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General Category => Yeast and Fermentation => Topic started by: guvna on November 11, 2009, 06:20:09 PM

Title: Rousing Yeast
Post by: guvna on November 11, 2009, 06:20:09 PM
If anyone's unfamiliar with the concept, here's a nice piece on it: http://www.brews-bros.com/index.php/topic/921-rousing-yeast-aka-swirling/ (http://www.brews-bros.com/index.php/topic/921-rousing-yeast-aka-swirling/)

My questions:

Even if we don't have a stuck fermentation, would rousing the yeast be a good thing? Other than keeping the yeast in suspension, would there be a benefit from driving off CO2 or sulfur components?

Also, even if there's still an active fermentation -- meaning there are frequent bubbles in the air-lock -- do we even need to worry about oxidation?
Title: Re: Rousing Yeast
Post by: bonjour on November 11, 2009, 06:37:28 PM
Rousing yeast is a great technique for keeping active yeast suspended in the wort as you are nearing the end of the fermentation of really big beers.  And No, as long as you are producing CO2 you shouldn't have to worry about oxidation.

Fred
Title: Re: Rousing Yeast
Post by: tony on November 11, 2009, 06:48:42 PM
But why bother rousing yeast if the fermentation is chugging along, unless you've selected
a strain that's unattentuative and a great flocculater?
Usually there's enough yeast in suspension to keep things going to finish.
Title: Re: Rousing Yeast
Post by: bonjour on November 11, 2009, 06:53:51 PM
But why bother rousing yeast if the fermentation is chugging along, unless you've selected
a strain that's unattentuative and a great flocculater?
Usually there's enough yeast in suspension to keep things going to finish.
When you are brewing really big beers, they all are

Fred
Title: Re: Rousing Yeast
Post by: guvna on November 11, 2009, 08:12:39 PM
But why bother rousing yeast if the fermentation is chugging along, unless you've selected a strain that's unattentuative and a great flocculater?
Usually there's enough yeast in suspension to keep things going to finish.

That was part of one of my questions. Is rousing yeast beneficial in other ways, i.e. does it help release CO2 and H2S from solution? One would seem beneficial for yeast health, the other for your beer's final flavor profile.
Title: Re: Rousing Yeast
Post by: guvna on November 11, 2009, 08:23:12 PM
I just found this little ditty: http://www.byo.com/stories/techniques/article/indices/8-aging/116-aging-gracefully (http://www.byo.com/stories/techniques/article/indices/8-aging/116-aging-gracefully)

Quote
Airing Out the Difficulties
One by-product of yeast is hydrogen sulfide (H2S). Hydrogen sulfide produces a strong rotten-egg smell and is sometimes noticed at the beginning of fermentation. It can be carried over into the finished beer if not allowed to vent from the fermenter. The CO2 produced during fermentation will scrub hydrogen sulfide from the wort, and open fermentation or the use of an airlocked carboy will allow its escape.

I heard that this was something utilized often in wine-making, and it seems to carry over into brewing. It really depends, I guess, just how much remains in solution even with the airlock, but rousing should definitely help remove some of those volatile compounds.
Title: Re: Rousing Yeast
Post by: bonjour on November 11, 2009, 09:22:32 PM
CO2 is a detriment to fermentation and it is toxic to yeast, so yes, getting excess CO2 out the fermenting wort is a good thing.
Title: Re: Rousing Yeast
Post by: bluesman on November 11, 2009, 10:11:16 PM
CO2 is a detriment to fermentation and it is toxic to yeast, so yes, getting excess CO2 out the fermenting wort is a good thing.

+1

I usually rouse the yeast on bigger beers near the end of the fermentation to keep it moving until the very last fermentable sugar baby is swallowed up and converted into goodness.... 8)
Title: Re: Rousing Yeast
Post by: homebrewdad on November 12, 2009, 09:33:00 PM
Wouldn't it be easier to rouse yeast just by screaming, "Hello, Yeast!"? :D
Title: Re: Rousing Yeast
Post by: The Professor on November 21, 2009, 07:30:15 PM
I rouse the yeast at least once a day throughout the primary ferment...the biggest benefit of doing so is probably not realized until near the end of the primary period, but doing do from the start certainly doesn't hurt. 
Title: Re: Rousing Yeast
Post by: denny on November 21, 2009, 07:58:34 PM
Wouldn't it be easier to rouse yeast just by screaming, "Hello, Yeast!"? :D

That would be my preferred method!
Title: Re: Rousing Yeast
Post by: bonjour on November 21, 2009, 08:29:55 PM
Wouldn't it be easier to rouse yeast just by screaming, "Hello, Yeast!"? :D
That method rarely worked with my kids
Title: Re: Rousing Yeast
Post by: dhacker on November 22, 2009, 02:05:26 PM
I do similar to the professor . . maybe not everyday, but often enough throughout the primary. I liken it to what happens when you make a starter with a stir plate . . . A better population of healthy yeast is the result. How could that be a detriment?  ???

Plus, it's good exercise!   :D
Title: Re: Rousing Yeast
Post by: ndcube on November 23, 2009, 12:36:35 PM
I do similar to the professor . . maybe not everyday, but often enough throughout the primary. I liken it to what happens when you make a starter with a stir plate . . . A better population of healthy yeast is the result. How could that be a detriment?  ???

Plus, it's good exercise!   :D

Isn't the purpose of the stire plate to make more oxygen available to the yeast?  Just curious because I don't use one at the moment.
Title: Re: Rousing Yeast
Post by: babalu87 on November 23, 2009, 01:38:02 PM
Wouldn't it be easier to rouse yeast just by screaming, "Hello, Yeast!"? :D
That method rarely worked with my kids

My kids listen better when I whisper  ;D

[whispermode] hey, yeast you gots more sugars to eat according to the fast-ferment test data..[/whispermode]
Title: Re: Rousing Yeast
Post by: ndcube on November 23, 2009, 02:51:22 PM
Wouldn't it be easier to rouse yeast just by screaming, "Hello, Yeast!"? :D
That method rarely worked with my kids

My kids listen better when I whisper  ;D

[whispermode] hey, yeast you gots more sugars to eat according to the fast-ferment test data..[/whispermode]


Glad my hearing's pretty good.  I almost didn't hear that. ;D  Reminds me I want to try it for the first time today!
Title: Re: Rousing Yeast
Post by: bonjour on November 23, 2009, 06:08:12 PM
Isn't the purpose of the stir plate to make more oxygen available to the yeast?  Just curious because I don't use one at the moment.
The stir plate performs several functions,  Keeps CO2 out, Adds O2, and keeps the yeast in suspension.

Fred
Title: Re: Rousing Yeast
Post by: dhacker on November 24, 2009, 08:33:00 PM
The stir plate performs several functions,  Keeps CO2 out, Adds O2, and keeps the yeast in suspension.

Fred

I wish someone would explain how a stir plate adds oxygen. I've heard this for years, but never could figure out how it's possible. If you mean making the already dissolved oxygen easier for the yeast to get at hence greater reproduction, yeah . . OK. But adding O2 as if pulling it into the flask from the outside world??  ???   Not convinced!
Title: Re: Rousing Yeast
Post by: a10t2 on November 25, 2009, 12:18:50 AM
I wish someone would explain how a stir plate adds oxygen. I've heard this for years, but never could figure out how it's possible. If you mean making the already dissolved oxygen easier for the yeast to get at hence greater reproduction, yeah . . OK. But adding O2 as if pulling it into the flask from the outside world??  ???   Not convinced!

It's because the yeast scavenge O2 that's dissolved in the wort. Leaving the flask open to the air keeps O2 going into solution to replace what they're using. So they effectively have an unlimited supply of oxygen.
Title: Re: Rousing Yeast
Post by: dhacker on November 25, 2009, 12:29:18 AM
I figured that was the reasoning. But is a vacuum actually created to draw in O2? How?

 If CO2 is heavier than O2, how would the oxygen have any chance of displacing the CO2 from above? I'm just sayin', the physics seem cloudy.
Title: Re: Rousing Yeast
Post by: bonjour on November 25, 2009, 02:09:01 AM
the top of a flask is not sealed with a bubbler, the volume of CO2 produced is low when compared to a beer.  thus the air will try to equalize with the CO2 rich air in the flask,  The stirring will allow the liquid to to maximize it's absorbtion.  You don't need much

Fred
Title: Re: Rousing Yeast
Post by: dhacker on November 25, 2009, 03:45:48 AM
the top of a flask is not sealed with a bubbler, the volume of CO2 produced is low when compared to a beer.  thus the air will try to equalize with the CO2 rich air in the flask,  The stirring will allow the liquid to to maximize it's absorbtion.  You don't need much

Fred

I know a lot of folks swear by their stir plates, I'm just having a hard time wrapping my head around the idea of air/ CO2 equalization ???, and that the stir plate is responsible for drawing in such minuscule amounts of oxygen and propagating the yeast cells so well as a result. As much as we harp on blasting the heck out of our fermenters with pure O2, seems odd a stir plate could do so much with so little!  :)
Title: Re: Rousing Yeast
Post by: dhacker on November 25, 2009, 01:37:42 PM
Someone sent this my way . . Thank goodness I was able to learn something today!  :D

http://www.chm.davidson.edu/vce/gaslaws/DaltonsLaw.html (http://www.chm.davidson.edu/vce/gaslaws/DaltonsLaw.html)

Title: Re: Rousing Yeast
Post by: tom on November 25, 2009, 10:28:14 PM
Yes, the Law of Partial Pressure for gases. Each kind of gas within a mixture will equalize its concentration/pressure between the 2 spaces. Thus, as the yeast use O2 inside the flask its concentration (and thus partial pressure) will decrease. Because the O2 partial pressure is less than outside the flask, oxygen will diffuse into the flask. This happens independent of other gases' partial pressure.