Dear Mr. Wizard,
I am an extract and partial mash brewer and I’ve encountered an odd phenomenon on a couple of occasions, the most striking example of which came this week. I started out to brew a brown porter, though my creativeness got out of hand and I ended up with an OG of 1.084, pretty high for anything that I typically brew. I often use the yeast from a previous batch, and this time, I used the yeast from a batch of pale ale, along with good doses of yeast energizer and nutrient, since the gravity was so high. The airlock started bubbling away about an hour later, which is typical when I reuse yeast. However, after three days, the activity slowed considerably, to one or two gurgles per half hour. Because I knew this is a pretty weighty beer, and because I was out of town, I left the beer in the primary fermenter. It has now been 18 days. I was ready to transfer to the secondary fermenter, when I noticed that the activity had picked up considerably. It’s now bubbling three to four times per minute, what I would expect on day four or five in a typical brew cycle. On a related note, the pale ale in the secondary fermenter had a resurgence of activity, as well. I’ve been monitoring the temperature, and there aren’t any changes from what my brews normally experience. Do you have any idea what might cause such a wide ebb and flow? How common is this? Is it an indication of a problem with fermentation?
The Wizard replies:
I have seen this type of unusual behavior before. Most of the cases I have personally noted have been due to under-pitching yeast. Most brewers agree that pitching rate should proportionally follow wort gravity and the ratio most often cited is 1 million yeast cells per liter of wort per degree Plato. Yeast slurry harvested from a previous batch is pretty dense stuff and right at 250 mL or 1 cup of slurry will give you about the right amount of yeast for five gallons (19 L) of 12 ºPlato wort (1.048). Your King Kong Porter weighed in at around 21 ºPlato and should have been pitched with nearly double the yeast as a normal batch. You may have done this and under pitching may not have been your problem.
Other contributors to the problem may have included poor aeration, old yeast, highly flocculent yeast strain or a contaminating organism. While a successful fermentation cycle is by no means a guarantee, if you pitch enough viable, healthy yeast into aerated all-malt wort and conduct the fermentation in the temperature range recommended for your chosen yeast strain, things normally go just fine. A contaminating microbe, either a second yeast strain or some type of bacteria, could cause an apparent resurgence of activity late in the game.
These are all pretty generic guesses because I haven’t been given much information to digest. While bubble watching is an indicator of fermentation, it doesn’t give one very important piece of data. . . gravity.
There is no way to know if a fermentation has halted early (“stuck fermentation”) or cranked down to finish gravity faster than expected without taking a sample for testing. I know that grabbing samples is something that small-batch brewers really like to minimize or even avoid because the sample is lost. After all, two to three decent samples add up to about two bottles of beer.
If you have a sluggish or hanging fermentation, rousing, adding more yeast or kräusening can rejuvenate the fermentation. I have found that kräusening high gravity lagers, specifically doppel bocks, is an especially effective way of fermenting to completion. I have had lager fermentations that seem like they will finish a little higher than normal and then begin to pick back up and move down to their finish gravity by doing nothing. I cannot explain why that happens but have several fermentation curves plotted over the years that demonstrate this trend. We made changes to how we handle our lager yeast at Springfield Brewing Company and have solved this problem.
The only other thing that comes to my mind is that you may have some other yeast strain or bacterial contaminant that is showing up late in fermentation and producing carbon dioxide. This is probably not your problem, but it is a possibility. Some yeast strains are known as super attenuators and are able to metabolize dextrins that brewing yeast cannot. The same is true with certain bacterial species. The fact that your pale ale and the porter both exhibited the same pattern may suggest that you have a contaminant in the yeast.
I would recommend taking a sample at least once and preferably twice when you think fermentation is nearing completion, because if you do have a stuck fermentation, you can do something about it before you bottle beer that is not completely fermented. Just a suggestion!