Originally Posted by wortmonger
Diacetyl is amplified with pressure.
I agree with this statement. My first batch under pressure ferment was an English Pale Ale at 1.040 OG headed for 1.014-1.012. I ran the primary at 5psig. It was awesome. Every hydro sample was tastier than the last. Best beer I had ever made.
When it got to 1.018 I cranked up the spunding valve hoping to get it to 25psi at room temp. I worked out that if i could get to 25psi room temp I would have 10-12psi in the keg once it was cooled, and I would be ready to serve. The ferment crapped out at 18psi, that is I got the final gravity before I got the pressure I wanted. So I forced it the last few psi after it was cold - but I started tasting diacetyl as the pressure went up in the end of the ferment; and the diacetyl flavor never went away.
So I went back to the wiki.
Here is the wiki article I am about to quote:http://www.homebrewtalk.com/wiki/index.php/Fermenting_Lagers
Here is the relevant text I followed:
Section 2 para 6, Conventional ... German
During the secondary fermentation (a.k.a lagering) the tanks are closed and the pressure build-up is controlled by a pressure sensitive bleeder valve. This system, called Spundungsapparat, ensures the proper carbonation of the beer during lagering. The German Purity Law prohibits the use of non-fermentation CO2 for beer carbonization. It is also more economical for a brewery to use the CO2 produced during fermentation.
So it must be my fault.
So I started another batch, a Roggen. I ran the primary at 25psi. All was right in the world, I scored a very tasty example recipe of a style that is going to go with lots of different food. Again, every gravity sample was awesome.
Target carb level is similar to a hefe, so I was looking for 32-35psi at room temp, so I could cool and serve. But two hours after I turned up the spunding valve the butter flavor was back.
My diacetyl was creeping up with the pressure again. So I looked at the wiki again, and just stared and stared at this drawing. And stared.
Here is the key:
solid line: temp in Centigrade
dot/ dash dot/ dash extract%, fermentable sugars remaining
dash/ dash/ dash/ dash diacetyl amount
dot/ dot/ dash/ dot/ dot/ dash gauge pressure x 10. bar.
So I turned the pressure down. Though it is early it seems like the diacetyl is going away. Probably I'll have to force carb the Roggen, but that beats pouring it out.
Does it seem to you that the picture and the text are pointing in two different directions?
What variables could change to make both sets of instructions correct some of the time?
EDIT: 02-09-08. After six days at 12 psi the diacetyl is gone and I love my Roggen again. I'll have to force carb it the last few psi to get it to serving pressure, but I am not going to have to pour it out. The week at 12psi not only got the diacteyl back out, but now I can taste the actual rye distinct from the barley that was last aparent in the mash tun. My gravity was 1.020 when I walked the pressure down from 25psi to 12. I have a gravity sample off gassing on the counter top now.