What is an ideal environment for saving yeast I have harvested from my fermentation vessels?
The lowest survivable temperature? Highest?
I recently found the top of my fridge to be too cold when I couldn't revive yeast that had probably neared freezing.
From White Labs Yeast Storage and Maintenance:
As yeast sit in storage, they consume their glycogen reserves. Glycogen deprivation weakens their cell walls, and makes them more susceptible to rupture. Cold temperatures retard this process, but you want to avoid freezing yeast, as ice crystals will also rupture cells. The ideal storage temperatures range is between 33-38 F. When yeast rupture, they release their contents into the liquid phase. Bacteria can feed off the nitrogen released, and multiply rapidly. So the yeast slurry needs to be as contamination free as possible when stored. Cold temperatures will also help retard bacterial growth.
It sounds like you're ok as long as it didn't actually reach freezing. Do you have a thermometer in your fridge to take the temperature? If your fridge isn't below 33 and you can't revive it, something else is wrong. How old was the yeast and how often did you feed it?
Store the yeast at normal fridge temps, but keep it away from the top shelf where the coldest air comes out of the vents (although its not the coldest part of the fridge). The bottom of the fridge is the coldest spot and stuff is less likely to freezer there.
Secondly, the optimum storage medium in the fridge is water. After you take a sample of yeast cake you should dilute it with water (boiled and cooled water). Then shake it up to help break up chunks. Heavy clumps of protein and hop debris will settle out first. You can then pour off the dark milk like yeast layer. Let it settle over night. Then pour off the liquid and replace with an inch or two of water. This greatly reduces any residual nutrients inthe container. Yeast will slowly try and consume nutrients in the medium if possible even at cold temps. Ultimately, they yeast stress themselves out too much doing so and it becomes difficult to revive them because the viability drops.
I've store yeast like this for 3-4 weeks with good luck. Stepping in the back up in at least a two step starter is good after prolonged storage. If less than a week you can pitch the slurry directly.
