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I recently brewed a batch and let it set in the carboy for 2 weeks. When it was transfered to the bottling bucket we noticed that it was a little carbonated already, which seemed odd. We ignored it and proceeded on to keg it like normal.

It measured a FG of about 1.002. Which was another red flag but it was not insanely lower than the projected 1.008.

The really odd part was when sugar was added to the keg for it to prime. Right when the sugar was poured into the beer it started overflowing the keg, and sort of foamed but mostly just gushed out, and was pretty neat/messy.

Any ideas what could have caused this effect?

We tasted the beer used to calculate the FG and it tasted good. No noticeable off flavors. One thought was that we did mash at 144, but I don't see how that could effect the carbonation.

Since I am writing this question a few days after the case and have had time to think I wondered if it could have been that the 3 piece was partly blocked from foam but not enough to push it off? I did not notice any hiss, or oddities from the 3 piece when I opened it, although I had "tasted" a few brews by that point.

EDIT: fixed FG to take temp into account.

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Not sure it's relevant to your CO2 question, but a F.G. of 1.000 is pretty much impossible for beer. You'd have to use 100% table sugar or a wine yeast or something to get down to 1.000. Are you sure your hydrometer is accurate? Did you forget to compensate for temperature? Take a reading of distilled water to sort it out. –  Henry Jackson Jul 30 '12 at 20:20
    
I did not take into account temp. Just ran it through a calculator online with a guess at wort temp, and it is actually 1.002-1.003. I will still test out my hydrometer regardless. –  corymathews Jul 30 '12 at 20:51
    
1.002 would still be suspiciously low, so check it out for future use. Most hydrometers are calibrated around 60 ˚F, so a measurement near there (e.g. fermentation temperature) shouldn't have too much error. If you try to measure the gravity of a hot liquid (e.g. the pre-boil gravity when doing an all grain batch), the difference is much larger. –  Henry Jackson Jul 30 '12 at 21:12
    
I entered it in the calculator for being 85. I ferment at 66, and it sat out at about 85-90 for probably an hour or so as it was transfered to bottling bucket then keg so I figured 80-85 is a good estimate. I use a refractometer for the SG reading which takes temp into account for me. –  corymathews Jul 30 '12 at 21:16
    
That's still a really low FG. Can you post the recipe, or at least the ferenetables (malts and sugars)? Could be that you introduced some bacteria that's been chewing away on the sugars that brewers yeast won't ferment. –  Tobias Patton Jul 31 '12 at 2:27
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1 Answer 1

up vote 6 down vote accepted

Recently fermented beer will have co2 still dissolved from fermentation at atmospheric pressure - ca. 14.5psi. The amount dissolved depends upon temperature, which is why priming calculations always ask for the fermentation temperature to estimate how much co2 is in the beer already.

When you add the sugar to the keg, the granules of sugar have a high surface area relative to their volume, and act as nucleation sites allowing the co2 to quickly come out of solution, creating thousands of tiny bubbles. This creates the large head of foam that you saw. It's the same as dropping mentos in diet coke.

EDIT: There are a couple of comments that doubt this is a frequent occurrence, so I'll try to expand the answer to clear that up.

With fermentation under an airlock, the fermentation vessel becomes filled with CO2 at atmospheric pressure which dissolves into the beer. Eventually the beer has absorbed all the CO2 it can at that temperature and pressure - it is saturated with CO2. The BYO article Priming with Sugar discusses this process in detail, and has a graph showing residual CO2 and temperature. At 66F, the residual CO2 is just under 0.9 volumes, so, just a little less than the volume of the beer itself. It's then not hard to see there's more than enough CO2 in the beer to fill the headspace several times over. The sugar falling through the beer causes a lot of the CO2 to quickly effervesce creating bubbles and foam which fills and overflows the headspace in the keg.

One way to avoid this is to boil the sugar in water - this sterilizes it and also greatly reduces the nucleation sites, reducing the amount of foam to almost none.

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You make it sound like this type of behavior is common place and completely logical...I have never had anything like this happen to me before, nor heard of this type of thing happening. Is this something that frequently happens to you? –  hartski Jul 31 '12 at 12:32
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Yes I think it's fairly common that beer is partially carbonated after fermentation. The colder the ferment the more carbonation, even more so if the temperature is stable, such as in a fermentation chamber. If you have rising temperature fluctuations, then the carbonation will be less since the higher temperatures reduce solubility of co2. –  mdma Jul 31 '12 at 13:16
    
So the wort was still actively fermenting when I decided to keg it? @hartski I was thinking the same thing. I have kegged probably 20 batches, and brewed tons more and never seen this happen. which is why I asked the question lol. –  corymathews Jul 31 '12 at 13:41
    
Do you have fermentation temp control? If not, then each ferment is potentially different, and the amount of co2 retained will vary. The low FG could be indicative of a wild yeast contamination, which will ferment slower, longer and lower. If that's the cause, then the beer could have still been fermenting when you kegged it. –  mdma Jul 31 '12 at 14:04
    
yeah I have a fridge for fermenting that is set at 66 all the time. –  corymathews Jul 31 '12 at 15:00
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