Why is my attenuation so low?

Question

Here's the recipe:

> Recipe Specifications
> -------------------------- 
> Boil Size: 7.37 gal Post Boil Volume: 6.38 gal Batch Size (fermenter): 5.50 gal    Bottling Volume: 5.50 gal
> Estimated OG: 1.037 SG Estimated Color: 22.8 SRM Estimated IBU: 21.3
> IBUs Brewhouse Efficiency: 72.00 % Est Mash Efficiency: 81.9 % Boil
> Time: 60 Minutes
> 
> Ingredients:
> ------------ 
> Amt                   Name                                     Type          #        %/IBU          
> 4 lbs 8.0 oz          Pale Malt, Maris Otter (3.0 SRM)         Grain         1        62.1 %  
> 1 lbs                 Brown Malt (65.0 SRM)                    Grain         2        13.8 %        
> 12.0 oz               Caramel/Crystal Malt -120L (120.0 SRM)   Grain         3        10.3 %        
> 2.0 oz                Carafa III (525.0 SRM)                   Grain         4        1.7 %         
> 2.0 oz                Chocolate Malt (350.0 SRM)               Grain         5        1.7 %         
> 12.0 oz               Invert Sugar #3 (65.0 SRM)               Sugar         6        10.3 %        
> 1.00 oz               Goldings, East Kent [5.80 %] - Boil 60.0 Hop           7        21.3 IBUs     
> 1.0 pkg               London ESB Ale (Wyeast Labs #1968) [124. Yeast         8        -             
> 
> 
> Mash Schedule: Single Infusion, Full Body, Batch Sparge Total Grain
> Weight: 7 lbs 4.0 oz
> ---------------------------- 
> Name              Description                             Step Temperat Step Time      
> Mash In           Add 9.75 qt of water at 164.0 F         154.0 F       60 min        
> 
> Sparge: Batch sparge with 2 steps (2.09gal, 3.75gal) of 168.0 F water

Final gravity is 1.014, giving an apparent attenuation of 65%

Fermentation was done at ambient basement temperature -- around 62 F.

After four days of fermentation, I racked the beer off the yeast even though the krausen had not yet fallen. SG was 1.015 and the beer was crystal clear.

I've tried a couple things to remedy the low attenuation. First I re-pitched a quarter cup of 1968 slurry thinking that perhaps I'd removed the beer from the yeast too early. This had no effect. Thinking that maybe the high flocculation yeast was the problem, I pitched a packet of Nottingham dry yeast. Again, no effect.

I have to conclude that there is something about the recipe that caused the low attenuation.

The invert sugar is home-made. I heated a sugar solution to 204 F. and held it there for 20 minutes. I then mixed in blackstrap molasses (20 grams to 480 grams of sugar) for colour and flavour.

My notes show that I hit my mash temperature of 154 F almost exactly, and after 60 minutes the temperature had dropped by 3 degrees.

The crystal only comprised 10% of the grain bill. I'd expect it to raise the FG somewhat, but 65% attenuation when 10% of the fermentatables are sugar is a bit ridiculous.

I'm going to drink the beer as is. It's mighty tasty if a bit too sweet. My concern is that I'm going to make and ESB with a similar grain bill and don't want the same attenuation problems.

Answer 1

Wyeast 1968 is not a high attenuator to begin with - 67-71%, and has a temperature range of 64-72F (source).

I think your temperature of 62F is the main culprit, especially if it's fluctuating, although there are other factors that contribute to the low attenuation. There are several changes you might consider next time:

• since you're using an English strain, let it show it's true colors by giving it some warmth! 62F is too cold for this yeast - you want to be at least 64, preferably 66 or 68F.

• if you can't increase the temperature, then increase the pitching rate to help get in the upper range of attenuation.

• reduce the crystal to 5%

• consider mashing at 152F to produce a more fermentable wort

• Your invert sugar is not likely to be the problem, although including simple sugars can make the yeast less inclined to ferment the maltriose, further decreasing attenuation. (At best, ale yeasts only ferment up to 1/3 of the available maltriose, which typically constitutes 15% of the sugars, low attenuating strains even less.)

I think temperature is the main cause, but these other points also contribute.

Answer 2

If you have a pH meter, I suggest checking the pH. With a beer this small, it's very possible the grain wasn't sufficient to bring the pH down to levels the yeast agreed with. And seeing your target OG was low and 37% of your malt bill is grain that's already converted, it's possible that you wouldn't have noticed a big dip in efficiency. This would explain the failure of the Nottingham to take off as well. Between 4-4.5pH is expected in finished beer.