Is conventional D-rest time way too short? A question on 'out-gassing' after a stable FG is reached

[stealthfixr]

I have noticed that after a 2+ day diacetyl rest @ 70F with a pressurized fermenter (Spike CF), that for a week+ after a stable FG is reached, the interior pressure will 'creep' upwards for almost a week or more longer with no change in terminal gravity. In the fermentation profile below, this out-gassing for this American Amber with Imperial Joystick yeast continued long after the 2-day D-rest ended on 24 Apr. From 24 Apr forward, I relieved the pressure to 6psi every morning (and reattached a spunding valve in addition to a PRV--no safety concerns here). From 24 Apr until today, the pressure had increased, sometimes a lot. But, this out-gassing has slowed over time, but this example is consistent with previous batches that exhibited the same characteristics.

I have noticed this with multiple yeast types, even lagers. What I am wondering is if the 2-day D-rest rule of thumb isn't shorter than it should be, by much more than just 2-days. I am not sure what exact activity is causing the pressure to increase with no chance to terminal gravity, but I have to think it is the yeast in cleanup mode. Any thoughts?

 

[Andre3000]

I'm not an expert, but I don't think off gassing of CO2 is necessarily related to uptake of diacetyl. Did you try a forced diacetyl test? Probably the most reliable way to tell if your beer needs more time. A mason jar works great:
https://escarpmentlabs.com/blogs/resources/the-forced-diacetyl-test

Not sure if I understand what you're talking about with regards to the pressure increasing. If the beer was at equilibrium at pressure higher than 6psi, it stands to reason that "off gassing" it to 6 psi every morning took a long time to stabilize to 6 psi. Dissolved CO2 can take awhile to come out of solution in a closed vessel. And if you've been bleeding it off, you never actually did reach its true equilibrium.

I'd be interested to hear what others think.

 

[Deadalus]

Your gravity is not flat until after ~4/27. It dropped slightly after 4/24, which you are stating is the end of a 2 day D-rest. You have a minor temp increase around 4/19 and then it looks like you warmed things up on 4/21.

What data did you consider to start to warm the fermenter? Looks like you decided it had hit ~75% attenuation. What's the range of attenuation for the yeast you used? It still dropped about 4 points after 4/21. From a mathematical curve standpoint, the curve wasn't stable there, the slope was negative there.

 

[gabrielrskin]

Best way to determine your diacetyl rest length is by taking a sample and tasting it, or doing the forced diacetyl test - instead of following a predetermined "x days" rule. If it's clean, move on with your process.

 

[Dland]

On some of my cold fermented lagers (primary ferm temp around 52F), CO2 pressure continues to increase a bit for up to two or two and a half weeks during spunding/D rest process. Normally max temp I reach during this time is 64-65F.

I used to go with a set 3 day ramp up, 3 day D rest 3 day ramp down, now I let it run until pressure stops rising. I generally don't keep spunding valve on kegs, I just check the pressure with gauge every other day or so.

Not sure how much difference it makes, but I know the beers I'm making now are some of the cleanest tasting I've ever made.