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I have heard yeast can mutate when you re-use yeast, and when it goes through many yeast-generations.

How long does it take for the yeast to mutate?

Which factors have an impact on the characteristics of the yeast?

Which characteristics will this mutated yeast have?

Is the yeast stable and healthy after being mutated?

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  • An alternate strategy would be to freeze several aliquots from the first generation and then thaw out one at a time to make new starters. The terms "stable" and "healthy" are not well-defined terms. They are by definition at least as healthy as the original yeast since they are survivors of whatever conditions you have arranged for them.
    – 42-
    Jun 8, 2019 at 22:33

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Yeast follow the laws of natural selection.

As a cell is budding the new mass may have some minor defects / mutations. If those changes give the yeast an advantage to survive it will get passed on.

You can have noted changes in just a few generations.

I've observed in my own washing methods strains becoming less flocculant just because my washing method discards the first yeast cells to drop out.

Also many breweries refresh yeast after a few generations or brews. To put it simply: Not that it's unhealthy yeast but rather they've grown a generations of spoiled brats that have gotten lazy and simply just don't make good beer anymore.

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  • Hehe, good one! Lazy brats :P ... That's a good answer, thank you :) So flocculation is one characteristic There are probably many other characteristics as well?
    – Rouse
    Aug 20, 2018 at 19:15
  • I imagine alcohol tolerance would be one characteristic that would increase on subsequent passages if you were making stronger beers.
    – 42-
    Jun 8, 2019 at 22:30
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There is no set time and no set form that the mutation will take. But in general, mutations take a long time to happen. Years is the time line often cited.

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    Perhaps this may be better applied to "stable" mutations. Mutations generally happen on replication so "unstable" mutations can happen at almost any time. It seems mitochondrial mutation ("petite variants") happen more frequently than in nuclear DNA replication.
    – barking.pete
    Aug 23, 2018 at 13:11
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From "Methods of modern homebrewing, Chris Colby"

In reality, given the increasing probability of contamination with each repitch and the presence of new mutations, five to seven generations is a safe rule of thumb. That said, some commercial brewers have strains they've repitched for years.

This guy is a biologist, so I assume he knows his stuff.

I've never reused up to the fourth generation, because I do not brew very often, and the third or fourth generation or so would be almost 10 months old.

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