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Fixing a stuck fermentation 101: Ground rules: Don't rely on your airlock alone as an indicator for fermentation progress, especially if you use a bucket. Your hydrometer is your most reliable tool for this purpose. - Are you sure the fermentation is stuck? Take hydrometer samples every two-three days for a week and correct for temperature. It's only stuck if your readings don't change for several days and are at least 5 points above your targeted final gravity. - Be patient - a stuck fermentation is not a time-critical flaw. Just sitting there for a few extra days is not going to hurt your beer. Think before your act, and give your actions plenty of time to show results before deciding to move on to an alternate fix.
Procedure: Slowly increase the temperature of the fermenter to near the upper range of the yeast's optimal range, and gently agitate the carboy to resuspend the yeast every few hours. Avoid splashing. - If that doesn't show a drop in gravity after a few days, consider one of the following options:
1.) Dissolve 3 oz of corn or table sugar along with some yeast nutrient in boiling water, cool it down to fermentation temperature and add this mixture *gently* to your fermenter (avoid splashing). Sucrose and dextrose are very easy for yeast to ferment, and adding some along with yeast nutrients can provide the yeast with just enough of a boost to resume fermentation of more complex sugars remaining in your wort.
2.) Pitch more yeast. This is best done with liquid yeast: Make a very large (half a gallon or more), very well-aerated starter with ample yeast nutrients, let it ferment to completion, decant the starter beer and gently add the slurry to the fermenter. If you want, you can add a second, much smaller starter (1 pint or so) and pitch it at peak activity (along with the starter beer) - that way, you provide a large volume of healthy inactive yeast along with a smaller volume of active yeast. DO NOT aerate or oxygenate your beer when pitching more yeast - that's why we made the large, well-aerated starter. Other options that I have not tried, but others have used with success is the addition of enzymes to the stuck beer. Amylase enzyme works on converting any remaining starches to sugars, and can help if the yeast are struggling with fermenting the more complex sugars, or the mash had incomplete conversion. An option of last resort is alpha galactosidase ("Beano"), which will convert otherwise unfermentable sugars. This will, however, significantly dry out the beer and affect head retention, mouthfeel and flavor. - If neither of these options works, it's time to consider more drastic options:
1.) Just call the beer done and see how it'll turn out after some aging. It may be fine, or it may need to be blended with another beer. CAUTION: Keg beers that finished significantly high. If you must bottle, use PET bottles. I do not recommend glass bottles due to the increased risk of dangerous bottle bombs. If glass bottles are your only option, be conservative when adding priming sugar, place your bottles in a "bomb shelter" - a sturdy container that can withstand exploding bottles and keep the resulting mess contained, and monitor your bottles for signs of over carbonation. Use caution when handling potentially over carbonated bottles - use eye protection, sturdy gloves and cover as much of your body as possible in thick clothing to protect yourself in case a bottle decides to blow up in your face.
2.) Consider pitching a highly attenuative, highly alcohol-tolerant yeast strain such as a champagne yeast if adding more beer yeast did not do the trick. Follow the starter/rehydration guidelines above.
Preventing stuck fermentations in the future: Always rehydrate dry yeast in warm water, ideally with a proper dose of Go-Ferm rehydration nutrient. Do not add sugars (including malt extract) or regular yeast nutrient/energizer while rehydrating yeast! - If using liquid yeast, make a starter, no matter what the directions on the vial/smack pack say. Refer to the Mr Malty Pitching Rate Calculator to see how large a starter you need. Rarely is a starter less than 1.5 qts sufficient to achieve significant yeast propagation. You don't need a stir plate or borosilicate flask - a simple one-gallon glass jug will do just fine (available from most grocery stores filled with apple juice). Shaking it up every hour or so will provide the yeast with plenty of oxygen to grow. - Know your water chemistry. Yeast needs calcium to be happy. If your water has less than 50 ppm calcium, add some, but don't exceed 150 ppm (this is less critical for extract brewers, since the extract itself contains minerals). - Temperature control is hugely important. Use a dedicated chest freezer or fridge with external temperature controller, a
swamp cooler (the "wet t-shirt method"), or - as I do - a picnic cooler:

Monitor temperatures closely and avoid swings of more than 2 degrees. Here is the important part: As fermentation winds down, allow the temperature to slowly rise to the upper range of the yeast's optimum (usually the low to mid-70s for ale yeasts). This will result in higher attenuation and less chances of premature flocculation, as well as more rapid reprocessing of undesirable fermentation by-products. Some highly flocculant yeasts may need to be resuspended by agitating the carboy occasionally. Because this temperature increase is done slowly and when fermentation is almost complete, no excessive esters, phenols or high alcohols will be produced. - Use yeast nutrients. While beer wort contains nearly every nutrient that yeast need to prosper, it can be short on zinc, and some nutrients are not very readily available. Providing external nutrients (usually added late in the boil, occasionally during fermentation) costs only a few cents per batch and is just one more tool to maximize yeast propagation, vitality and viability.
Summary: With adequate temperature control, water chemistry (especially calcium), proper yeast rehydration (or a large starter if using liquid yeast) and proper supply of labile yeast nutrients, it's almost impossible to get a stuck fermentation. Preventing one is much easier than fixing one.
Can someone clue me into the why of this statement? (mainly the do not add sugars part)
- Always rehydrate dry yeast in warm water, ideally with a proper dose of Go-Ferm rehydration nutrient. Do not add sugars (including malt extract) or regular yeast nutrient/energizer while rehydrating yeast!
How to Brew has a brief explanation for this on page 73, and Palmer has also discussed this on the BrewStrong podcast, I believe.
Basically, it has to do with depleting yeast nutritional reserves for adapting to a starter wort that differs from the main wort and the yeast's inability to control what passes through the cell membrane while rehydrating.
Pretty damn good!!!
Although I would like to see you add a couple more tricks to fixing stuck ferm, and then I would even use it as a resource to link people to (on those rare times we really find a stuck fermentation
). There's one that I personally would employ before the corn sugar sugar one. I know 3 oz of corn sugar isn't a lot in the scheme of things, but I'm not necessarily a fan of just adding some more fermentables. I think I would add the energizer alone.
I am a big fan of adding yeast hulls or "yeast ghosts) to help bring down the fg. There's some great source material on that.
I had great success with bringing my saison down from 1.018 to 1.009. I actually have a small amount of dead bread yeast (that was pretty much baked to death after being improperly stored.)
It can also be added to the boil in the last 15 minutes of the boil to act as a yeast nutrient. You can either buy it from an LHBS as yeast hulls, get it at a health food store as "brewer's yeast" or kill off a few packs of yeast by exposing them to high heat (perhaps by microwaving them for a minute or so? Mine was just dead for being stored for weeks in a hot cupboard, and no-longer made bread.)
If you are adding it to the boil, you don't have to do anything. In fact if you have any old or unmarked yeast that maybe came in some crappy kits, or just some bread yeast, you can just add a 1/2 tsp or so to the boil, the heat will kill it.
Another "interesting" fix for a stuck fermentation is from Evan! https://www.homebrewtalk.com/f13/psa-foolproof-stuck-ferment-fixer-72072/
It is a little drastic, but if you happen to have a cake lying around on bottling or racking day, you may want to try it.
Or similarly if you have anything else at "high krauzen" like a starter for a new batch or another batch of beer that is going at it gangbusters, you could take some of that krauzen with a sanitized spoon and add that.
These are a little "drastic" and I would surely go with your energizer/corn sugar route before employing Evan!'s idea, but I thought it would be a good idea to have that info in here.
Anyway good job, great and useful info!!!!!
I give this 4 mugs for usefulness!!!!