Freezing Yeast Question

[Dusan Kovacevic]

Hi All,

Firstly I'm interested to see is anyone still banking yeast? There seems to be serious lack of discussion in recent years, all I can find is posts from few years ago.

There are a few good threads, blogs and articles that explain the process of freezing yeast so I got that part covered, but there are a few more questions I can't find the answer to. So here they are:

1. Most articles and forums are talking about freezing in 15ml vials. Is there any problem in using larger containers (besides the fact that you need more storage space)? To be more precise I'm thinking about using 50-70ml vials.

2. Preferred cell freezing rate is 1C per minute. Folks are claiming that placing vials in 90%+ alcohol and than in freezer achieves that. Did anyone test that claim in regular home freezer?
Did anyone test any other method of achieving 1C per minute freezing rate?

3. Are you guys preparing a starter for freezing from a bigger sample or are you streaking plates and growing a starter form a single cell colony and why?
Furthermore, if you are streaking the plate, do you select single cell colony and grow from that or do you mix in a few single cell colonies for genetic diversity?

4. There are quite a few people having discussions about microscopes but has anyone bothered to measure maximum cell density in solution for different yeast strains?
To be more exact, is there a spreadsheet with yeast strains and their maximum cell density per millilitre of X Plato wort?

5. There is a general consensus that yeast should not be used beyond 3-5 generations because there is higher chance of mutation, but there is no definition of what "a generation" means.
I believe, originally, one generation meant one complete fermentation after which the brewer would wash and reuse the yeast.
Are we not making a distinction between yeast washing, top cropping, and propagating the yeast for 12 hours in very light un-hopped wort? Are we saying that in each of those methods yeast went through one generation and is equally likely to mutate after that method is repeated 5 times?
Besides, whatever method you use there will be a few, actual, generation of original yeast there, so that's a bit confusing as well.

I hope there are people with answers to these questions and I would really appreciate links to articles, blogs or threads that answer the questions as well.

D.K.

 

[Taket_al_Tauro]

I'm doing this since a few years now with great results so far. I revived and propagated 2+ years old yeast samples and brewed with those without any issues. I haven't tested samples older than that yet, but I certainly will in the coming years.

I use a final concentration of glycerin in the vials of about 15%.

I always make a starter, cold crash it and then take a few samples of that slurry for freezing (2-4 depending on how often I plan on reusing that yeast).

You could certainly freeze in containers larger than 15 ml if you have the space.
I see only advantages: larger starting culture and more protection against possible temperature fluctuations during storage.

To prevent possible temperature fluctuations issues, I store my vials in a box containing an alcoholic solution (in my case some cheap booze I had around and did not want to drink), that should provide some insulation.

I initially freeze my vials also using the same kind of booze in a small container, to allow for a slower freezing rate. Did not test whether it's close to 1 C min or not, but it seems to work!

Good luck!

Edit: I checked my notes and in fact I use a final glycerin concentration of about 10-11%. Not that it probably would change much from using 15%...
I had read somewhere that this concentration range seems to be best for freezing yeast under usual home freezer temperature conditions (-18 to -20 °C).