Preserving yeast cultures
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Short term storage
Yeast cultures are stable for 1-2 weeks when refrigerated. Petri dishes should be sealed or in plastic bags.
Medium term storage
Yeast can be preserved for up to a year on YPD agar medium slants in screw cap tubes. Previous versions used mineral oil to overlay the medium, but this is no longer recommended and is messy. Incubate with the cap loose, then after growth seal the tube and refrigerate.
Long term storage
Freeze dried cultures protected with skim milk and sealed in glass ampules works for at least 10-20 years.
More straightforwardly, cultures are grown in normal medium for 28-48 hours, brought to 50% glycerol and frozen in cryotubes. Cultures should be vortexed just prior to freezing to re-suspend the cells. Yeast frozen at -20C will last for several years, but for long term storage, -80C storage is required. Routine culturing is done by scraping small amounts of medium without thawing the entire culture. Cultures can be recovered from a freezer failure by re-culturing and storage again.