Author Topic: Today's Stupid Question: What Causes Fermentation to Cease in Beer?  (Read 11537 times)

Offline Cicerone

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I've been brewing mead and beer for some time now, so I'm embarrassed to be asking such a stupid question, but what exactly causes fermentation to cease in beer?

I'm aware that different strains of yeast have different alcohol tolerances, and will die when the alcohol level reaches their tolerance, but my understanding is that most beer yeast strains' tolerances are far above what most beers make it to (10-11% range).

I'm also aware that if the wort were to run out of sugar, that would also cease fermentation.

But neither of those limitations were reached with a recent scotch ale I brewed, that made it to probably 6.5% ABV and was still VERY sweet. Why did fermentation stop, when there was plenty of sugar left to ferment, and the beer hadn't even come close to the alcohol tolerance of the yeast?

Thank you for your help,
Brian

Offline 88Q

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #1 on: February 09, 2011, 08:56:11 AM »
You named the two that I know of.... 1)yeast tolerance to alcohol, and 2) all the sugars being converted.
I also see how mashing at too high a temp MAY leave some complex sugar chains behind, that the yeast can't work on.

Having said that (and there MAY be more) I just did a Cascadian Dark Ale that turned out the same way. I ended up with an OK alcohol level, but VERY sweet. I am contributing that to the 20% roasted malt and carapils I added. I even tried to rouse the yeast in the conical late in the game, no luck.

 It finished some 19 points higher than BS calculated, but everyone likes it. Some have said I need to enter it in a local competition. But Nah...........

I'll drink it up and try again with some changes till it's right.
88Q

Offline dogma46an2

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #2 on: February 09, 2011, 12:00:38 PM »
Did you have a starter?
The yeast could have just been a old batch and not had enough in the brew to get the full Attenuation .
As well like 88Q said very well could have been to high of a mash
The lack of nutrients for your yeast
What was your O.G ?
and what yeast did you use?
As well yes the alcohol could have been a factor damaging the yeast
Normally if I have high gravity brew and a weak strain then i will add Champaign yeast
just to make sure i ferment out all the way ...
also you may not had left it long enough on the yeast did you let it ferment all the way out in the primary?
doing a transfer to soon could have f'd it up by taking the beer off the yeast and not leaving it with enough to continue and via the alcohol and stress , killed off the strain ...
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Offline stevemwazup

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #3 on: February 09, 2011, 12:06:23 PM »
     If your yeast are not happy, they won't ferment.(Fully)
Pitching the right amount of yeast, not to little and to much. Too little and they won't have a big enough army to tackle a large amount of sugar. Too much and they won't be able to grow, to give off the ester profile of the yeast.
Yeast love to have nutrients, Wyeast has a yeast nutrient blend you can add 10min to the boil.
The proper amount of Oxygen. Just shaking the carboy, might not be enough Oxygen for certain levels of gravity. Yeast need Oxygen to grow, without the proper levels they might not be healthy enough throughout the entire process of fermenting.
Temperature, I like to start out a little lower in temperature while the yeast are moving around like crazy, creating heat of their own, once they slow down, I'll gradually increase the temp a little each day, to help keep them awake and finish their job.
Time.Sometimes I'll gently move the carboy back and forth, just to help get the yeast back in the beer, and just wait.
Hope this can help.

stevemwazup.

Offline MikeinWA

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #4 on: February 09, 2011, 09:53:23 PM »
It doesnt sound like Cicerone is having a problem with his final gravity.  I think the question is why the yeast don't go past the final gravity and keep going until all of the sugar is completely gone.  Can the beer still taste malty with all of the sugars digested?

Offline jomebrew

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #5 on: February 10, 2011, 09:34:56 AM »
Generally, yeast continue to do their stuff until all the fermentable sugars are depleted or their time on this earth has expired, whichever comes first.  They start to die in about 3 days and continue up to about 15 or 20 days. As the oxygen is depleted the yeast become anaerobic which is a slow death phase.  They continue to convert sugar and produce ethanol but are slowing running out of energy. Heartier ones can survive in longer alcohol though it is a toxic environment which makes it harder for them to survive and be healthy.  They do this by consuming the nutrients released by dead brethren. Eventually, they succumb too. 

You can search technical papers about the life span of brewery yeast for more technical descriptions searching for "Saccharomyces cerevisiae life span".


Offline dogma46an2

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #6 on: February 10, 2011, 09:54:58 AM »
Jo is right on the money ... I am thinking its either a temp and or a starter issue
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Offline jomebrew

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #7 on: February 10, 2011, 10:52:53 AM »

But neither of those limitations were reached with a recent scotch ale I brewed, that made it to probably 6.5% ABV and was still VERY sweet. Why did fermentation stop, when there was plenty of sugar left to ferment, and the beer hadn't even come close to the alcohol tolerance of the yeast?

Thank you for your help,
Brian

Most likely, the environment was not conducive for the yeast to consume all the fermentable sugars.  While yeast can work in high gravity environments does not mean they will.  Your yeast must be healthy and have been provided the nutrients to build strong cell walls.  There must be an abundant supply of oxygen.  There must be an abundant number of cells.  They must not be stressed.

Your under attenuated beer is likely a combination of all of these.  Under pitched cell count for the gravity, not enough oxygen and not enough yeast nutrients.  Additionally, yest perform well in a constant temperature.  If you do not have temp control and think it was between 68-70 all the time, it is likely the temp had a bigger swing than that.  A 2F swing can stress the yeast causing then to die off early.  Of course this is all a guess since I don't have all the details of your brew and fermentation process.


- Joe


Offline 88Q

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #8 on: February 10, 2011, 11:42:33 AM »
Quote
I just did a Cascadian Dark Ale that turned out the same way. I ended up with an OK alcohol level, but VERY sweet. I am contributing that to the 20% roasted malt and carapils I added. I even tried to rouse the yeast in the conical late in the game, no luck.

 It finished some 19 points higher than BS calculated

Reviewing my notes on that cascadian dark ale, I recalled that the fermenter fell to 22deg F during fermentation. My wife accidently turned the heat off in the BrewHouse.
 I tried to rouse the yeast (stirring/O2) but never got another bubble.

So I definitely swung more than 2deg F!!

Oh well, still tastes pretty good!
88Q

Offline Cicerone

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #9 on: February 10, 2011, 01:37:10 PM »
Thanks guys, that was very informative and helpful.

I think my problem was swings in temperature and other stressors for that batch; for reasons I'll spare you all, I was forced to drive 1000 miles with the wort in primary, sitting in my front seat.

For higher alcohol brews, is it useful to leave the wort in primary for longer than six days, or will this just lead to an unwanted yeasty flavor without any additional fermentation?

Offline MikeinWA

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #10 on: February 10, 2011, 04:38:06 PM »
So, yeast can die out when the oxygen goes so  the less O2 in the primary to start, especially in high gravity beers, the sooner the yeast will die out and not ferment all the sugars.

I don't think you can get unwanted yeasty flavor as long as the yeasty flavor is the yeast you added.  I typically transfer to secondary after the bubbles are more than a minute apart.  I don't know about the people who don't use a secondary fermentor.

Offline mhenry41h

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #11 on: February 11, 2011, 08:50:56 AM »
Thanks guys, that was very informative and helpful.

I think my problem was swings in temperature and other stressors for that batch; for reasons I'll spare you all, I was forced to drive 1000 miles with the wort in primary, sitting in my front seat.

For higher alcohol brews, is it useful to leave the wort in primary for longer than six days, or will this just lead to an unwanted yeasty flavor without any additional fermentation?

   My limited experience tells me that if your beer is only on the primary for 6 days or less, then you probably didn't leave it long enough.  My experiences are telling me that anything short of 14 days isn't long enough.  I have a chocolate porter that today has been on primary for 19 days and still isn't quite ready.  Im going to run this thing all of the way until next Weds or 24 days before I bottle it.

Offline econolinevan

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #12 on: February 26, 2011, 06:24:47 PM »
I use  primary and secondary ferms.  I don't let it stay in primary past it's welcome.  Now that my wife gifted me a refractometer, I can closely monitor the process(without the large samples).  When gravity ceases to drop, it's time for the secondary, where depending on style of beer, I will keep it there for a week to several weeks before bottling. 

Offline Ziggybrew

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Re: Today's Stupid Question: What Causes Fermentation to Cease in Beer?
« Reply #13 on: February 27, 2011, 05:56:41 AM »
As a recent convert to "seeing the light" with yeast.... I say get the book, YEAST, by Chris White and Jamil Zainasheff. It will open your eyes and explain everything, in detail, maybe more than you'd ever expect, and then more. There is BS and then there is truth from an experienced scientist.