What do you do With a Stuck Fermentation

Stuck fermentationWhat do you do when you get a stuck fermentation?  For some the process of fermentation is a bit like the dark arts, what really goes on in that churning barrel is a mystery, but when it has finally finished working its magic the results are fantastic.  But whilst that magic is working its best left well alone.  However, for others fermentation has to be constantly monitored and tweaked to ensure absolute quality in the final beer.  For me, I sit somewhere in the middle of the two groups.  I find that there is always a sense of reassurance gained from the gentle gurgling sound that emanates from the fermentation lock.  As long as that sound starts and doesn’t finish too quickly I am happy that the fermentation is moving along quite happily and I don’t have to interfere risking potential contamination.

However, there are occasions, and I have to be blunt, usually only when I am extract brewing, when the re-assuring gurgle starts and stops a bit too quickly or, even more ominously, is conspicuous by its absence and then I have to do something about it rather than risk losing a whole batch of beer.

Be re-assured fermentation is a fairly straightforward process and when everything is right, it is rare for it to get stuck but in the unlikely event that it does the question that often gets asked is what can you do.  Can the situation be rescued and a perfect brewed beer produced?  The answer has got to be yes.

First things first what causes a stuck fermentation?  Generally if fermentation stops too early it will be down to two main issues, temperature and nutrients.

Fermentation is a biochemical process where sugar is converted into alcohol by yeast.  To do this the yeast needs to be healthy and provided with the correct conditions and all the nutrients required to sustain fermentation.  If any of these components are missing fermentation will stop.


Some home brewers will worry if a vigorous fermentation has not started within hours of adding the yeast.  However there is no need to be unduly concerned if fermentation is sluggish in the first 12 to 24 hours, especially if you are using dried yeast.  It is usual, when using dried yeast, for fermentation to take a little longer to become established as the yeast goes through a phase of re-hydration.  Let’s face it you would be a bit sluggish if you had been freeze dried and then suddenly re-hydrated.  However, after 24 hours your fermentation lock should be merrily bubbling away.  If it isn’t you could try giving the whole fermenter a good rouse with a bit of vigorous stirring.  This will drive any yeast that has sedimented out to the bottom of the fermenter back into the wort but also the oxygen forced into the wort through the process of rousing is essential during the early stages of fermentation.  Why is oxygen important at the start of fermentation?  We discuss this further in an article on wort oxygenation and olive oil which is worth reading if you would like more information.

If your brew is still not fermenting it might be that the nutrient balance in the wort is wrong.  The two main factors to consider with nutrients are free amino nitrogen and the sugar profile of your wort.  If you are using extract then you are very much stuck with what the extract manufacturer has produced.  However, if you are whole grain brewing, and you have a stuck fermentation you might need to review your malt analysis and mashing profile.  To increase the levels of FAN in wort it might be necessary to mash in at 45ºC to 52ºC to encourage the proteases in your malt to liberate FAN.  However, if you don’t have a mash tun where you can control the temperature then this can be difficult to do.  If you think FAN might be an issue check your malt analysis certificate if the figure is over 150 mg/l then you should be ok.  However, if it is too low and you can’t mash in at the lower 45ºC – 52ºC mashing in temperature then consider changing your malt.

In terms of the sugar profile, if the temperature of your saccharification rest is incorrect then fermentation can stop early as the level of fermentable sugars is too low.  The ideal mashing temperature for ensuring a well balanced sugar profile is 65ºC and you should ensure that this temperature is maintained throughout mashing.  Mashing for too short a period can result in insufficient conversion of starch to fermentable sugars.  If you think this may be a problem then next time you mash consider doing a starch end point test.  Many brewers use this simple test to confirm the end of mashing.  The test involves taking a small portion of the mash from the mash tun, placing it on a white surface and mixing with a drop of iodine solution.  If there is unconverted starch in your mash you should see the presence of a black colour where the iodine has stained the unconverted starch.  If this occurs continue mashing for a further 10 – 15 minutes and then repeat the test.

If all else fails and you believe that the wort that you are trying to ferment is suffering from nutrient deficiency then it is possible to save the day by adding some yeast nutrient and a gluco-amylase both of which can be purchased from most home brew shops.  The yeast nutrient provides a blend of nitrogen, vitamins and minerals which should be sufficient to restore vigour to the sorriest of yeast.  Gluco-amylase is an amyloglucosidase which will breakdown larger sugars, which are not fermentable such as maltotriose and dextrins into glucose which is highly fermentable.  These actions should kick start the most stubbornly stuck fermentation.  Although be warned once added, unless you are going to pasteurise your beer the amyloglucosidase will keep on working releasing fermentable sugars until they are all consumed.  This will lead to a very dry beer but also can lead to some unwanted surprises if you bottle too early.

Finally the temperature of fermentation is critical and should be maintained as much as is possible at a consistent level.  Most instructions will refer to a fermentation temperature between 18 – 25ºC and this is ideal.  If the temperature drops too low then fermentation will slow and there is a strong possibility that it will stop.  Keep a thermometer by your fermenter, if you notice the temperature drop move the fermenter to a warm location.

With a stuck fermentation there is no quick fix and you might have to experiment with a few of the suggestions discussed above to get your fermentation started.

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