Let's start with two pieces of useless advice. The first came from a doctor who, when I was in my fifties, diagnosed a fractured foot and told me I should have done it when I was 17. The second is to tell you that with all-malt worts you shouldn't have stuck fermentations, which is very little help when that is exactly what you have and you want to know what to do about it.
What is a stuck fermentation?
Brewers generally use the term "stuck fermentation" to mean a fermentation that started normally, but then stopped at a gravity significantly higher than the targeted gravity. But I think I should include in this discussion fermentations that either have not started, or have started very slowly, since these are fermentations that have "stuck" right from the beginning.
Some of the causes of these problems may be similar or at least closely related, but the means of getting fermentation satisfactorily under way may be different, so I shall deal with them separately.
These are generally the result of one or more of the following causes:
1. Yeast quality:
The sample used was too old or had been mishandled, and just contained few or even no viable cells. If a starter had been made, this lack of viability would have been obvious prior to pitching, and a fresh sample could then have been used for pitching. The answer is to re-pitch with a fresh starter (more on
2. Lack of nutrients to promote growth:
This is generally not a problem with all-malt worts, but can be so if high proportions of sugars (those not derived from malt) or non-malt adjuncts (such as rice or corn) are used. In that case, adding ammonium phosphate-based nutrients or proprietary nutrients such as Servomyces from White Labs will help, although these should probably be added along with a fresh yeast starter.
3. Insufficient oxygen dissolved in the wort:
Yeasts need oxygen in order to permit sufficient growth of new cells, which are what are going to do the work of fermentation. If fermentation hasn't started at all, then try aerating or oxygenating it again, and preferably re-pitch with a fresh batch of yeast. Whatever your technique may be (splashing, using an aeration device, or direct oxygen with a carbonation stone), make sure you have sufficient oxygen in the wort when you pitch your next brew.
4. Wrong temperatures:
If you pitched into wort above 90 °F (32 °C) then you may have drastically reduced yeast viability, or perhaps even killed the yeast off completely. If the wort was too cold, then that too can cause problems with initial growth. If you pitch an ale yeast strain into wort below 50 °F (10 °C) its growth will be at best sluggish, and it may even give up the ghost entirely. In that case you need to bring the wort temperature up to 65–70 °F (18–21 °C) and then re-pitch with a fresh, active yeast sample. Do pay close attention to the supplier's recommendations for pitching and fermentation temperatures for the yeast strain you are using. Good brewers are those who can brew a particular beer to a consistent quality, and control of fermentation temperature is important in achieving consistency. Temperature-controlled conical fermenters are available to the homebrewer, although at some cost; simpler solutions, such as a heating band or an insulated box heated by a bulb can be quite effective in this regard.
5. Not enough yeast pitched:
This will generally result in a very long lag time before fermentation is visible (that is CO2 bubbles can be seen on the surface. And by long lag time I mean more than 12 hours). It is true that in such cases, assuming none of the earlier problems are the cause, that fermentation will probably get under way eventually, and many homebrewers are content to leave it at that. In most instances the resulting beer will be disappointing for a number of reasons. The first is that wort sitting around at ambient temperatures is an inviting target to beer spoilage organisms; remember that if you encourage them once they may spoil not only the present brew but also later ones! The second is that if the yeast has to struggle in the early stages it may not perform properly as the fermentation gets under way. It may, for example, produce too many esters for the style, and it likely will not handle reduction of diacetyl so that the finished beer has too high a concentration of that chemical and an overly buttery taste. Also the yeast may be too tired to give the desired level of attenuation, so that the beer is overly sweet. This is not an easy problem to solve, for you are unlikely to have a fresh starter on hand to re-pitch the wort, and it will take a day or two to prepare one. Nevertheless, pitching a fresh starter is the best way to go. While you are preparing the starter, make sure the wort is kept tightly covered (with an airlock in case fermentation does start); if you are using an open fermentor all you can do is pray!
6. Very flocculent yeast:
Some yeasts, even when healthy and pitched in sufficient amounts — will flocculate so readily that fermentation will be slow to start and will be sluggish. Some brewers approach this by frequent rousing of the sediment in the fermenter, and that can sometimes work. This is essentially what happens with the traditional Yorkshire Stone Square fermentation system, where the yeast is continually pumped from the bottom of the vessel back to the top of the beer. At least one English beer that I am familiar with is fermented in this manner, and the result is that it has a significant concentration of diacetyl, a flavor that is by no means to everybody's taste.
Just to pull this together, if you have a fermentation that doesn't start or is very slow to do so, you have a serious problem and in most cases the only way out is to add a fresh, active yeast starter. Even that may not work if your wort has picked up unwanted organisms while sitting there virtually unprotected while you prepare the new starter. The latter may finally get a good fermentation going, but those devilish bacteria or wild yeasts will be waiting in the wings to spoil the beer. Every brewer should hold it to be an act of faith to pitch a sufficient quantity of active yeast into properly aerated or oxygenated wort. To me that means preparing a starter, for that will not only give sufficient yeast it will also prove that your sample is viable. How big a starter? Well that is a separate topic and one that I dealt with in the "Techniques" column from March-April 2012. Or you can check at www.mrmalty.com, which will easily allow you to calculate how much pitching yeast you need.
The yeast was pitched and it took off satisfactorily, and then a day or so later everything seems to have stopped, with no more CO2 being evolved. You immediately think you have a stuck fermentation and wonder what to do about it. The very first thing to do is to ask yourself whether it really is a stuck fermentation. Just because the airlock isn't bubbling doesn't mean nothing is happening.
It might just be that you have a sluggish fermentation, which is often the case with high gravity worts, for example. Or, conversely it may have been a very rapid fermentation that reached completion more quickly than you bargained for. Or perhaps there has been a sharp temperature drop, and all you need to do is bring the fermenter into a warmer place. One thing is certain, however, and that is that you must check beer gravity before bottling or even kegging to avoid development of excessive pressure in that final container. If the gravity is the same on three consecutive days, and is in the target zone for finishing, then you are safe to bottle.
But suppose you have taken a gravity reading (take care to remove all gas from the sample) and you get a result greater than a third of the original gravity? Remember, that in most beers you would be looking at a finishing gravity about 15-30% of the original gravity. If this was an all-grain brew it's possible that you mashed at too high a temperature and have too high a level of non-fermentables. If that is the case there is not much you can do about it, you just have to accept it and resolve to take more care when you do the next mash. The only real way to determine if you have a true stuck fermentation is to do a forced fermentation by taking a sample, pitching with an excess of yeast and fermenting warm, then measuring the final gravity and checking it against the gravity of the wort in the fermenter. That's not too practical for the homebrewer, so you have to assume that you do have a stuck fermentation and need to take some action.
If the yeast has given up the ghost it could be that there was insufficient yeast nutrient present in the original wort, or that the yeast has flocculated very rapidly. It would therefore seem that a good solution would be to add some more nutrient and to rouse the yeast thoroughly, a remedy that many brewers espouse. In fact, rousing at this point may cause all sorts of oxidation problems, as well as increasing the concentration of diacetyl in the beer, without actually doing anything to improve attenuation.
What we need to add is some fresh yeast that can pick up the baton from the tired cells and complete the race. But just tossing in more yeast is unlikely to work, because there is no oxygen present to help yeast growth, but there is sufficient alcohol present to "poison" the yeast and reduce its vitality. Instead, the yeast must be in active, vigorous growth phase when it is added to the beer. And that means (yes, you've guessed it) adding a fresh starter. This technique is called kräusening.
If you have space, I suggest you make a 2-quart (2-L) starter for a 5-gallon (19-L) batch (10-15% kräusen is normal for the traditional method). If you do not have the space in your fermenter, a 1-quart (1-L) starter will usually work too, but a larger number of active yeast cells is better. Normally for a starter you would decant the liquor before pitching the sediment to the wort. You can do that here but since you want to add the yeast at full kräusen I think it is preferable to add all the liquid in the new starter.
Take 4 oz. (113 g) of dried malt extract (DME), and add water to a total of 1 quart (1 L), and stir until the DME is dissolved. Add a pinch of yeast nutrient and boil the solution for 20 minutes, then top up with boiled water as necessary and cool to about 70 °F (21 °C). Aerate thoroughly (oxygenation is better) and pitch with a fresh yeast sample; if you have a stir plate keep the pitched wort continuously agitated. Maintain at room temperature until it is fermenting vigorously (the so-called high kräusen stage), then add this starter to the beer. For best results the beer should have been left in the fermenter during this time so that much of the dissolved CO2 will have escaped.
Note that if your new yeast comes in a "smack pack" it can be added directly from the pack without smacking if you wish. However, it is preferable to smack it to break the inner pack and allow the yeast to become active in the pack, and then add its contents to the starter solution so that you get as much growth and as many active cells as possible in the starter.
Note also that some brewers prefer to use very lightly hopped wort to make a starter, usually at 10-15 IBU, which in this case would amount to a maximum of 1 gram of hops at 5% alpha acid.