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I saved the yeast cake off my last batch, and tomorrow I'll be brewing again. The cake filled a growler, and after a week in the fridge has settled to about 2/3 yeast, 1/3 beer.

After siphoning off the beer, and pitching however much in the new batch, is there anything I need to do to keep the rest of the saved yeast viable, since it won't have the layer of beer on it?

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You can pour sterile, de-oxygenated water on it. De-oxygenated water is just boiled and cooled water.

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  • why does the water need to be deoxygenated?
    – mdma
    Feb 9 '14 at 6:29
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    I had read that deoxygenated water was ideal to keep the yeast dormant for longer term storage. See, for example, this blog post from Brewing Network or this one. However, after some google fu, I see this statement being made several times without any scientific backup for it. In any case, I have been using boiled, cooled water - for example, today I poured off the beer from a pint of slurry I had saved, added some sterile water, mixed up the slurry, pitched half, and added more sterile water to the rest. If this is wrong, I will correct. Feb 10 '14 at 7:54
  • Perhaps it isn't the removal of oxygen in the water that you're after. Perhaps the the sterilizing of any bacteria, or the removal of chlorine if your water is treated. The former of the two would help prevent you from inoculating a contaminant into your next batch, the latter I'd just rather not have come in contact with my yeast.
    – Scott
    Feb 12 '14 at 0:44
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"The cake filled a growler, and after a week in the fridge has settled to about 2/3 yeast, 1/3 beer." That's 2/3 trub. The only way to know how much of it is yeast is to use a microscope. I wouldn't recommend just pitching in a new batch. Since you didn't wash it, it is going to have high amounts of non-yeast trub (proteins, lipids, et al). If you make a starter using some of your trub, once it has propagated; that is formed krausen and begun to settle, you can swish it, let it settle for 10 minutes, decant it into a sanitized container, let that settle until it is trub and beer, decant and pitch the slurry. That will avoid re-pitching some of the non-yeast trub. Ideally, you should do it with a large starter (at least 2 liters) since you will be ditching some of the trub.

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