After having plated and selected yeast colonies on solid media, you need to preserve them in time.
You might store yeast on Petri dishes or slants in the refrigerator, but the shelf life of these media is limited, and after 3-4 month you need to reculture the yeast.
I was looking for a more permanent storage method, when Joseph Kish on the AHA Forum brought to my attention an article by M. D. Graham on the sourly missed Brewing Techniques magazine.
The article was about an underestimated method of preservation of yeasts and other fungi: distilled water.
Though skeptical at first (the words reverse osmosis shock do come to mind) I decided to give it a try. Since then I have adopted it as my standard long term storage method.
It is extremely simple. First you fill a culture tube with distilled water and autoclave it to ensure absolute sterility.
Then you transfer one or more colonies of yeast from a plate to the tube, using solid to liquid sterile transfer technique.
The amount of yeast carried over must be minimal: the suspension inside the tube should become only slightly blurred. If it gets milky white, then there is way too much yeast in suspension.
When you need to use the yeast, plate a sample on Petri dishes, select then proceed to make a starter.
Authors suggest that you can store this master culture at room temperature, but I prefer to keep it refrigerated. After 6 months, I counted a percentage of viable cells around 70% on all my strains, with no apparent sign of mutation.
References
(1) M.D.Graham, "A Simple,Practical Method for Long Term Storage
of Yeast", Brewing Techniques 5, March/April (1997), pp 58-62
(2) S.Castellani,"Viability of Some Pathogenic Fungi in Distilled
Water", Journal of Tropical Medicine and Hygiene 42,
pp 225-226 (1939)
(3) M.R.McGinnis,A.A.Padhye,and L. Ajello,"Storage of Stock
Cultures of Filamentous Fungi,Yeasts, and Some Aerobic
Actinomycetes in Sterile Distilled Water", Applied
Microbiology 28, pp 218-222 (1974)
(4) F.C.Odds, "Long Term Laboratory Preservation of Pathogenic
Yeast in Water",Journal of Medical and Veterinary Mycology 29,
pp 413-415(1991)