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How many times can yeast be reused?

I have read in a couple of places that you should only reuse yeast a couple of times - most say up to 5 times because you would get an infection eventually.

I now wonder if there is any indication for when to throw away the old yeast and get a new vial. I am obviously trying not to reuse until I get a batch that I need to throw away...

Would it be possible to detect this kind defect already in the starter maybe?

  • I prefer to buy fresh yeast most of the time, but look into "washing your yeast". Its a technique that uses a mild acid to remove any rogue bacteria from a yeast slurry without hurting the yeast themselves. – Graham Sep 19 '12 at 12:25
  • If that's not exactly what you're looking for, feel free to edit the question and I can open it back up again. – baka Sep 19 '12 at 13:06
  • This one may answer some questions, as well: homebrew.stackexchange.com/questions/1746/reusing-yeast-at-home – baka Sep 19 '12 at 13:07