Freezing Yeast At Home For Long Term Storage/archiving

[Wolfy]

Storing yeast on malt-agar slants is easy enough to do, and it works well, however it takes time and effort, especially since the yeast should be re-slanted every 6 months.
Since last year, I've been freezing yeast samples with the aim of reducing the work required to archive yeast strains and keep them for long periods. If done right, the frozen yeast should be able to be stored for at least 2 years and maybe even 5+ years.

The method I have been using pre-dates the publication of the 'Yeast' book, however, it is very similar to what they suggest and I did get a few pointers from JZ.
Other than using malt-agar slopes and plastic test-tubes and vials there is no need for special lab-equipment, but a centrifuge and YPD solution are recommended in the book.

The idea behind freezing yeast is that the lower the temperature the longer the yeast should keep without mutation and changes. Cryo-storing at the yeast -80degC would be ideal since at that temperature the yeast should (if frozen successfully) keep for an indefinite period, however like most people all I have is a normal (but non-frost-free) freezer that sits at about -18 to -20degC.

Unlike BribieG's first attempt to freeze yeast, the method I use is not focused on storing pitch-able quantities of yeast, so it's a little more time-consuming since the yeast is slanted (to confirm and check viability) both before and then after it's been frozen and then a starter is made from the slant prepared from the previously frozen sample.

The viability and vitality of the yeast before it is frozen plays a factor with how well it can be stored, so I always start with a freshly inoculated slant of yeast:

10ml of sterile wort (about 1.035 gravity) is prepared in 15ml plastic test-tubes.
I make the wort and fill the tubes in batches before putting them in the pressure cooker to 'autoclave', once that is done they should then be sterile and keep indefinitely.

The sterile wort is tipped into the slanted-vial and the yeast scraped off the malt-agar with an inoculation loop.

Tubes are capped and then shaken to aerate:

Over the next 3-5 days the yeast should fully ferment out in the 10ml of wort.
Test tube caps are tightened to shake the samples frequently, but then loosened so that pressure does not build up that might harm the yeast.

.... ctd next post

 

[Wolfy]

When fermentation has finished (usually 3-5 days), the tubes are put into the fridge for another 2-3 days, this forces the yeast to settle, and should let the yeast build suitable glycogen and trehalose reserves.
For most yeast strains there is usually about 0.5ml of yeast sediment:

I decant most of the spent beer leaving a total of about 2ml of yeast and wort:

Rather than risk possible contamination by transferring, decanting or adding additional storage media, all I do next is to add about 1ml of glycerin.
Mixing anywhere from 25-50% glycerin should work, and this way there is a mix of about 16% yeast, 50% wort/beer and 33% glycerin:

Shake the test tubes to uniformly mix the beer, glycerin and yeast:

and then store in the fridge for another day to let all the air-bubbles settle out.

Seal with (florists) parafilm and enclose in a zip-lock bag:

and store in a non-frost-free freezer.
If you do not have a non-frost-free freezer then you'll need to protect the yeast from the defrosting freeze-thaw cycles.

I have found that by mixing in about 25-50% glycerin with the yeast/beer that the samples do not actually freeze solid, so this may also help protect the yeast and prevent the cell walls from rupturing.

To re-culture the frozen yeast samples, remove them from the freezer, remove the parafilm and dip in a sterile inoculation loop into the yeast sediment and inoculate a fresh malt-agar slant.
Since the samples do not freeze solid, they do not need to be thawed and the test tubes can be returned to the freezer as soon as the sample is collected.

Last year I froze 20 different strains of yeast using this method. I recently tested and inoculated each of the frozen samples onto malt-agar slants, and as per the photo above most all of the samples were still viable. Some strains (the two on the left) appear to be very viable and have suffered very little during the freezing process, however with some strains (those two on the right) the viability is noticeably less. I'm not sure if the loss of viability is related to my process and procedures or the yeast strain, while all the samples were stored exactly the same way there was some minor differences in how they were prepared.

Until I am sure the frozen yeast will remain viable for long periods (which might take years from now to test), I still keep 2 malt-agar slants of each strain, and plan to investigate storing the samples under distilled water as another method of long term storage.

 

[Punkal]

There has been so much good yeast info recently... I probably didn't need to buy the yeast book, ill be getting right into this as soon as i get home...

 

[Yob]

plan to investigate storing the samples under distilled water as another method of long term storage.

Have you done this yet Wolfy?

My next build is a stir plate and am keen to know your furtherd results and observations of viability over time.

:icon_cheers:

 

[Wolfy]

Have you done this yet Wolfy?

Nope, got lazy, slants work fine, just need a heap of effort to re-do each year.

 

[Yob]

lol... when are you ready to re-slant? Could be a good time to pop over h34r:

 

[punkin]

Do you have a flowbench or do you do your sterile work in a glovebox?

 

[stillscottish]

Do you have a flowbench or do you do your sterile work in a glovebox?

:blink:

 

[punkin]

Glove box...




Flowbench...