Cold crash bottling

I haven't had any bottle conditioning issues after cold crashing. There are multiple reports from others scattered around HBT that have the same experience.

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No, you should not adjust the priming sugar amount because of cold crashing. The starting CO2 level in the beer prior to bottling will be determined by the highest temperature the beer experienced while fermenting, and that is the temperature that should be entered into a temperature compensating priming calculator (like the one at BrewersFriend.)

In order for cold crashing to significantly change the CO2 level in the beer, you would need to cold crash for a week or longer, and have to keep replenishing the CO2 in the headspace as it got absorbed into the beer. Neither of these conditions is usually present during homebrew cold crashing. The explanation is a little involved, but if you are interested, then read on.

Let's do an example: In a typical fermenter the headspace is about 20% of the beer volume, so if all of the CO2 in the headspace were to dissolve into the beer, it would add 0.2 volumes to the carb level of the beer. If the fermentation finished at 68°F, then we would have a carb level in the beer of 0.75 volumes (according to the BrewersFriend calculator.) Now we cold crash to 34°F, where the equilibrium carb level at 0 psi is 1.56 volumes. If half of the CO2 in the headspace were to dissolve into the CO2, then the carb level would be 0.75 + 0.1 = 0.85 volumes, and the CO2 pressure in the headspace would drop to -7.35 psig (= 14.7 / 2 = 7.35 psia.) However, the CO2 pressure required to achieve 0.85 vols at 34°F is -7.1 psig = 14.7 - 71 = 7.6 psia, so we cannot even get half of the headspace CO2 to dissolve into the cold crashed beer. And, equilibrium would require about 3 weeks to achieve.

If you were looking for a carb level of 2.5 volumes in your bottles, and entered a beer temp of 34°F into the calculator, it would say that you need to add enough sugar to create 2.5 - 1.56 = 0.94 volumes. However, your actual starting carb level is not 1.56 but somewhere between 0.75 and 0.85, closer to 0.75. Your bottles would end up with something like 0.76 + 0.94 = 1.7 volumes. Thus they would be severely undercarbed.

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I did not consider the fermenter a sealed container. Any reduction in CO2 pressure (from temp drop or absorption into the beer) will cause air to suck back thru the airlock. So, the total pressure in the headspace remains at 14.7 absolute psi absolute, but the only thing that matters for carbonation is the CO2 partial pressure (the fraction of the total pressure due to CO2.) At the end of fermentation, no more CO2 is being generated, so the amount of CO2 in the fermenter (beer + headspace) is nearly constant. CO2 that absorbs into the beer will reduce the amount of CO2 in the headspace, and thus also reduce the CO2 partial pressure.

I say CO2 is nearly constant because if the fermenter is subject to thermal cycles (heating and cooling) then during heating headspace gas will be expelled thru the airlock, and during cooling air will be sucked back into the headspace. Headspace gas expelled during heating will contain some CO2, causing the total amount of CO2 in the fermenter to decrease. I did not take this possibility into consideration for my calculations. A small amount of CO2 will also escape by diffusion thru the water in the airlock. This was also not considered.

Does this answer your question?

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Correct.

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