1 Posted: Thursday, October 12, 2017 3:15 PM

Wanted to report on my accidental yeast freezing experiment, I have mentioned on another thread here that I accidentally froze a 750ml passata jar of US05 that I had recovered from a pretty strong IPA, (some 350G of hops used, 7.5%abv).

The US05 was 5th generation from first use in Feb 2017. Yeast in jar was slurry recovered from yeast cake as per my process now, see bottom of post*

So yeah I accidently froze this jar of yeast solid, I won’t bore you with how that happened, but suffice to say, I am not perfect in every conceivable way, and have actually made mistakes in my life, this was one of them. I am guilty as charged.

Anyway I thought, I’ll just set the fridge back to the temp it should have been at, say 2-3c and let it thaw out over a couple of days which it did. It did some strange things as it thawed out, like the liquid on top became murky as yeast was disturbed, probably by CO2 bubbles popping up through the slurry. Anyway I simply left it in the fridge, thinking I may re-use, possibly with a starter.

The next ale I made was several weeks later, a Centennial/Galaxy pale, I took a risk and just lobbed the yeast in (all 400ml compacted slurry) with no starter to test/boost viability.

So that yeast was now 40 days or almost 6 weeks from harvest, had been frozen and thawed early on in those 40 days (probably day 1-2). As per usual I did not warm up the yeast from fridge storage, I just dumped it in to 18C 1.057 wort, from a 2c fridge temp. So pitched in the evening, around 8pm, by 7am there was a healthy 1cm krausen and it was rocking along just fine and was at FG of 1.010 in three days

Have read that freezing yeast (without glycol etc) may only result in as little as 10% cell death, not sure what length of time or thawing procedure used but I can verify that it didn’t seem to bother my yeast one bit. So I may consider freezing for longer periods if I don’t plan on using a particular yeast for a while, and experiment with viability after months of being frozen. I have actually frozen yeast with glycol before autoclaving the small jars and glycol with my poor man’s autoclave (aldi pressure cooker) but have not brought back to life to date.

I bottled the ale last night and it tasted like it will be one of my best ever.

My yeast harvesting process:

*Rack pretty much all the beer off the cake to bottling vessel, pour in 750ml boiled cooled water. (I also boil the 750ml glass passata jar in this water, starting cold of course not to crack the jar)
Swirl up the cake for 30 secs or so (helps degas it and mixes it up) then left to settle with FV tilted on side towards tap, while doing the bottling.
Approx. 40 mins later rack 750 ml slightly settled slurry (using only 750ml water I end up with very little settling after this time, maybe only 2mm of clear diluted beer as top layer and no trub/yeast separation evident)
Put jar in fridge, next morning settles down to maybe 2/3 then to ½ the jar or around 350 to 400ml compacted slurry after a couple of days at say 2-3c.
Use up to 4 months later with no starter

2 Posted: Thursday, October 12, 2017 3:27 PM

You've made mistakes in your life?


I, on the other hand, am great, having never made a single mistake ever….

On a more serious note, this doesn't really surprise me, but it would be interesting to see, how many cells are killed off in the process.

Kelsey, feel free to run this experiment.



3 Posted: Thursday, October 12, 2017 9:35 PM

The list of experiments is growing… This one at least can be quick, just freeze a bit of yeast slurry for a week or so and then thaw it out and stain test it. Might even take some of the dregs from the 1272 starter after pitching it into a batch on Saturday. Or I might not, I'll make that decision on Saturday . Either way, it would be interesting to test it.

4 Posted: Friday, October 13, 2017 1 AM

I read somewhere that freezing yeast kills off about 50% of the cells. IIRC somewhere around the six month point you are better off freezing than storing in the fridge. It would still be interesting to see what kind of results you get though Kelsey.



5 Posted: Friday, October 13, 2017 2:01 PM

Would be great to see the results of that experiment Kelsey!

Christina, 50% sounds closer to what may be true, maybe Kelseys experiment can offer some data here..

But with the amount of yeast I pitch, if 50% of 400ml compacted slurry was alive and well, then that would have been a very tidy amount to start the brew with, and the dead cells being great nutrient for the live ones.

Plus once frozen, if it lasts for ages (years) then would be a great simple method for hibernating your specialty yeast to use next season for eg. Witbiers, octoberfests etc that you might only brew once a year.

Last edited by headmaster (Friday, October 13, 2017 2:01 PM)