Priming: Dissolved CO2 vs Temperature with one really low temp day

[everfrost]

EDIT: Yeah I got it backwards...thanks to dontman for clarification, but I'm leaving my original post below as is...

I know when figuring the amount of priming sugar, one aspect to look at is the amount of dissolved CO2 already present in the beer. This of course is based on temperature...

At one point for a day or so in the past (during a month or so secondary) my temperature dropped a lot lower than usual (thanks to a power outage and winter weather). Let's say it dropped to around 42 to 45F, but usually the temp was around 52 to 55. So the question basically is this...which temp do I use for figuring the amount of priming sugar? My assumption would be the lower one since, I would think that, once released from solution, the CO2 is gone whether the temperature goes back up or not.

This is probably a moot point, but I've worked hard on this brew and don't want it over or under carbed. Thoughts?

Thanks,

Chad

 

[dontman]

You've got it backwards. As the temp lowers dissolved co2 increases, so you should use the higher temp. But you are correct, once the co2 is gone the beer does not reabsorb it if the temp falls.

This is because the higher the temp the lower the amount of co2 in the beer. So you always use the warmest temp that your fermenting beer stabilized at for a couple of day period to determine priming sugar quantity. A good example is a lager which spends most of its fermenting period at around 50. If you do not do a diacetyl rest then 50 is the proper temp to use to determine priming qty. But if you raise the temp of the beer to 70 for 72 hours for a diacetyl rest then you should use this as the temp for calc because it will dump a lot of the co2 that was held in suspension while it sits at 70 degrees. When you drop the temp back to 36 for the lagering period it does not reabsorb the co2 that it lost.

 

[everfrost]

You've got it backwards.

LOL...Too much coffee and not enough beer...Sorry for the brain fart and thanks so much for confirming my suspicions. I knew what I was trying to say but it came out backwards...

 

[Bobby_M]

...When you drop the temp back to 36 for the lagering period it does not reabsorb the co2 that it lost.

I'm wondering if this 100% true. I understand why the beer absorbs CO2 during fermentation. It's bubbling up through the beer at atmospheric (or a very slight amount more) pressure. Raising the temp would certainly encourage some CO2 to come out of solution. I wonder though, what if you lowered back to the original temp and waited say 3 weeks. You're no longer bubbling CO2 through the beer, but the headspace is still CO2 at atmospheric pressure and given enough time, it should absorb, just like kegging set and forget methods. The only curious thing here is that there isn't a constant new supply of CO2 in a closed container so I guess the headspace goes to a vacuum state pretty quickly.

I have no idea. I really haven't put much thought into the residual CO2 concept.

 

[everfrost]

I have no idea. I really haven't put much thought into the residual CO2 concept.

I smell experimentation!!!!!!!!!!!!!

 

[dontman]

I'm wondering if this 100% true. I understand why the beer absorbs CO2 during fermentation. It's bubbling up through the beer at atmospheric (or a very slight amount more) pressure. Raising the temp would certainly encourage some CO2 to come out of solution. I wonder though, what if you lowered back to the original temp and waited say 3 weeks. You're no longer bubbling CO2 through the beer, but the headspace is still CO2 at atmospheric pressure and given enough time, it should absorb, just like kegging set and forget methods. The only curious thing here is that there isn't a constant new supply of CO2 in a closed container so I guess the headspace goes to a vacuum state pretty quickly.

I have no idea. I really haven't put much thought into the residual CO2 concept.

Bobby, you're right that the beer will reabsorb some readily available co2. The reality is that there is only a miniscule amount of co2 to reabsorb and the pressure will never drop below atmosphere so that leaves even less available to reabsorb. For all intents and purposes then using the warmest temperature that the wort sits at is going to be closest to accurate.

Especially when you look at the numbers. Using my lager numbers Beersmith calls for only 2.3 oz of priming sugar to achieve 2.4 volumes final carb. But if you did a diacetyl rest at 70 for 72 hours and use that as the temp then BS calls for 4.4 oz of priming sugar. I guarantee if you primed based on the 36 degree temp with only 2.3 oz of sugar you would know instantly that your beer is nowhere near 2.4 vols of carb.

The numbers are just as dramatic with force carbing. When I do my beers I always do a 1-2 week cold crash/ bulk age immediately prior to kegging and I then rack to keg cold at 38 degrees. If I were to follow BS recommendations for a English Mild and used the temp at kegging to determine force carb pressure then it says I should set my reg at .7 psi to achieve 1.5 levels of carb. In practice this just doesn't work. Even if I up the carb level to 2.0 vols BS is telling me to set my reg to 5.9 psi. I've done this and the resulting beer was just not carbed to 2 vols.

I realize that I am citing anecdotal rather than scientific evidence but I trust my palate and believe I am correct here.

 

[Bobby_M]

I trust you. I'm just running through the mental exercise. I hardly ever bottle condition so it's almost a moot point for me and I never brew lagers without doing a diacetyl rest anyway so 65F is always my reference point.