said.

1. Problems with incomplete fermentation before storing.  

2. Remedy for #1 is a. allow to ferment until krausen is gone in starters     and    b. off-gas the container every few days until it's not building pressure.

3. Dan put me on to the Chlorine Dioxide thing and I just used 1/4 tablet in about 20 ml of slurry before making my starter to put back up.







Posted 34 days ago.

ingoogni
nl
314 Posts


Most of my yeasts are stored that way after fermentation, they all build up pressure in the fridge. Even when they have been in there for half a year or longer. On reuse so far no signs of contamination. I just build up a new starter with a thee spoon of the yeast. Some are awfully smelly after half a year. There is always some digestable sugar left in a beer or a starter.

No beer ferments fully as most yeast start flocking (out) before there are no more sugars left. It is one of the main reasons why fermentations stall, the ratio of cells to sugars and nutrients is a signal for the yeast to flock and how much or how fast they flock depends on their flockability genes. Actually dropping out of suspension depends mostly on the turbulence in the beer. Stirring fermenting beer helps a lot with heavy flockers like WLP002, WY1968, they ferement faster and deeper then.
Paper on stirred fermentation: 
"Flocculation of Brewers' Yeast:
quantification modeling and control" --Hamersveld

repository.tudelft.nl/assets/uuid:3d5...

When I'm concerned with a contamination I don't wash but plate and select a new starting colony.




Posted 34 days ago.
Edited 34 days ago by ingoogni

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