A stuck fermentation is one that has stopped before hitting the expected final gravity; in particular the specific gravity of the must has not changed in at least 48 hours. Stuck fermentations are often proceeded by sluggish fermentations, which is when fermentation slows dramatically earlier than expected to a rate of less than 1.001 points per day. If your fermentation never started, refer to this section
As the saying goes, an ounce of prevention is worth a pound of cure. Closely monitoring the change in specific gravity as your must ferments can help you fix a stall before it even happens. For this reason, it can be helpful to check the specific gravity of your must during fermentation every 24-48 hours.
It is very important to act quickly once you notice a sluggish or stuck fermentation. Sluggish fermentations are easier to remedy before a stall actually happens. The longer you wait to act once a fermentation has become stuck makes it less likely that you will be able to get it to restart.
Pitching more of the same yeast. It will not have any tangible effect in almost all cases. Yeast grow exponentially and have large colonies established by the time a stall happens. Adding more yeast is adding a small amount to the existing colony of otherwise viable yeast. Secondly, the issue is almost never the yeast themselves and the new yeast have been thrown into the same problem the original ones had.
Racking. Do not prematurely rack the mead because of a stall. This is actively harmful and will make things worse. Many of the yeast on the bottom are still healthy and viable. Rousing them is the answer. Racking removes them from the equation and makes it more likely the stall will become permanent. Growing a new yeast colony is more difficult the closer to done a mead is. Rely on the yeast already there.
First step is to do a full assessment of the mead.
The last one is a catch-all for strangeness. Perhaps a new and rare adjunct or unusual attempt with a wild ferment, etc. Attempt this protocol and consider posting on the subreddit if the last case applies to you.
In some situations, restarting fermentation can be difficult or impossible. As a general rule of thumb:
If you fall into the difficult to impossible territory, it is often advised to dilute your must and add additional sugars to meet the 'easy' category. A blending calculator is helpful. This can often restart fermentation on its own.
The best way to resolve a stall is to determine why it stalled and rectify that problem; this will often restart fermentation.
This is generally the most common cause. Many hobbyist meadmakers often overlook this as a lot of bad information or it outright excludes this. Do not blindly add nutrients if you are on schedule. There might be other issues, see the other sections for other possibilities.
If not, then see nutrition for a primer on it. Follow the instructions and add the required nutrients. Monitor for changes and reassess after 24 hours, if needed.
Next possibility is it wasn't enough. Many homebrew shops will sell small packages of nutrients with instructions. These often aren't enough for mead and are aimed at country fruit winemaking, which has some nutrients from the fruit available.
See nutrition and add the difference. Monitor for changes and reassess after 24 hours, if needed.
A less common occurrence but not excluded. Likely doesn't apply to you if you are fermenting at room temperature in a living space. Ensure that yeast are within the temperature range they need. Causes are fermenting outside/in a shed/etc where temperature swings happen or temperature controller failures.
Consider alternate causes as well. A bucket on a bare concrete cold basement floor may be colder than the room's air temperature suggests. Temperature controller probes might be misplaced or temperature coolers/heaters plugged into the wrong plug. Double check all connections if you have temperatures you are not expecting.
It is normal for temperature-controlled cold fermentations to have their temperature raised during the final phase of fermentation. This allows for fermentation off-flavor byproducts to be cleaned up by the yeast. Keeping the temperature cold will often cause a stall. The common equivalent beer-brewing technique is called the Diacetyl Rest.
A rare case, typically not an issue for those who didn't add any adjuncts that lower pH. Many low pH musts will ferment (classic example of Skeeter Pee). Fermentations as low as 2.9 are known to complete without other issues. Acid types make a far bigger difference in fermentation kinetics than overall pH. Yeast is largely resistant to pH problems and often needs no direct addressing. Small amounts of certain acids can
An example of why pH is a tough topic: citric acid is more toxic to brewer's yeast at pH 4.5 than pH 3.0 (Neilsen, 2007). It has to do with the changes the compound undergoes as pH drops and it loses more protons (up to 3 since it is triprotic). The more dissoicated and ionic, the more toxic. It is a weaker acid therefore will retain its protons at lower pH.
[To do: add section about acetic acid after reading Fleet, 1994]
A general cause of fermentation problems isn't acids in general but excess fatty acids. They are synthesized by yeast and used in reproduction but also dissolve when ABV increases and waxes dissolve. They are known to cause stalls. Yeast hulls up to 0.5-1g/gal or 0.5-1g/4L is the recommended course. Substitute boiled yeast at the same rate since we only want their hulls. A small amount of YAN is introduced but that is a side effect instead of intent and is small enough to safely be ignored.
Attempting titration (balancing acids/bases) is not recommended. It is very difficult to do because it does not scale linearly. Neutralized acids will also leave minerals and salts in the brew which may compromise the flavor. Attempt balancing of the mead when finished instead.
Instead, have a proper fermentation plan using appropriate nutrients, temperature, gravities, etc. It is much easier to prevent an acid problem than it is to solve it on the backend.
It is appropriate to use yeast hulls as outlined in the fatty acid section. It is also okay to pitch 2g/5L of potassium carbonate (not calcium carbonate). It contributes a very small carbonate buffer and potassium for the yeast (See Bray's One Month Mead section 6b. for info on this).
Note: 2g/5L adds about 300ppm of carbonate ions to the water profile. For those familiar with water chemistry, this is the high side of some mineral-heavy water profiles like historic Munich and Dortmund profiles (before decarbonation). More is not better and can have strong effects on taste since it raises alkalinity. The intended purpose is not specifically for alkalinity. Stick to the max recommended amount of 2g/5L per Bray Denard's protocol.
Some problems will slip past our eyes and hydrometers. This assumes the assessment was done and other causes were eliminated.
Gently agitate/swirl/stir to rouse lees from the bottom. Some of the yeast down there are still good and can be brought back up.
Excess long-chain fatty acids are common stall instigators. They can be scavenged by 1g/gal or 1g/4L of yeast hulls. Substitute boiled yeast at the same rate for their hulls.
As a last resort, you may pitch a new yeast to attempt to restart your stuck fermentation. It is important to not use the same yeast that you originally pitched. Yeast selection here is very important; brewing yeasts are glucophilic, preferentially consuming glucose in the must first, meaning that stuck fermentations have much higher levels of fructose. While yeast can process fructose, it is more difficult, especially in hostile environments likely to be found in stuck musts. Some yeasts have a particularly high fructophilic index, meaning it is easier for them to process fructose. Ideal restart yeasts are vigorous fermenters with a high fructophilic index. Examples of such yeasts are:
It is recommended to follow the restart protocol below for the best chances of restarting your fermentation:
Gravities above 1.15 are difficult to begin fermentation on. Dilute to a reasonable level (like 1.12) and repitch. Consider step feeding to obtain higher ABVs without having to have too high SGs.
It is somewhat common for off-the-shelf juice products to have stabilizers in them to prevent fermentation. First recommendation is to avoid these products since it will make everything difficult. Fermentation may or may not start but may often stall.
Stabilizers include (but not limited to): Sorbates (or sorbic acid), benzoates (or benzoic acid)
The exception are sulfites. They include anything that has metabisulfite or a product that says it contains sulfites. Sulfites can be removed by aeration (thus why we readd sulfites at each racking). Give extra attention and shake, stir, aerate prior to yeast pitch.
This is a rare possibility and usually only possible when acidic adjuncts (like large amounts of lemon juice) or acids were added directly into the must pre-fermentation.
Buffering can be attempted but will likely affect flavor via electrolyte additions. Baking soda is one example that will increase pH but also raise sodium levels in the must. Add small amounts, stir, re-check pH frequently. The non-linear scaling means you may easily overshoot your result. Too high a pH is worse than too low. You only need to hit 3.0 to get your yeast started.
This is a losing battle. Prevent needing to fight this one by planning out a better process to begin with. Consider all direct acid (e.g. citric, malic, acid blend, etc.) post-fermentation as part of balancing.
This is rare in most cases. Exception is using an already open package of yeast or an existing yeast cake. Assume something wrong elsewhere before looking here. Dry yeasts typically have expirations listed as 1 year after production but often last up to 3 years if still sealed (use as quickly as possible once opened, keep in a closed container and use within 6 months). Liquid yeasts expire much more quickly. Follow your manufacturer's guidelines on viability and necessary starters for aged liquid yeasts.
The second error is rehydrating yeast in too hot water. Rehydrate in 104F/40C water but not higher. It is not uncommon for people to accidentally cook their yeast during hydration. It is fine to rehydrate in tepid or room temperature water if you cannot measure the temperature. Err on the side of lower instead of hotter.