Prevent a Stuck Fermentation

I’ve been working on this recipe for an Imperial Breakfast Stout (yeah I know, Imperial anything is sooo 2008).  The problem is that’s when I brewed it!  For almost 6 weeks now my silky smooth, chocolaty, coffee-buzz-inducing friend has been stuck in the fermenter.  Stuck at 1.034 specific gravity – not exactly what I was aiming for.

When something like this happens I start wracking my brain trying to figure out what could have gone wrong.  But, before we can figure out what caused the problem we need to be sure there actually is one.

Brewers measure attenuation – to calculate the percentage of the sugar in the wort was consumed by the yeast – using the formula:

Apparent Attenuation = (Original Gravity points – Final Gravity points)  / Original Gravity points x 100.

Drop the 1. from the front of your gravity readings to determine the points – 1.085 is 85 points.

For my breakfast stout, the starting gravity was 1.085 and it’s now stuck at 1.034. So, (85-34)/85 x 100 = 60% Apparent Attenuation.

The yeast manufacturers provide a range of attenuation for each strain.  In this case the strain I used should have resulted in 73-77% apparent attenuation.  Not good.  If I had reached the minimum attenuation level for the yeast, the gravity should be about 1.023 – really not good. If you’re within 2-3 points on either side of your target that’s ok. In this case we’re off by 12.

This is called under-attenuation.  When the final gravity is lower than expected then you have over-attenuation.  The terminology can seem kind of backwards if you’re just looking at a final gravity number.  You could think of it as the yeast under-performed, or over-performed.

So what could have gone wrong?  Here are some common causes for attenuation problems – and what you can do to prevent them:

• Yeast Pitching Rate.  A yeast cell count that is too small can stress the yeast and lead to incomplete fermentation.  Jamil at Mr Malty has an excellent guide for yeast pitching rates.
• Yeast Strain.  Each strain has a range of attenuation, measured in %.  Try a strain with a different Apparent Attenuation range.  You can find the ranges on the manufacturers’ websites.
• Fermentation Temperature.  Each yeast strain also has a recommended temperature range.  Sometimes fermenting at or below the bottom of the range can cause a sluggish fermentation – particularly if you didn’t pitch enough yeast.
• Inconsistent Fermentation Temperature.  Rapid fluctuation in temperature – several degrees up and down during the day – will cause the yeast to slow down or go dormant before fermentation is complete.  If you have a programmable thermostat in your house, be sure your fermentation area stays constant during it’s cycles!
• Incomplete Mash Conversion (AG/PM only).  Not all of the starches in the mash were converted to sugars.  Be sure there are no large clumps of grain in the mash tun.  Allow plenty of time for all of the starches to convert.  Try an iodine test to check for complete conversion.
• Mash Temperature (AG/PM only).  The saccrification rest temperature will affect the fermentability of the wort.  Lowering the temperature a few degrees (3-5ºF) will give you a more fermentable wort.  Conversely, raising it by the same amount will lower the fermentablity.  For more information on mash enzymes and fermentability check out this excellent episode of BrewStrong.
• Extract Brand.  Not all brands of extract have the same level of fermentability.  The brewer that produced the extract selected a specific mash temperature which affects the fermentability of the extract.
• Simple Sugars.  Cane, beet, or candi-sugar are almost 100% fermentable so they can be used to raise attenuation beyond the normal range when substituted in place of a little malt or extract.  Don’t exceed 10% of the total grain bill or they could introduce some additional flavors which may not be desired, depending on the beer style.  If you have problems with over attenuation, and your recipe contains simple sugars, try reducing the amount.
• Amylaze enzyme (or Beano). Can be used to further break down sugars, making the wort more fermentable.  Use it in the mash or pre-boil with extracts.  Boiling will denature the enzymes and stop the reaction, but the results can be unpredictable, so this isn’t really an ideal solution.

And some things you can do with a beer that didn’t ferment correctly:

• Pitch Actively Fermenting Yeast.  Make a starter and allow the fresh yeast to become active before pitching.  This will only work if there is actually some sugar left for them to eat.
• Blending. This is probably the best way to fix attenuation problems post fermentation.  Brew up another batch of a similar or complementary style with a lower final gravity.  Then blend small amounts of the 2 batches in varying percentages to find a good flavor profile.  Apply that blending percentage to the full batches.
• Amylaze enzyme (or Beano).  You can use them at this stage, but it’s not ideal.  Adding enzymes to the fermenter will eventually cause over-attenuation as there will be no way to stop the reaction.
• Malto-dextrine or Lactose. Unfermentable sugars can be added to an over-attenuated beer to return some body and mouthfeel.  Malto-dextrine doesn’t really have a flavor, but lactose is used for milk or sweet stouts.  Choose whichever one is appropriate for your recipe.

One final note on over-attenuation. Don’t try to fix it by stopping fermentation early.  There are a number of other compounds present in the beer that the yeast will try to clean up after they have finished munching on all of the sugars.  If you try to artificially stop fermentation early then you might end up with some off flavors.

So what was the problem with my breakfast stout?  A combination of things.

1.  Incomplete conversion.  The 10 gallon mash tun was completely full with 20 lbs of grain and 6.25 gallons of water.  This made it tough to stir and there were lots of clumps of grain near the bottom.

2. Inconsistent fermentation temperature.  I was out of refrigerated fermentation space but the temperature in the house had been perfect for several days leading up to this brew.  Of course the weather changed a few days in and it didn’t hold steady.

To fix the problem, I’m going to brew up another batch of session strength oatmeal stout and mash a little lower so that it dries out a bit more.  Then I can blend them to get back to the original result I was looking for.  I could also use some for a stout-reduction sauce, or stout floats!

Tagged as: attenuation, brewing, fermentation, fixes, home brew, over-attenuation, stuck, under-attenuation