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What's the maximum amount of time for primary fermentation, assuming the beer is going straight to bottles next? Will it over ferment and then fail to carbonate, if left too long in a carboy?

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It takes three to nine days for yeast to ferment a typical wort. After yeast consumes all the available food (or produces too much toxic alcohol) it goes into a dormant stage, flocculates and drops out of suspension. At this point it does not produce alcohol or CO2. Priming sugar is used to give the yeast a little more fuel so they will wake up and carbonate the bottle. This means that you can not over-ferment your beer.

I do not transfer my beer from primary. You can leave them for months with little flavor impact.

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The danger of off-flavors caused by leaving beer on the yeast cake for too long is something of a boogey-man. The biggest concern is autolysis which produces flavors starting at "yeasty" & going through "meaty" to "rubbery". I left a brew on the yeast for six months without negative impact. This is, of course, anecdotal. I get some of my information from Basic Brewing Radio and The Jamil Show. –  Dean Brundage Dec 10 '09 at 21:40
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I've heard that autolysis is usually only a problem in commercial breweries, because the weight of hundreds of gallons of beer focused on a small yeast cake in a conical fermenter will actually kill the yeast. –  TMN Oct 1 '13 at 14:44
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I've had beers in the primary bucket for as long as a 5 weeks (busy, lazy, whatever), and never had a carbonation problem in bottles or kegs, upon addition of priming sugar. There will always be live yeast hanging around. Never had real problems with off-flavors from sitting on the trub for too long either.

I don't think there are any hard-and-fast rules, but I do try to get it into the secondary in 1-2 weeks, if it's a big beer. For a regular session beer, it goes to bottling/kegging after 4-5 days anyway, as soon as fermentation is done.

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I had one batch in the primary for about 3 months and it turned out great. I was worried something "wrong" would happen but everything tasted great with no off flavors or anything out of the ordinary.

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I have had the same experience. As long as your yeast health was great when you started most of the issues with "long" primary ferments becomes a non-issue. –  brewchez Jan 20 '10 at 19:16
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I have had mine in there for 3 weeks probably max. I tend to follow the 1-2-3 rule and that's 1 in the primary - 2 in the secondary and 3 in the bottles.

The yeast don't "die" they just go dormant so when you add some fuel (ie sugar) they will get active again until they have burned that up and thus carbonate the beer.

The only real reason I even move to a secondary is out of probably an unfounded fear of off flavors from the beer sitting on all those proteins etc, but to be honest it's probably not really necessary. It's more of me just wanting to mess with it and not having the patience to leave it alone for 2-3 weeks haha.

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Best sanitation practices and healthy yeast let my beers sit in primary for many months. As I tend to over pitch, the extra time is not wasted by our single cell friends. Crystal clear (I stopped using secondaries some years back) and no off flavors. Sorry, I don't know how this would affect bottling as I haven't done that in over 30 years. Agree, great yeast is out there, grow it up if need be before pitching. Or just get a growler full of it from from your local micro-brewery.

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@ Hokiesguy95: your beer was not a failure because of excess time in primary (three days by your count), that would have nothing to do with soapy, unpleasant flavors. It would take WAY longer than ten days to produce any sort of soapy flavors from yeast autolysis.

The old advice of getting the beer out of primary as quickly as possible is antiquated and need not be repeated ever again. Just because there are no visible signs of fermentation does not mean the yeast is finished working; even with absolutely zero airlock activity the yeast are still cleaning up the byproducts of fermentation (including acetaldehyde and diacetyl), and allowing proper time for this work to be done will result in a more refined beer.

My fermentation process is a 21 day primary (under controlled temperatures), and then straight into a serving keg or bottle for most beers and into a lagering vessel for any lagering or long-term aging (barleywines, etc.). This extended primary has only resulted in much better beer, not worse, and no off-flavors or undesirable yeast byproducts. Oh, and some ribbons to boot. :)

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I've read where there is a risk of adding some off flavors if you let your beer sit in the primary fermenter too long. Typically when you see that there isn't any bubbling or activity for a day then consider racking to a secondary or going to the bottling bucket and add priming sugar. Or if you don't want to worry about bottle conditioning, consider kegging your beer and force CO2 into it. No worries about carbonation then.

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That's really an old brewers' tale at this point. Yeast quality improvement is likely the reason no one really worries about this anymore. –  Ell Apr 13 '12 at 14:19
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I left a cider in primary for 1 year... it turned out dry, crisp, and tart. Definitely no autolysis, socks, meat flavors there, nothing for it to hide behind.

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If you're using a fresh yeast is should be fine. If you're reusing yeast in the carboy and you're on your third cycle and you leave your beer in the primary on a huge 4 month old cow pie then you're certainly risking some off trooby flavors.

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I do not understand how people have left their beer in primary for more than 7 - 9 days. My first ever attempt was a failure because I left it in primary for 10 days. It was an Amber Ale and it ended up being very soapy tasting and unpleasant. Could possibly be the style was not meant for longer, but I would not recommend leaving in primary once fermentation has ended and one should transfer to secondary if they are going to age or condition it before bottling.

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Don't know enough to disagree with your post since I am very new to brewing. However, as to why brewers might leave the beer in primary so long, instructions for the kits I've worked from said to leave the primary for 2 weeks. I think they are hoping that the yeast will settle in that time and make it easier to bottle. –  GSP Oct 3 '13 at 21:13
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How do you know the soapy flavor was from leaving the beer in the fermenter too long? –  Denny Conn Oct 8 '13 at 19:57
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The real problem with leaving beer in primary for too long isn't the yeast, it's the hops. If you're dry-hopping, leaving the hops in the fermenter for too long can cause vegetal or grassy off-flavors. If you're not dry-hopping, then I don't think there is a problem. I've left several batches in primary for a couple of months and haven't noticed any ill effects.

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I have disproven this many times by dry hopping for months. It seems to be dependent on the hop variety you use. Continental varieties are more likely to exhibit this. –  Denny Conn Oct 8 '13 at 19:56
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