Rescuing a Stuck Fermentation

We've all been there... You've meticulously planned your recipe, put your heart and soul into a brew day, and finally tucked your precious wort into the fermenter with a nice pitch of yeast. Things kick off well, then all of a sudden everything just stops short of your expected final gravity... Damn.

Here's some advice of where to go from here to see if there's any way to kick the yeast back into action.

Confirm You're Dealing with a Stuck Fermentation

Before you take too much action, it's a good idea to do a test to determine that you are not at terminal gravity. You can do this with what's called a forced ferment.

1. Draw off some of the beer into small PET bottle, fill to the brim/squeeze out air. Shake well and leave somewhere warm (MUCH warmer than your fermentation temperature).
2. Do the same again except leave the bottle half empty and shake well to aerate the beer.

After 2-3 days you should be able to feel pressure build up if the yeast has not reached terminal. Take gravity readings and compare the difference between the aerated sample and the un-aerated one.

Ways of Kicking of Fermentation

If you've still got some sugars left to ferment out, try some of the following methods (or combinations of) to see if you can get things going again.

Agitation

Some yeast are just highly flocculant and can settle down before they're done. If you're using a simple fermentation bucket, just open the lid and give it a good stir with a sanitised spoon/paddle. If you're using something a bit more fancy like a conical or a pressurised vessel like a fermzilla, if there's a port at the base of the fermenter to attach a CO2 disconnect or tube, give it a blast of CO2 to knock the yeast back into suspension.

Raise the Temperature

If you're most of the way through the fermentation and are just looking to eek out the last few points, just raising the temperature of the beer/wort by a degree or two could be enough to help things along - this shouldn't give any off-flavours or other issues as most temperature related off-flavours originate from the growth phase of the yeast.

Combine this with agitation for an even better chance of kicking things off.

Pitch More Yeast

If temperature and agitation don't do the trick, your yeast might just be knackered.

Either use the same yeast as before, (cultured up to a good pitching rate if using wet yeast), or a neutral, high-performing yeast such as US-05, simply just pitch more yeast and see if that helps things along.

What If That Doesn't Work?

If none of the above methods work, then you probably are actually at terminal gravity and you've had some kind of mash issue where the mash has biased for less fermentable sugars. So you now have the following options to correct the fermentation:

1. Rebrew the same beer with a higher fermentability (lower mash temperature for longer) and blend the 2 finished beers together.
2. Dry hop if the style is appropriate to drop a few points.
3. Use glucoamylase in carefully dosed amounts to break down some of the long chain sugars and let fermentation resume.

NOTE: glucoamylase dosages are insane - 0.2ml in a 50L batch will take you down 5-15 gravity points.

ALSO NOTE: If you are using yeasts that get a bit fussy about fermentation conditions (i.e. some Belgian and English yeasts), amylase based solutions are not the answer.