We've been planning this one for a while. We wanted to make a big stout, better than ever before. We wanted to see how far we could push our house yeast, just to see what it would do.
This was to be brewed with a peated malt I'd come by. However, we don't have a mill and my initial over optimistic view of the task soon turned out to be somewhat false. The food processor just didn't do the right job and so I set to with a rolling pin to manually crush 25kg of malted barley. 8 hours over 2 days saw significant temporary RSI.
We hoped it would get to 10% ABV. We aimed for an OG of 1100, or there about. In the end we managed 1098.2, which we thought to be close enough.
Of course, the final ABV of any beer is dependant on how well the yeast attacks the sugars. This in turn is dependant on a number of factors including enzyme action in the mash, yeast pitching rate, oxygenation of the wort, nutrients in the wort and no doubt a few more bio-chemical things that I don't understand.
As our team consists of not only me, with my 10 years of experience but also Scott, who is good at analysing past performance of brews and successfully improving on what we've already achieved. And then we have Sarah, who has done a degree in microbiology specialising in brewing yeast. Between us, after several discussion over coffee, donuts and QA samples of Azimuth, we devised a mash temperature, glucose dosing, aeration and pitch rate program that we though should do the job.
We took gravity readings every 12 hours, as is good practice in any yeast management strategy. We also remembered to write them all down carefully in perfectly legible writing every single time1.
Most importantly we watched them gravity drop down towards the target that we needed to achieve our goal of 10%. Using the HMRC recommended method we calculated the target PG (present gravity)
(OG-PG)*0.133 = ABV (at target ABV)
So, by rearranging the formula;
ABV/0.133 = OG-PG also known as attenuation.
ABV/0.133 = 75.2 degrees of attenuation.
PG = OG - 75.2 = 1098.2 - 75.2 = 1023
I was a little bit nervous. It was the first time we'd pushed our house yeast this far3. Would it fall over as the alcohol pickled the yeast? Would the yeast run out of the nutrients it would desperately need to keep going? Had we grown enough fresh new daughter yeast cells with the vigour needed to get the job done?
The gravity closed quickly towards 1022.4, and then romped on past it. Downwards past 1020, 1019, 1018 and further. Actually, it went incredibly fast, so fast that I wondered if it would become a stupidly dry beer, like Granite 2013 ended up to be. But no, all of a sudden the yeast stopped fermenting and the PG remained steady at 1016.6. The thing is, not only does the extra attenuation mean more alcohol, it also means that we need to use a different "f" number to calculate it;
ABV = (OG-PG)*f = (1098.2 - 1016.6) * 0.134 = 10.93%
As it'll probably do a small amount of further maturation in bottle, we felt it appropriate to declare 11% dead.
I'm keen to now have a go at pushing our yeast still further. At 1016.6, starting at 1098.2 I have a fair degree of certainty that yeasty has mopped up all the available fermentable carbohydrates. I was scared the yeast would give up due to alcohol poisoning, but no, those little critters have done me proud.
In related news, we've are so proud of what our house yeast has done that we've given it a twitter account. Follow @HardknottYeasty and you might get an insight into what the yeast is up to in the tanks.
1Yes, you are perfectly right, that was a lie. Mostly we have a collection of people who are very intelligent, well educated and very fluent in both English2 and Mathematics. However, calligraphy ain't our strong point.
2Well, Scott is fluent in a version of English that has gotten corrupted by them people over the other side of the big pond. Still, he's quite eloquent with it, for an American.
3Previous beers that were up at this ABV we've used a combination of yeasts to get to the ABV, generally including dried yeast. We've only been using our current strain of yeast for just over 12 months.