Many HBDers like to store yeast slurries. Storage under finished beer or deionized water at refrigeration temperatures is common. Some homebrewers reuse these stored slurries after as much as 60 days of refrigeration ! I have very grave concerns about the vitality and viability after such long storage and think that 30 days storage is pushing the envelope.

Yeast viability drop during storage and measurement methods To make a long story short, last year I did some experiments and viability drop tests with the methylene staining procedure and I posted the results to this list for comments some time ago, and come up with to my limited understanding some conclusions but I really would like your more authoritative opinion on this.