High Gravity Fermentation
TroubleShooting
Ben Kalota - Jacksonville, Florida asks,
I am currently brewing a high gravity beer (1.097 OG). after 6 days I noticed there were no bubbles in the airlock. I took a gravity reading of 1.034. I poured the sample back into the fermenter and checked the temperature two hours later. The airlock was busily bubbling away at 5 to 6 second intervals. Two days later there were no bubbles in the airlock. I checked the gravity again and got a reading of 1.026. Same thing, two hours later the airlock was bubbling away. Have I discovered a way of unsticking a stuck fermentation? I did use a yeast starter. Is there a percentage of gravity drop before racking into the secondary fermenter? Also I have read about diacetyl rest, does this take place in the primary or secondary fermenter?This is one of the questions that makes me scratch my head, and for more reasons than one. My first response actually has nothing at all to do with the question. And that response is “stop pouring samples back into your fermenter!” Taking a sample from a fermenter to check gravity can easily result in the sample being contaminated with bacteria or wild yeast from your gear, therefor returning it to the fermenter is not a practice I would choose to use. I do advocate taking gravity samples to confirm that fermentation has ended and I understand why homebrewers are tempted to return their samples to the fermenter. If you ferment in glass then you can visually determine when things are slowing down. I prefer watching the airlock and the appearance of the fermenter and then taking two or three samples towards the very end to confirm that the gravity has stopped dropping. But that is a different subject.
OK, I am still scratching my head about the observation because I am quite sure that taking a sample, checking its gravity and returning the sample to the fermenter has nothing to do with causing the fermentation to pick up activity. Perhaps pouring the sample back in the fermenter caused enough turbulence to release some dissolved carbon dioxide from the beer, resulting in the bubbles. Or the airlock may have a leak between the rubber stopper and the top of the carboy that was sealed by removing and replacing the airlock. Or maybe you somehow aerated the sample and that really did have an effect on the fermentation. None of these answers are very satisfactory. But one thing I am fairly sure about is that you have not discovered a way to remedy a stuck fermentation!
When brewing high gravity batches there are a few things that are very important. While pitching rate and proper aeration are the obvious things to be mindful of, the importance of these two topics cannot be stressed enough. Yeast strain is also something to carefully consider. Some strains do not perform well with high-gravity wort. Yeast suppliers do an excellent job of describing the strains they sell and I suggest choosing a strain that is described as performing well in high-gravity wort. The use of a yeast nutrient containing zinc is also something to be considered.
Knowing the right point in fermentation to rack a beer to the secondary is hard to determine if you have never brewed the beer before, that is; if one chooses to rack to a secondary fermenter. If you have a beer that you want to move off of yeast for prolonged aging I would wait until the fermentation slows and is about 4-6 points above the expected final gravity. This will allow enough carbon dioxide production during secondary fermentation to purge the headspace of the fermenter of air.
If you are brewing a beer that is not being aged in the secondary, but really just being moved prior to bottling I have a different opinion. I would hold my beer in the fermenter until fermentation is complete and the gravity stops dropping. At this point I would hold the beer for an additional four days for the diacetyl rest, remove the airlock and seal the top of the fermenter with plastic wrap and then move the fermenter to a refrigerator that is set to 34-38 °F (1-4 °C) and leave it in the refrigerator for at least two days. This will help clarify the beer prior to racking. If you have a Cornelius keg you can purge the keg with carbon dioxide before racking and either carbonate in the keg for draft dispense or add sugar and yeast before bottling. Or you can rack to another carboy or bottling bucket. In my opinion, there is really no real benefit to racking beer into a secondary fermenter for a short aging duration.
