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> Stuck Fermentation, a discussion, Submitted by sidney porter
post Mar 25 2008, 11:39 AM
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The following is not a FAQ per se. However, it is hoped that it will be helpful for homebrewers to attempt to understand what a stuck fermentation is, what causes it and what can be done about it. As usual, prevention is the best cure.

Stuck Fermentation

If you browse intent forums dedicated to homebrewing you will frequently see questions of how to handle stuck or stalled fermentation. The answers always varied. Some are based in tried and true processes while others could probably be categorized as throwing it up against the wall and seeing if it sticks, no pun intended.

In order really understand how to fix stuck fermentation we first need to define the causes, identify possible solutions, and finally discuss how to prevent it in the first place. We can divide the causes up into three main categories: Yeast selection, handling, and wort characteristics. But remember that the causes can often be multiple. Luckily the solutions often work for multiple causes.

Yeast Selection
  • Alcohol intolerance – Most beer yeast can easily handle 12%; some can handle up to 18%. So normally this is not the cause of stuck fermentation. You will frequently see advice to pitch champagne yeast. Champagne yeast is not a magic bullet to fix stalled fermentation. It does not do anything that normal beer yeast can't do. Champagne yeast is a valid solution if you are making a beer greater than 10%; if you are less than 10% I think we need to look at something else.
  • Low apparent attenuation of yeast strain – Most strains have published apparent attenuation of 60-80%. These are published based upon a standard recipe with a standard pitching rate, fermented at a standard temperature. The standards differ between manufacturers and they are not published. However, it gives you an idea of what you are going to get. For a 1.050 beer at 75% attenuation, finishing gravity should be around 1.012. If you are making a larger beer say, 1.090, it would be expected to finish around 1.022. Remember that the bigger the beer the more yeast and O2 you need. The solution to this is really to prevent it in the first place. Use yeast with a higher apparent attenuation, mash at lower temps than the standard, or use adjuncts that are 100% fermentable. However, like alcohol tolerance, this is not usually the main cause. A solution would be to pitch more yeast; we will address how to pitch more yeast in the following sections.

Yeast Handling
  • Low fermentation temperature – Yeast will have a published temperature range. If you are below the minimum the yeast will work slow or not at all. Warm up the fermenter.
  • Premature flocculation – Most common with British strains, in combination with lower temperatures. Shaking the fermenter seems to work. Some commercial breweries have stirrers within in their fermenter to keep the yeast in suspension.
  • Insufficient amount of yeast pitched – This is probably the number one cause. It is true that you can successfully brew a batch of beer without making a starter, under optimum conditions. These optimum conditions: fresh yeast, OG 1.050 or less, 5 gallons or less, pitching temperature 70 degrees or higher, and plenty of O2. In reality a lot of beer is brewed outside of optimum conditions. I have been asked why can’t the yeast just grow in the wort, the brewer being willing to extend lag times to avoid pitching the proper amount of yeast. Not even addressing the potential for contamination with the smaller pitching rate, this method doesn’t really work; the issue is that you can only expect so much growth out of the yeast.
  • Insufficient amount of O2 dissolved in wort – Yeast needs O2 for the growth phrase, if there is not enough O2, the yeast will not properly grow. If they don’t properly grow there will not be enough yeast to ferment the wort.

How to Add More Yeast

Several of the above problems can be solved by adding additional yeast. But what we need to keep in mind is that we cannot use normal pitching processes since we have partially fermented wort. Normally we would say make a starter, add plenty of O2 to the wort, this will allow us proper lag time, growth and fermentation.

But since the beer is partially fermented we really don’t want to add O2. We need to add yeast that has completed the lag and growth stages and is in the process of fermenting. We could make a starter and when it is at high krausen we could add it to the wort, but you are going to want to make a huge starter, since you are not going to get much growth, the required size is unrealistic.

The solution is to make a second batch of beer, when that beer is at high krausen remove 5% and add it to the stuck beer. This should be plenty of yeast and the yeast has completed their growth stage and they are actively fermenting the wort. If there are fermentable sugars in the stalled wort these yeast will eat them.

Wort Characteristics
  • Unfermentable sugars – This is an ingredient and/or recipe issue. The brewer used too many unfermentable specialty grains, mashed at a high temperature or most likely used an extract that is created to have a higher finishing gravity.

    The only real solution is to change the unfermentable sugar into fermentable sugars. The brewer would have been better to handle this in the mash by mashing at a lower temperature. If you are reading this it is probably too late to correct the mash. But we can add the enzyme that would have been working in the mash. The enzyme that we are looking for is beta amylase; this is the active ingredient in the digestive product Beano. If you crush up the Beano tablet and add it to the stalled wort the enzyme will start to break down the unfermetable sugars into fermentable sugars. This process is not going to be overnight; it will take weeks. The problem is that you really have no way of knowing when it will stop nor is there an easy way to stop it. You could crash chill the wort and chill all the beer, then keep it cold once you get to your target gravity. This really needs to be done in the keg, since you would not be able to naturally carbonate the cold beer. This method doesn’t really stop the enzyme but rather stalls the yeast. You could go one additional step and filter the yeast out. Another option would be to denature the enzyme. In the mash this would be completed by a mash out at 170 or in the boil. If you heat the beer to 170, in effect pasteurizing the beer, you will denature the enzyme and in the process kill the yeast. Keg the beer and artificially carbonate the beer. The problem is the effect on flavor when heating the beer. I personally do not have experience in doing this.

    Extract manufacturers cater to two different buyers, the 1st group is the brewer that will attempt to get 100% of their fermentable sugars from the extract. The 2nd group of brewers make recipes that call for the extract and then a percentage of sugar. If we make one beer with 100% malt extract and a starting gravity of 1.050 and we make a 2nd beer with the same 1.050 starting gravity but 75% came from extract and 25% came from sugar, the 2nd beer will finish at a lower gravity, and drier in flavor. Without knowing who will buy their product the manufacturer needs to create a product that can be used by both. On smaller beers, say 1.060 and smaller, it is usually not a problem, it becomes a problem once you get above 1.060 OG. Larger beers are more susceptible to many of the causes above so it is important to keep the potential problems in mind. If I were to make an extract beer 1.060 or greater I would use 5-10% simple sugars.

In summary, when experiencing stuck fermentation.
  1. Identify if it is really stuck or was it an issue with grain bill, extract used, or yeast selection.
  2. Warm and shake, the fermenter.
  3. Start another batch of similar beer, when it is at high krausen transfer 5% of the new beer to the stalled beer.
  4. Add Beano, but you need to have a plan on how you are going to stop it once it get to the gravity you are shooting for.

Thanks sidney porter, for contributing this article to our knowledge-base.
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