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Old 07-13-2009, 05:32 PM   #1
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Default Was I correct to re-pitch?

I've got an American IPA going right now. It sat in primary for about 10 days and had been in secondary for about 10 days when I decided to go ahead and bottle it. I didn't take a gravity reading until I got the beer into the bottling bucket on top of the priming sugar. It was only then that I realized my gravity was currently at 1.020 (OG was 1.060). So, I figured I'd give it more time to come down to 1.015 at least, if not 1.012 or 1.010 (yes, I know that will be dry, but I like it that way ).

I was thinking that maybe I had a stuck fermentation, so I went ahead and pitched a full pack of SafAle US-05 (the yeast I used before, too) back in with the beer in secondary. That was almost 2 days ago, and it's been bubbling away pretty good in the airlock since then.

1. Like I said, I won't mind if my FG gets down to 1.010, but is it possible for the new yeast to take it too low?
2. What would I look for, or should I just take a gravity reading this afternoon?
3. If it IS where I want it to be and still bubbling/fermenting, what then?
4. Would I add a yeast killer, let the yeast die and then re-pitch a small amount of yeast when I bottle?
5. Should I just expect the beer to stop fermenting at the appropriate FG?

This is my first time where the beer didn't get down at least reasonably close to the correct FG, so I'm sort of flying blind here. Thanks to anyone who has some experience to share with me.

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Old 07-13-2009, 06:21 PM   #2
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1. I'd be surprised if it dropped that much at this point but I guess it's possible. There's other factors at work here - was it an AG or extract recipe? Some beers just won't get down that low, especially extract recipes.

2. I'd give it another 10-14 days and then check it again. If you did restart the fermentation process it will need the time to finish and clear again. If they didn't finish it out in 20 days before, you can't expect them to do it in 3 now.

3. It's only going to ferment as far as the available sugar will let it. If you get 2-3 consistent gravity readings over several days, it's done - even if it's still higher than the recipe predicts.

4. No

5. Yes, and you may just have to accept that as being higher than you want. Sometimes there's nothing you can do to get the gravity to drop further (unless you infect it with something other than yeast).

In the future, you should take a gravity reading before moving it into secondary and delay racking if it's not where it's supposed to be. In the primary on the original yeast cake is where it has the best opportunity to completely finish. Re-pitching yeast later doesn't work very often because the conditions in mostly fermented beer are nowhere near as good as they are with fresh, oxygenated wort.

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Old 07-13-2009, 06:26 PM   #3
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Since it is bubbling away you only have 2 choices. Kill the yeast, and repitch again when you bottle (not recommended) or just wait it out. Most likely the beer will finish pretty close to your expected FG unless it got infected (not likely). Keep in mind as long as the yeast are fermenting you can not bottle it. If you did it would produce over carbed beer (best case) or bottle bombs (worse case).

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Old 07-13-2009, 06:44 PM   #4
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Your attenuation was about 65.5% so it was a little low, however, the amount of unfermentable sugars in the wort could also make for a higher FG.

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Old 07-13-2009, 07:35 PM   #5
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It's an AG recipe.

From all appearances, it is fermenting again. I'm content to wait it out, but I was curious whether the addition of a new packet of yeast would be too powerful, dropping the FG too low.

I guess this brings up another question that's flummoxed me for a while: if a yeast's alcohol tolerance is higher (say, 11%) than the beer I'm trying to brew (say, 5-7%), WHY does fermentation stop at 75% attenuation, rather than continuing until all fermentable sugars are eaten?

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Old 07-13-2009, 08:04 PM   #6
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If re-pitching the same yeast is producing a noticeable fermentation, and your initial fermentation resulted in insufficient attenuation, then you initially under-pitched or under-aerated. Yeast cells can only double (create daughter cells) so many times before they run out of sterol reserves synthesized during the lag phase in an aerobic environment. What apparently happened is that your yeast consumed as many of the fermentables as possible before running out of sterols, and then a stuck fermentation resulted. Since you added dry yeast, which already have a limited amount of sterol reserves, they were able to begin fermenting the remaning fermentables. So, if this second package of yeast is healthy enough, you should reach your target final gravity. I would be more worried about reaching your final gravity than exceeding it. You are not going to exceed your final gravity unless the initial fermentability of the wort was greater than you expected. In other words, a specific yeast strain is only able to ferment a given wort to a given degree of attenuation, regardless of how much yeast you add. At a certain point, attenuation will not increase as additional yeast is added, because there are only so many fermentables in the wort to ferment, and each yeast strain has a different tolerance for the level of ethanol that it can tolerate.

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Old 07-13-2009, 08:08 PM   #7
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Cool. Thanks for the reply! How long would you expect the new fermentation to take before I could rack to another secondary and/or bottle?

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Old 07-13-2009, 08:15 PM   #8
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Lots of factors can affect total attenuation - fermentation temperature, aeration level, fermentability of wort, pitching amount, nutrients in the wort, fermentation time, it goes on. The specs on a specific strain of yeast are typically just average results given optimal conditions. Yeast are living organisms, not robots - and they are affected by their environment. That's why we do everything possible to optimize conditions and improve the chances for a good complete fermentation.

I would wait another week at least and then check the gravity before doing anything.

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Old 07-13-2009, 08:37 PM   #9
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Yeah, unless you have a reason why you need to bottle it right away (i.e. an upcoming party, gift, etc.), you may as well give it a few weeks to condition before racking into secondary or bottling. In the first couple weeks after fermentation is complete, the residual yeast will continue to clean up some undesirable by-products of fermentation, so it is desirable to leave the beer exposed to the yeast cake. Little autolysis should occur during this time if the temperature of the fermenter is not extreme. If you are in a hurry to bottle, then take a hydrometer reading every few days until you notice no change in the gravity, and then you should be safe to bottle.

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Old 07-13-2009, 09:02 PM   #10
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Not in a real hurry, but it's the next brew to be bottled and I'm running somewhat low on bottled beer . Also, I'm somewhat limited in space, so with 3 brews currently running, I'm pretty much done brewing until this one pops.

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