Recently, I brewed a classic Irish stout and to my dismay, my pre-boil gravity reading was about 2 points below what it should be. This was the second time brewing a stout and it was the second time my gravity came in low. My calculated efficiency that I was getting was only about 62%, which is about 10 points below my usual 72%. What’s going wrong? The problem was that I was ignoring the diastatic power of the mash.
Discovering Diastatic Power
Well, I asked some friends and I did a bit of research, and I discovered that the diastatic power of my stout mash was probably low. (Just when I thought that brewing beer couldn’t possibly get any more complicated, there is the diastatic power of the mash to think about.)
The diastatic power (DP) is the ability of the mash to convert starches into sugars. Brad Smith does a fine job describing the importance of DP in the mash, so I won’t go into too much other than to say that there appears to be a direct relationship between the diastatic power of the mash and conversion efficiency. According to Smith, DP has to be at least 30 Lintner (°L) to convert properly, and others would argue that it has to be as high as 40°L.
Maris Otter formed the base malt in my recipe comprising only 62% of my malt bill, and I was surprised to learn that Maris Otter typically has a Lintner value of about 50. (There is conflicting information about the DP value in Maris Otter, and unfortunately, Munton’s does not provide the value for us. But my research put me anywhere from 40°L to 62°L). If I use a Lintner value of 50 for the Maris Otter, then calculations of the total Lintner for my batch was only 29.4°L. This is likely the reason why I’m getting much lower efficiency than what I’m typically used to: the mash simply didn’t have enough enzymes to convert the mash in the 60 minute time-frame. I would likely require a 90 minute to a 120 minute mash to do the job that I typically do in 60 minutes!
Wow! That’s what I love about this hobby. I never stop learning.
Batch Lintner is calculated using the following formulae:
US Customary Units:
Example Grain Bill:
Assuming a specific gravity of about 1.042
5.7 lbs of Maris Otter @ 50°L = 50 x 5.7 = 285°L
2.5 lbs of Flaked Barley @0°L = 0°L
1.5 lbs of Roasted Barley @0°L = 0°L
285 + 0 + 0 = 285
5.7 + 2.5 + 1.5 = 9.7
285 / 9.7 = 29.4°L
Europeans would know the Windisch–Kolbach (°WP) units:
For those using °WP, you would need a total batch DP of about 120°WP to convert the mash in 60 minutes.
Inventing a New Formula
Disclaimer: there is no science to back this up, so take it for what it’s worth. But, there is a formula buried in there somewhere.
It stands to reason that if a mash requires 40°L to convert a batch in 60 minutes, (assuming a linear model which probably is not very accurate), half of the Lintner value would take twice as long to convert the mash. Twice as many would theoretically convert the mash in half the time. Using that as a basis for my data, I can create the following formula:
Where L = Batch Lintner
Update – 8 January 2016
Proof of my theory: the stout batch as described above didn’t attenuate quite like I planned, and it confirmed that the mash likely didn’t convert like it should have. The final gravity of the batch came to about 1.011, and I was expecting something in the order of 1.008 or even lower. The calculated attenuation was about 69.5% and I was looking to get between 73% to as high as 85%. The beer is still delicious, and I’m happy with the results, but I can’t help but wonder if it would taste even better with a longer mash. I’ll certainly make the Guinness Stout Clone recipe again, and I’ll report back the results on this page.