Ok folks, here's the deal: CO2 is created a molecule at a time during fermentation, and those molecules are in solution. In order for bubbles to form, the CO2 content in the beer must be higher than the amount that would be in equilibrium with the CO2 partial pressure in the headspace (plus hydrostatic pressure from the liquid column and the
Laplace pressure inside a bubble, but let's just ignore those since it doesn't affect the analysis.) So, bubbles only form if there is excess CO2 in the beer w.r.t. the CO2 in the headspace. CO2 can also escape (or enter) the beer, without any bubbling, via diffusion across the surface. Again if the CO2 content in the beer is in excess of the amount that would be in equilibrium with the headspace CO2 partial pressure, then there will be a net diffusion of CO2 out of the beer. Conversely, if the beer CO2 content is lower than the equilibrium amount, then there will be a net diffusion of CO2 into the beer (this is what happens during forced carbonation.)
When fermentation starts, the headspace is air, with very little CO2 content. During fermentation in an airlocked vessel, all of that air is swept out of the headspace. I did a detailed analysis of how effective the fermentation is at removing air from the headspace
here. At the end of fermentation, the headspace is essentially pure CO2 at atmospheric pressure - 14.7 psi absolute pressure or 0 psi gauge pressure (unless a spunding valve was used.) Thus at the end of fermentation, when no more CO2 is being produced, CO2 will diffuse out of the beer until the carb level is in equilibrium with CO2 at 14.7 psi partial pressure, or 0 psi gauge pressure. This carb level is what the charts show for 0 psi at the temp of the beer.
Now if you chill the beer, you will reduce the partial pressure of CO2 in the headspace, and if the headspace partial pressure is higher than the equilibrium pressure for the current carb level and beer temp (spoiler: it is) then more CO2 will absorb into the beer. But, as you absorb more CO2 into the beer from the headspace, the CO2 partial pressure drops even more. At some point (after a few weeks) everything comes to equilibrium. So, the "excess" carb level due to CO2 in the headspace after extended cold crashing will be lower than if the headspace CO2 partial pressure was maintained at 14.7 psi. The detailed analysis I did for this is
here. The takeaway is that if you use the beer temp after a short cold crash (little additional CO2 absorption) to calculate your priming sugar, you will under carb your beer by about 0.8 - 0.9 volumes. On the other hand if you did a cold crash long enough to reach equilibrium, and you use the fermentation temp to calculate your priming sugar, you will over carb by less than 0.1 volume. If your cold crash is only a couple of days, then your over carb will be much less. So, use your fermentation temp in the priming calculator. Calculators that tell you to use the current beer temp (if colder than the ferm temp) do so out of ignorance.
Adding a balloon complicates the analysis (how much CO2 does the balloon hold, how much O2 & N2 diffuse into the balloon during fermentation [rubber balloons are terrible diffusion barriers, and O2/N2 diffusion into the balloon is not affected by the CO2 pressure in the balloon], how much CO2 diffuses out of the balloon, etc.) But, unless you do a long cold crash, you still won't pick up much additional CO2 in the beer.
The amount of CO2 in the balloon isn't really that much. You will probably absorb some of it in the beer but not enough to affect how much priming sugar you should use for carbonation. I would disregard the amount of CO2 in the balloon and prime the beer as you were planning.
Yes, except use the fermentation temp.
Regardless of where it comes from (headspace, balloon, Mars), the lower the temp, the more co2 will absorb into the beer. Why do think air is being sucked into the fermenter?
I haven’t been let down with using calculators that ask for the temp of the beer.
Unless you have a sealed system that backfills with only CO2, you suck air back into the vessel (see the third link above) An airlock is not a seal. Use the ferm temp in the calculator (also the third link above.)
Air is being sucked in because gas pressure in the headspace is directly related to temperature, not because beer absorbs more CO2. Beer is already oversaturated and will not increase its CO2 content once fermentation is done. Considering that beer that fermented at 20°C has about 0,8 volumes of CO2 and beer at 0°C would have 1.6 volumes of CO2 it would have to absorb a further 0.8 vols from the headspace. Unless the headspace in your fermenter is disproportionate to the amount of beer there simply would not be enough CO2 to achieve equilibrium. If in this example (ferment at 20°C, bottle at 0°C) you were to prime based on 1.6 vols you will be undercarbing your beer. If this is not the case then your beer simply hadn't reached FG and the residual fermentable extract more or less exactly made up for the difference. In other words, you got lucky.
Mostly correct. Beer will absorb some additional CO2 during cold crash (third link above), but it is insignificant in normal cases.
Okay, but I was asking for a citation regarding the solubility limit of CO2 in beer. This seems to be the focus of your argument, no? That no more CO2 is dissolved into the beer as the temperature drops, because saturation has already occurred (requiring additional pressure input to increase dissolved CO2 levels).
When the beer is cooled it is no longer saturated with CO2. Little additional CO2 is absorbed when cold crashing due to the drop off in the CO2 partial pressure as I discussed above, even after you reach equilibrium at the colder temp, and reaching equilibrium takes much longer than typical cold crash times.
Like Mr. Wolf I was curt because time spent typing is a factor, sorry if this led to some misunderstanding.
What I meant with beer is already oversaturated is that it has exactly the amount of CO2 dissolved that you would expect in a 100% CO2 atmosphere with a pressure of 1.103 hPa. But since thankfully for us we don't live in a saturated CO2 atmosphere it is oversaturated for the environment the fermenter is placed into. As you lower the temperature the beer is theoretically able to absorb a lot more CO2 but this CO2 has to come from some source since the environment is not capable of providing it. Besides that, this process is really slow (especially at cold crash/lager temperatures) so it would take some time for you too see its effects. What you do observe pretty quickly is a drop in pressure due in part to the gas becoming colder and in part to the beer contracting as it cools (somewhat mitigated by the vessel contracting as well). This is what one seeks to counteract with their choice of "cold crash CO2 gizmo".
Let's say your beer is cooled from 20°C to 0°C. This would take us from 0,8 vols post-fermentation to 1,6 vols or exactly double. If there are no more fermentables we can rule out fermentation CO2 as a further source, so all the CO2 has to come from external sources. If all we have is CO2 trapped in the headspace one of two scenarios will play out:
- the fermentor is air and pressure tight and is sealed as soon as fermentation is done. Once beer is cooled it will start absorbing CO2 from the headspace, but as it does that pressure in the vessel will decrease (colloquially referred to as vacuum) and the equilibrium point will shift towards less than 1,6 vols of CO2 being able to be absorbed. Depending on the beer to headspace ratio CO2 absorption will stop rather short of the expected 1,6 vols. In fact unless you have an inordinate amount of headspace you'll see that beer can absorb a rather small amount of additional CO2. This scenario is just a though experiment as nobody in their right mind would seal a pressure-rated fermenter without residual extract (spunding) or without attaching an external CO2 source (force carbonation).
- the fermenter is not pressure-rated and since it cannot be effectively sealed it will start pulling in air so as to equalize pressure and not implode. Assuming no CO2 escapes (rather unrealistic since if air can get in CO2 will certainly be able to escape as well) CO2
partial pressure will decrease exactly like in the sealed fermenter scenario (assuming identical fermentor geometry) and CO2 absorption will fall rather short of our goal. With the added "benefit" that beer will be exposed to O2 as well...
The only way to avoid this is to attach an external source of CO2 able to maintain a set pressure, but that's just what we call force carbonation.
In the other realistic scenario (leaky vessel) two things are certain:
- there will be oxygen ingress given time
- it's wrong to assume that beer has reached CO2 saturation for the cold crash/lagering temperature and if we use that temperature for calculating priming additions we will systematically undercarb our beers. Unless of course the beer still has residual fermentable extract that will compesate for it.
Correct. The third link above does a detailed quantitative analysis of this qualitative description.
Brew on
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