Author Topic: Rehydrating dry yeast  (Read 756 times)

Online denny

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Rehydrating dry yeast
« on: December 14, 2017, 09:15:26 PM »
Yeah, I know...it's been debated to death.  But I'm not interested in whether or not to rehydrate.  I'm curious if anyone knows the science behind it.  We're told that rehydrating means that you're pitching more viable cells than if you don't.  But why?  What's going on to increase the viability?
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Offline SiameseMoose

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Re: Rehydrating dry yeast
« Reply #1 on: December 14, 2017, 09:34:30 PM »
The story I was was told relates to osmotic pressure. For the cell to achieve an active metabolic state it has to pull a lot of water into itself to restart its cellular functions. In a solution full of sugar, the sugar tends to pull water into the solution, out of the cells. It is therefore harder for the cell to draw water in. Many cells do not survive the hydration step, and it is assumed (maybe been measured) that more cells die when trying to rehydrate in wort.

Of course, when you pitch a rehydrated cell into wort, the osmotic shock will kill some number of cells. Don't ask me which is the healthiest way for the poor yeast cell.
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Online denny

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Re: Rehydrating dry yeast
« Reply #2 on: December 14, 2017, 10:41:38 PM »
The story I was was told relates to osmotic pressure. For the cell to achieve an active metabolic state it has to pull a lot of water into itself to restart its cellular functions. In a solution full of sugar, the sugar tends to pull water into the solution, out of the cells. It is therefore harder for the cell to draw water in. Many cells do not survive the hydration step, and it is assumed (maybe been measured) that more cells die when trying to rehydrate in wort.

Of course, when you pitch a rehydrated cell into wort, the osmotic shock will kill some number of cells. Don't ask me which is the healthiest way for the poor yeast cell.

Thanks, Rob!  Makes sense to me.
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Re: Rehydrating dry yeast
« Reply #3 on: December 15, 2017, 12:42:50 AM »
“As we learned, yeast do not have the ability to regulate what passes through their cell walls as they come back to life. The same holds true when yeast are introduced directly to wort. Sugars and other compounds are able to pass into the yeast cell, killing it. For this reason, direct pitching is not an ideal situation. A loss of 30% in viable yeast cells, possibly more, could be the result.” ...”it needs to be hydrated in tap water with some hardness. None of this bottled/filtered crap. Just good, old fashioned, American tap water. Distilled or filtered water will decrease cell viability. In high gravity beers or lagers, rehydrating at the proper temperature and slowly cooling (to avoid temperature shock) is the best way to get viability, and these types of beers are where it really matters.” Homebrew Digest 17 Apr 2000 http://www.hbd.org/hbd/archive/3301.html#3301-4


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Offline mabrungard

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Re: Rehydrating dry yeast
« Reply #4 on: December 15, 2017, 03:13:42 AM »
As mentioned in one of my recent forum posts, magnesium is the preferred cation for yeast rehydration. Epsom salt in distilled or RO water is a good medium. I don't recall the dose of epsom salt, but it is substantial. It was something in the 100 ppm range, so much higher than we would use in brewing water.
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Offline stmf

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Re: Rehydrating dry yeast
« Reply #5 on: December 15, 2017, 09:49:45 AM »
Yeah, I know...it's been debated to death.  But I'm not interested in whether or not to rehydrate.  I'm curious if anyone knows the science behind it.  We're told that rehydrating means that you're pitching more viable cells than if you don't.  But why?  What's going on to increase the viability?

I found this a while back.
https://koehlerbeer.wordpress.com/2008/06/07/rehydrating-dry-yeast-with-dr-clayton-cone/

Very good read and what was a litte news to me is how much the temperature of the rehydration fluid makes.

Offline mabrungard

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Re: Rehydrating dry yeast
« Reply #6 on: December 15, 2017, 01:15:50 PM »
It's interesting that Clayton Cone found that slightly elevated water temp produced better viability when there are other studies that found the opposite. I err on the safe side and rehydrate at a temperature that is closer to room temp.
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Offline hopfenundmalz

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Re: Rehydrating dry yeast
« Reply #7 on: December 15, 2017, 02:40:55 PM »
It's interesting that Clayton Cone found that slightly elevated water temp produced better viability when there are other studies that found the opposite. I err on the safe side and rehydrate at a temperature that is closer to room temp.

It depends on the yeast. If you poke around manufacturer web sites you can find info.

Fermentis has lager rehydration temps at 70-77F, Ale rehydration at 77-84F.

I just looked up Lallemand 71B, that is at 40C=104F. Dr. Cone’s area was wine yeast.
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Offline Robert

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Re: Rehydrating dry yeast
« Reply #8 on: December 15, 2017, 03:01:38 PM »
As mentioned in one of my recent forum posts, magnesium is the preferred cation for yeast rehydration. Epsom salt in distilled or RO water is a good medium. I don't recall the dose of epsom salt, but it is substantial. It was something in the 100 ppm range, so much higher than we would use in brewing water.

Any connection with the recommendation I've seen to use hard water for yeast rinsing?
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Offline mabrungard

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Re: Rehydrating dry yeast
« Reply #9 on: December 15, 2017, 04:46:55 PM »
Any connection with the recommendation I've seen to use hard water for yeast rinsing?

I believe there is, but the research I saw showed that calcium isn't really beneficial and magnesium is.
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Offline Robert

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Re: Rehydrating dry yeast
« Reply #10 on: December 16, 2017, 05:13:30 PM »
Any connection with the recommendation I've seen to use hard water for yeast rinsing?

I believe there is, but the research I saw showed that calcium isn't really beneficial and magnesium is.
Any idea what the beneficial level is? Next time I might try DI water and Epsom salt and see if I notice any difference.
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