et'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.
Non-starting fermentations
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
this later).
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