Brewers make wort - yeast make beer. In light of that, it's a bad idea to focus too much on recipes and wort production and neglect your fermentation process, which is why so few good brewers do it. But even the best brewer will occasionally have a problematic fermentation.
Diagnosing and treating a failed, slow, or stuck fermentation takes a bit of guesswork, but in the end you should nearly always be able to get things moving. This is a robust process, and once you get the yeast rolling they'll usually get back to what they're bred to do. After all - apparently it's cliche day here at Beer Simple - wort wants to become beer.
The questions are these: do you have a problem, how can you address it, and is it worth it?
Do You Have a Problem?
Step One is figuring out if you actually have a problem.
Lots of new brewers panic when they don't get quick action in the fermenter. I like to see some krausen forming within 12-24 hours (though with a good dose of oxygen I've had it in as few as 6), but I don't start sweating until about 72 hours, and maybe not even then, even if I don't see action in the airlock. Don't panic - just take a gravity reading. Sometimes your yeast crank through primary fermentation very fast and leave little evidence of it - these "phantom fermentations" are rare, but they happen. Your gravity reading will tell you. If it's still at your OG, then you have a failed pitch.
Diagnosing a stuck fermentation follows this same basic method, but asks for some analysis. It requires taking gravity readings at two times (say, 12-24 hours apart); if both are identical, then you may have a stuck fermentation. It's not certain, though. The gravity might be static, and higher than you anticipated, but it's possible you are finished. If you mashed warm, or had a recipe that included a lot of unfermentables, or over-yielded on your efficiency and started with a higher-than-expected OG then your FG calculations might be off (consider this possibility if you're over 50% attenuated). Maybe you're done and you don't even know it. How do you know? Taste your beer - if it's not particularly sweet, then you could very well be finished. Those residual sugars often don't taste as sweet as simple sugars, and your palate is very sensitive to sweetness. Sweet beer is usually still-fermenting (or fermentable) beer.
And then there's diagnosing a slooooooooow fermentation. Sometimes a beer is still fermenting, just very, very slowly. Sluggish yeast are a pain. You take your gravity readings, and it's dropping...but only by a couple of points every couple of days, even with 20-30 points to go.
If you have any of these, then you might consider taking action.
For a failed fermentation (no activity, and no movement on your OG):
- Step one is to check your temperatures. If you're freezing your yeast, they might have simply gone dormant - anything below 50F is a risk for that, though I've fermented at 45F without problems. If you're flirting with that number, put your fermenter somewhere a little warmer and see what happens..
- If nothing happens then, make sure your wort isn't too hot (over 90F) and re-pitch. You can wait it out, but if you don't see activity within 4-5 days and there's no bubbling in the airlock and your gravity hasn't dropped, then your yeast are likely dead (or so few are alive that they'll struggle like crazy, creating lots of off-flavors), and they need the cavalry to come to the rescue. If you wait too long, every other thing in your house will try to get in there and establish a foothold.
Stalled fermentations invite all kinds of tricks, but they have some uncertain results. They can work, and you should try, but don't get your hopes too high (though this isn't the end of the world, as we'll discuss a little later). Your options include:
- Just like a failed start to fermentation, check your temps - and increase them. Warmer yeast are more-active yeast, and if you catch them in time this might get them off the picket line and back to work. Go all the way to 90F - if you're already through the initial fermentation stages, the hot temps won't be nearly as likely to produce off-flavors.
- Shake it up. You could also try rousing the yeast, either by shooting CO2 into the sediment or old-fashioned swirling or agitation.
- Repitch. This can be with the same yeast strain, a more-aggressive yeast like champagne strains, or even bugs that might keep on chewing (especially if you think the problem was caused by an excess of unfermentables).
- Re-feed. If you think that the problem is a bunch of unfermentables and you're not comfortable introducing Brett or its ilk into your brewery, you can also spike your beer with simple sugars (honey, maple syrup, table sugar, etc.) that the yeast will consume. You'll add alcohol, thinning out the beer, but at the possible cost of new/off flavors and hotter alcohols.
And for slow fermentations, well...
- Increase temperature and wait. It will end someday. Go on vacation. May I recommend Campobello Island, New Brunswick, home to an Joint US-Canadian International Park that FDR used as a retreat?
Do you REALLY Have a Problem?
Before you take any of these steps, though, ask yourself if you really have a problem - or, at least, one that's worth fixing.
Taste your maybe-unfinished beer. If it's soured or funked already because the yeast never took hold, then you might consider dumping it, cooking with it, or making some vinegar depending on the flavor.
If it's a stuck fermentation but it tastes OK, then consider just carbonating it, claiming victory and departing the field - the odds that someone can taste the difference between a 1.030 and a 1.020 beer are pretty slim (just be sure you're actually stuck - otherwise you could be making bottle bombs).
I've seen a lot of brewers fight their beer for those last few points. It's not always worth it, and the cure can be worse than the disease, so ask yourself some tough questions about whether you even want to try.
But what you definitely shouldn't do is what we tried with an early group-brewed batch before we knew much about brewing: don't dilute your beer with club soda. I mean, it tasted alright eventually...about eight years later.
Keep it simple.