Culturing yeast cakes...

I've read that it's possible to reuse the yeast at the bottom of the fermenter. How would I go about doing this?

There is a Stickie over in Fermentation that addresses it.

https://www.homebrewtalk.com/f163/yeast-washing-illustrated-41768/

Yeast on my friend

Probably should've looked first.

How can I be sure that I did everything correctly? I don't want to screw something up and ruin an entire batch by pitching bad yeast.

Make a starter out of the washed yeast once you are ready to brew

NOISEpollution said:

Probably should've looked first.

How can I be sure that I did everything correctly? I don't want to screw something up and ruin an entire batch by pitching bad yeast.

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So the truth is, you can never be absolutely sure. But you can do some common sense things. Cacaman suggested making a starter, which is a great idea. I actually taste a little of my repitch starters just to make sure i don't taste or smell anything funky going on. Don't re-pitch if you have reason to believe the fermentation the yeast came from was suspect.

The downside of repitching is that you're also propagating any bacteria or wild yeast that made it into your first batch, but if you're careful and practice good sanitation, there's no reason you can't successfully repitch for multiple generations.

One thing to keep in mind, as you continue to repitch yeast, getting the yeast adequate oxygen is more important than on the initial pitch. When you buy your yeast, they're all stocked up on their sterols (which are necessary for reproduction and good healthy cell walls), but that can be depleted during fermentation. If you don't give repitched yeast enough O2 to rebuild their sterol supplies, you can end up with problems.

discnjh said:

If you don't give repitched yeast enough O2 to rebuild their sterol supplies, you can end up with problems.

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And how would I go about doing that?

NOISEpollution said:

And how would I go about doing that?

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1) make a starter that you shake up and swirl around a lot. doing it on a stir plate is ideal, but i don't have one, I just make a point of regularly shaking the starter.

2) Use an O2 tank to inject pure O2 to your wort prior to pitching yeast. Or use an air pump to inject air. Or shake the living crap out of your carboy. Those are listed in their order of effectiveness in getting O2 into your wort. My understanding is that you can't actually get the optimal O2 levels in wort with air alone, O2 injection is required. That said, many people get good results without O2.