[3wood]

I have never re-used a yeast cake.  So I made a pumpkin ale a few weeks ago and was not ready to make another batch yet so I poured the hops and yeast cake into a food saver bag, sealed it and set it in the freezer.  I plan on racking and bottling the pumpkin ale next wednesday and possibly making a christmas ale with the left over yeast in the secondary and thawing out the previously frozen cake.  Will this work?  Or should I just start from scratch again?

thanks

[a10t2]

Freezing will have killed >99% of the cells.

[bo]

Freezing will have killed >99% of the cells.

I'm not so sure of this. I'd give it a try..

[jaybeerman]

Freezing will have killed >99% of the cells.

I'm not so sure of this. I'd give it a try..

umm, no.  this is what's meant by yeast freezing http://www.homebrewtalk.com/wiki/index.php/Freezing_yeast
(didn't read the page and assume that it covers the basic idea)

[The Professor]

Freezing will have killed >99% of the cells.

I'm not so sure of this. I'd give it a try..

Me too. 
25 or 30 years ago I was given some  Budweiser yeast. which was kept frozen...frozen solid for several months in fact....and it performed quite impressively when I got around to using it.   Maybe some was killed off, but there sure was enough left  to get a good ferment going!

[jaybeerman]

Freezing will have killed >99% of the cells.

I'm not so sure of this. I'd give it a try..

Me too. 
25 or 30 years ago I was given some  Budweiser yeast. which was kept frozen...frozen solid for several months in fact....and it performed quite impressively when I got around to using it.   Maybe some was killed off, but there sure was enough left  to get a good ferment going!

ale vs lager?

[tschmidlin]

Freezing yeast without some form of cryoprotectant leads to massive die off.  That said, when you have trillions of cells even if you kill off 99%+ you can have billions of cells left to make a starter and brew with.  Treating the yeast in different ways can help them survive freezing better.

For example, if you refrigerate the yeast for a few days and then move it directly into the freezer you will have better survival than if it was warm and put in the freezer.  When they are cold for a while they anticipate freezing temps and will create the sugar trehalose which is protective.  Trehalose is broken down within minutes of warming, so you really need to go straight from fridge to freezer.  If you add glycerol as a cryoprotectant then you get even better survival.

Slow freezing is better than fast freezing.  Part of the yeast's protective mechanism is to eject cell contents.  This may be so that when the internal water expands with freezing they are not automatically exploding.  It may also cause internal freezing point depression and smaller ice crystals due to the reduction in water content. Their ability to survive freezing in this way is highly dependent on the strength of their aquaporin genes, which will let them push water out of the cell more easily and then rehydrate during thawing.  I have not found any viability difference between yeast that is slowly thawed on ice vs. room temp vs. quickly thawed in a heat block.

Bottom line - everyone is right

You probably got massive die off, but might still have enough cells to make beer with.  I would make some serial starters and try to get rid of dead cells.

[phillamb168]

Freezing yeast without some form of cryoprotectant leads to massive die off.  That said, when you have trillions of cells even if you kill off 99%+ you can have billions of cells left to make a starter and brew with.  Treating the yeast in different ways can help them survive freezing better.

For example, if you refrigerate the yeast for a few days and then move it directly into the freezer you will have better survival than if it was warm and put in the freezer.  When they are cold for a while they anticipate freezing temps and will create the sugar trehalose which is protective.  Trehalose is broken down within minutes of warming, so you really need to go straight from fridge to freezer.  If you add glycerol as a cryoprotectant then you get even better survival.

Slow freezing is better than fast freezing.  Part of the yeast's protective mechanism is to eject cell contents.  This may be so that when the internal water expands with freezing they are not automatically exploding.  It may also cause internal freezing point depression and smaller ice crystals due to the reduction in water content. Their ability to survive freezing in this way is highly dependent on the strength of their aquaporin genes, which will let them push water out of the cell more easily and then rehydrate during thawing.  I have not found any viability difference between yeast that is slowly thawed on ice vs. room temp vs. quickly thawed in a heat block.

Bottom line - everyone is right

You probably got massive die off, but might still have enough cells to make beer with.  I would make some serial starters and try to get rid of dead cells.

Science!!!

[euge]

That was a very cogent, clear and comprehend-able explanation by Dr Schmidlin there. I will be freezing more yeast in the future.

[tschmidlin]

Honestly euge, I would recommend against freezing your yeast.  Labs use a portion of glycerol as a cryoprotectant, in my case I use 20% in the mix.  But I also freeze at -80C, and your fridge/freezer is more like -20C.  The yeast will degrade some even at that temperature.

A better bet to avoid die off is to keep it from freezing in the freezer.  If you use ~40% glycerol by volume the freezing point should be depressed enough that the solution will remain a liquid in your freezer.  This is probably the homebrewer's best option for long term storage of yeast.  It gets unwieldy (and expensive) for storing cakes, but for storing small samples of cultures to grow up later it will work well.

[weithman5]

where do you get the glycerol?

i have always just kept the slurry in a clean jar in the fridge for months. then build up a new starter.  the other thing i have done is when i get a new pack of yeast i pour it into a few sterile specimen cups and keep those refrigerated and build a starter from these.

[tschmidlin]

glycerol = glycerine

Most pharmacies will have it in the make up section, or you can get it from soap making places.

http://www.cvs.com/CVSApp/catalog/shop_product_detail.jsp?skuId=483707&productId=483707
http://www.bulkapothecary.com/glycerin.htm?gclid=CPSI1M_OnawCFUkZQgodBVG8CQ

If you buy in quantity, I would portion it out into small bottles and pressure can them for sterilization.  Or at least pasteurize it before using.  Or at an absolute minimum, plate your culture when you use it and pick singles.

To do this for real, I would recommend 15 ml falcon tubes, then fill each with 5 ml of glycerol and sterilize in batches.  When you want to store a culture, add 5 mls of slurry, shake well, and put it in the freezer.

For the freezer, I usually recommend putting them in a small (6-pack) cooler to minimize temperature swings for the yeast, plus it helps keep them in one place.

[euge]

Good advice! I want to be able to stockpile various strains that might not get used for a good while. I guess it doesn't matter if the glycerin is food-grade or not?

[tschmidlin]

As long as it is 100% glycerin it should be fine.  I'm not sure what food grade means in this context, since it's not something you normally eat.

[tubercle]

If you buy in quantity, I would portion it out into small bottles and pressure can them for sterilization.

 Would a pint jar in a pressure cooker be safe. I've never dealt with glycerol. By this, I mean it doesn't have any weird expansion characteristics or something?

 I always thought it would be cool (pun intended) to recover some yeast about a mile deep from the Antartica ice pack to brew with. Paleo-yeast.