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After forgetting to place the yeast starter in the fridge overnight I placed it in the freezer this morning for a short while to do a quick cool crash. I misjudged the time and the water above the yeast cake has begun turning into a frozen slurry - though it is not completely frozen. I had a 1.1L starter in the freezer at -17ºC for roughly 1 and 3/4 hours.

I've read on other questions that if frozen it will rupture cell walls and viability will have been seriously affected. Given that this did not completely freeze over am I OK to brew? At what point during freezing is yeast ruined?

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Yeast can survive freezing. You will have to bring the yeast back to room temperature without over heating them, so naturally let them warm up.

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    Sure? Don't the yeast cells burst, when the freeze? Because of the ice-crystals forming inside the cell.
    – Oliver Hörold
    Dec 4, 2018 at 18:35
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    It can really depend on the situation so you should still look to re-pitch but the main option is to slowly thaw. There could be an extreme freeze that takes place, then there would be cases where they would ice-crystal burst, but not all of the yeast will be affected. To the OP's questions at an extreme case of freezing, you can re-use the yeast, but keep a re-pitch open if necessary. This seems accidental so there should not be any problems. (dehydrated yeast will survive, yeast that is used for brewing is not dehydrated and can burst with freeze crystals, but can still survive).
    – Robbye Rob
    Dec 5, 2018 at 16:59
  • Thanks @Robbye Rob ! So next time I don't have to throw away the yeast when I accidentally freeze it! :)
    – Oliver Hörold
    Dec 6, 2018 at 7:37
  • When in doubt, always make a starter. This will tell you if the yeast is still alive before pitching anything into our precious wort :)
    – Philippe
    Dec 7, 2018 at 13:45
  • I disagree with this idea. Some of the yeast in a slurry will certainly survive. And you can make a starter. But you will still have a significant amount of dead or dying yeast going in to the beer. Freezing yeast just on its own is not a normally good practice for these reasons. Taking some of it and doing a series of stepped up starters might make more sense so you aren't tossing a huge slug of dead yeast cells in the beer.
    – brewchez
    Dec 9, 2018 at 18:04

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