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 Home Brew Forums > The effects of cold crashing on priming sugar needs
08-31-2009, 09:02 PM   #21
Gremlyn
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 Originally Posted by Revvy ...there are more than one way to calculate carbonation...OR maybe Palmer/Beersmith took your way do doing things (figuring in excess co2) and came up with their calculators...and the numbers ultimately are close enough for government work.
There can't be more than one way to calculate carbonation, ultimately. I'm sure you can find a few different equations to fit the bill if you wanted to, but the end result will ALWAYS be the same. Calculation of gas in solution isn't subjective like calculating IBU's. You have ONE equation for finding out how much CO2 will be in your headspace in an ideal situation which is:

PV = nRT

Otherwise known as the ideal gas law, Pressure x Volume = moles of gas x R (gas constant) x Temperature.

Say you have a sealed container filled with only CO2, a sealed steel box. As you heat said box, the volume of the gas cannot increase so the pressure inside builds up. If you happened to take up some of that volume in the box with beer, the CO2 in solution would start to come out into the headspace. Cooling it back down would push CO2 back into solution.

Now say this container is a balloon. As you heat the balloon, the volume is allowed to expand, as well the pressure increases. The amount of each of these increases depends on the external atmospheric pressure and the elasticity of the balloon (I'll stick with just theory instead of launching into calculations on this... just for you Revvy ).

Now say this container is your fermentor with an air lock on it. The container is for the most part immovable by the gas expansion. So like the box, as you heat it up you would build pressure inside. BUT, now you've got the variable of the air lock. The air lock has the ability to keep in gas to a certain pressure level, but once that threshold is reached the gas comes spilling out, never to return.

Now I don't doubt that the calculators are close enough for the average brewer, but Revvy... you're not the average brewer. If you were to bottle straight from your cold crash, those calculators cannot apply to. The best bet is to use the temperature that the beer reached at it's highest, so you can accurately measure residual CO2 in the beer. If you fermented in a container with a pressure valve that would keep the pressure inside at, for instance, 10 PSI and then cold crashed and bottled that without ever breaking the seal, then yes you would be wise to use the temperature you are bottling at.

Maybe your original point is right, but that doesn't mean your methodology is.

Quote:
 Originally Posted by conpewter I wonder if this would need to take into account the headspace filled with Co2. So if I have beer at 70* with a certain headspace sitting at 1 atm, then move it to the fridge at 40* at 1 atm then it should absorb Co2 (slowly) from the headspace while pulling in air through the airlock (after sucking the water/sanitizer/vodka in). So if I have a huge headspace it should equalize out to having the amount of Co2 in solution that you would expect at 40*, if there was not enough headspace or time, then you'll have less Co2 in solution than you would calculate.
If you have sucked all of the airlock fluid in and the fermentor is no longer sealed against air flowing in, then you're likely going to end up with little residual CO2 in your beer. The CO2 will escape out to equalise with the air now in the headspace and with air outside the fermentor. If that has happened to you, it'd be best to consider the lowest amount of residual CO2 (likely none if left long enough).
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09-01-2009, 05:53 AM   #22
Malticulous
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I always disliked not really knowing the actual residual CO2. That is one thing I don't like about a D-rest. The worst thing about bottling cold it the stiff hoses. I just adds a day or two to carbonation.

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09-01-2009, 04:30 PM   #23
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Given that some of the stuff I wrote is referenced in I may as well chime in and add my 2c worth.

This is a difficult subject and what matters is actually the temp of the beer before bottling. But there are 2 problems: stopped fermentation and CO2 contained in the head space.

Especially w/o doing a fast ferment test and a precision hydrometer it is difficult to asses when fermentation is truly complete. I have noticed gravity drops in ales while it was sitting a cold conditioning temperature of 50F. Granted, I started cold conditioning early b/c I new how much sugar was left in the beer. But I think that you still have enough residual fermentation when you crash to cold conditioning which will create more CO2 that is retained in the beer. If not, you actually have another problem.

If at the start of cold conditioning the beer completely stopped fermenting and no more CO2 is produced, the lowering of the beer temperature and the resulting greater capacity for CO2 will consume some CO2 from the head space. This means that air will be sucked in beyond the extend that can be accounted for by the temperature dependent shrinkage of the head space gas. Now you got air into your head space with the possibility of oxidizing the beer. If we assume that the CO2 lays on the beer like a blanket it offers the beer CO2 at atmospheric pressure and the beer will eventually take up enough of that CO2 blanket to reach the CO2 equilibrium that is shown in the tables and normographs that you mentioned.

In the end it doesn’t matter what the beer fermented. What matters is the current beer temp.

BUT, you need to keep in mind that the CO2 exchange between beer and environment is a slow process and that those tables are only accurate if the beer sat long enough on its current temperature for the equilibrium to be reached. That means that you can’t expect the beer to have 1.1 volumes if it has only been sitting at 50F for a day or two. In addition to that active fermentation generally leads to more CO2 in the beer than what is to be expected from the equilibrium equations.

That’s my take on it. When I prime in the bottle I base the current CO2 content of the beer at the current temp and I also account for the residual amount of fermentable sugars that are still in the beer and the targeted amount of residual sugars. But that latter half is just my geeky self and I don’t think there are many that do that.

Sorry of I repeated points that have already been made. I skimmed over the posts to try to avoid this.

Kai

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09-01-2009, 04:44 PM   #24
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Quote:
 If we assume that the CO2 lays on the beer like a blanket it offers the beer CO2 at atmospheric pressure and the beer will eventually take up enough of that CO2 blanket to reach the CO2 equilibrium that is shown in the tables and normographs that you mentioned.
If I crash in secondary there is (practically) no head space therefore there can not be any C02 added into solution. I've never noticed the gravity change.
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09-01-2009, 04:48 PM   #25
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Ok, I looked at the existing responses more closely. And until now I didn’t notice that we are in the science forum.

If we consider a system where air is allowed to be sucked in (carboy w/ airlock) it becomes somewhat complicated. CO2 will be absorbed by the beer but because no pure CO2 is coming in through the airlock the partial pressure of the CO2 will be lowered as it is diluted with air. At some point an equilibrium will be reached with less than the CO2 content than expected from the tables and more than what you would have had at the temp the fermentation stopped at. I don’t want to do the math for that. I also don’t think that you’ll get a nice CO2 blanket since there will be diffusion between the CO2 and the air.

But at this point you allowed air into your head space. What happened to the concerns about oxidation?

As a last point, the pressure created by the water column in an air lock is very small. It is equivalent to an elevation change of about 50ft.

Kai

09-01-2009, 09:22 PM   #26
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I see two possibilities for this. Cold crashing in the primary or the secondary. For this example we’ll consider that both were fully fermented before cold crashing and that we are cold crashing an ale from 68 degrees to 35 degrees. In the case of a secondary there is little to no CO2 in the head space to be absorbed, but in the case of a glass primary there may be a gallon of head space. I have cold crashed a lot of beers from the mid 60 degree range and brought them down to the mid 30’s and never had my airlock sucked completely out. I can see that negative pressure is present, but not enough to pull the fluid through in either a primary or secondary situation. This would indicate to me that very little of the CO2 in the head space has been absorbed and that thermal contraction could account for the negative pressure that I do observe. In the zero to slightly negative pressure environment of the head space, is there enough CO2 present to bring the volumes from the .9 to 1.5 that the chart uses for reference? If so, you say it is a slow process, how long would it take with no new CO2 being introduced?

From what I’ve observed in kegging, if you charge the headspace with CO2 and remove the gas the pressure in the headspace will drop as the CO2 is absorbed. If the CO2 in the headspace of a carboy is absorbed wouldn’t a similar drop in headspace pressure occur? Without the introduction of pure CO2 to maintain a constant pressure equilibrium (in this case zero) how would an exchange take place?

As for the scenario where the airlock is sucked out, I would compare that to putting a bottle of beer in the refrigerator opened. Initially the CO2 coming out of suspension will blanket the top of the beer as you describe in the carboy. But in the end we all know that an open beer, even if chilled will go flat.
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09-02-2009, 02:59 AM   #27
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Quote:
 BUT, you need to keep in mind that the CO2 exchange between beer and environment is a slow process and that those tables are only accurate if the beer sat long enough on its current temperature for the equilibrium to be reached.
I think this is the important point. Given enough time, the colder beer will absorb CO2 back into solution until equilibrium is reestablished, based on Henry's Law. The problem is knowing how much time it takes? I'm not sure, given the chemical complexity of beer, that there is an equation out there that can determine this. It is probably something that needs to be empirically determined. Given sufficient time, then yes, the amount of CO2 in your beer is determined by the current temp of that beer (thanks to Henry's Law). But, as Kaiser mentioned, it won't be at equilibrium after only a couple of days, especially assuming the beer is not being sloshed around or stirred.
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09-02-2009, 06:26 AM   #28
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here's something I've never seen. A label on volumes of co2. Volumes is almost as good as saying handfuls. What is the unit we're dealing with here? L, cu. ft?

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09-02-2009, 06:46 AM   #29
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Quote:
 Originally Posted by JLem Given sufficient time, then yes, the amount of CO2 in your beer is determined by the current temp of that beer (thanks to Henry's Law). But, as Kaiser mentioned, it won't be at equilibrium after only a couple of days, especially assuming the beer is not being sloshed around or stirred.
But you also have to take into account available CO2 in the headspace. Temperature doesn't matter one bit if you haven't got he CO2 to absorb in the first place. Equilibrium is also reached much more easily when increasing temperature than lower, it's an entropy thing.
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09-02-2009, 07:01 AM   #30
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Quote:
 Originally Posted by JLem But, as Kaiser mentioned, it won't be at equilibrium after only a couple of days, especially assuming the beer is not being sloshed around or stirred.
And then the CO2 has to reach an equilibrium within the beer. Keep in mind that there are no currents in beer that is sitting still during cold conditioning. CO2 gets around by diffusing from one spot to another.

This is a very complicated subject and my best advice is to settle on one procedure and adjust your CO2 content target based on previous batches. I myself use a spread sheet to get the right amount of Kraeusen and such for priming but in the end I doubt that I'll be able to tell the difference between 2.2 and 2.4 vol CO2 in the beer.

Kai
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