Hmmm...while I would defer to many here with more scientific knowledge on the subject than I, I know that yeast (in any form-dry or wet) can only survive within a specific temperature range. I make bread a lot and carefully store my dry yeast sealed within a heavy ziploc (with as much air removed as possible) inside the freezer. I have successfully stored yeast for a year in my deep freeze, wrapped in freezer paper and similarly sealed within a ziplock. (Am actually using that batch now...)
I always portion out the amount of yeast I need at the time, return the remainder to the frezer, and allow the portioned amount to come to room temperature before using.
I also know that yeast, a living organism, has a finite lifespan. The longer it is stored, by any method, the more of it ages and dies. The stuff becomes less and less active. I use yeast so much and so often that my deep-frozen "backup supply" is usually not stored more that 3 or 4 months.
A colleague considers himself as a microbrew expert, having made very good ale for several years. He bought a vacuum sealer to store his portioned amounts of yeast, which he keeps in a tupperware container inside his refridgerator. He swears by the method and, as I said, his ale is good -- really good.
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