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Carbonation Tool and Sugar Amount: Keg vs. Bottles

sengsational

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When I look at the carbonation tool, I noticed that the amount of bottling sugar was MUCH reduced when naturally carbonating in a keg; the carbonation tool shows 2oz sugar for kegging and 4oz sugar for individual bottles.

It would seem to me that a certain amount of sugar would provide a certain amount of CO2 dissolved in the beer. Apparently something else is going on.

Why would it take 1/2 as much sugar to create the same amount of carbonation if the batch is naturally carbonated in a keg versus bottles?
 

brewfun

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Headspace.

Carbonation requires equilibrium between a container's headspace and the beer. With bottles, there is more headspace per ounce.
 

sengsational

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My quickie research indicates that there are folks that don't follow the "half sugar in keg conditioning" rule, but instead prime the same as bottling, and get good results.  There are others that say they get foam/overcarbonation if they prime kegs at the full sugar amount.  Since experience is all over the place, I'm looking for the SCIENCE behind the recommentadation.

As for the headspace theory, that falls flat with me for several reasons, but an obvious fault in that suggestion is that the keg priming sugar calculators do not say anything about how full you fill your keg (i.e. 4.5 gallon batch [14% headspace] or a 5.0 gallon batch [5% headspace]).  In other words, if priming sugar was sensitive to headspace, then why don't the calculators have a "total volume of keg" and a "volume of beer added to keg"?

But for arguement's sake, let's say they presume exactly 5 gallons of beer.  According to this post: http://www.homebrewtalk.com/f35/actual-volume-corny-keg-250954/#post4560039, there is considerable headroom in a corny keg after 5 gallons has been transferred:
The 5 gal mark is indeed right at the weld line, 1 to 1.5 inches from the bottom of the gas dip tube.

Rough numbers here, but out of the total, say, 21 inches of corny height, 1 inch is headspace.  1/20 = 5% headspace.

I just weighed the contents of one 12 ounce bottle of beer, filled to normal level (with head space): 358g.  The contents when filled to the brim (no headspace): 381g.  So we have 6% headspace in a 12 oz bottle.  PS: Someone should do the weight experiment with a real corny to get better numbers than my rough numbers, above.

Why would a one percent headspace difference equate to half the bottling sugar?  I'm still not seeing it!

 

brewfun

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sengsational said:
My quickie research indicates

Mixed results! On the internet?  ::)

I'm looking for the SCIENCE behind the recommendation.

Well, stand back then....

But it really isn't all that sciencey. Carbonation is pretty mechanical...

  • The headspace in a corney keg can be measured as a percentage of the volume.
  • A Corney is 9" in diameter.
  • That means the headspace is about 96 in^3.
  • The beer is taking up the remaining 19.5 inches of height for a total of 1240 in^3.
  • Thus, the headspace represents 1.57% of the total volume.
  • Translated into ounces as a percentage of 5 gallons, the headspace holds 10 ounces.

A standard American beer bottle designed to hold 12 ounces of beer is actually a 13 ounce container. A 5 gallon batch makes 53 bottles, ideally. That's 53 ounces of accumulated headspace.

But, WAIT!
...There's more!

Fermented beer has CO2 in it even when still.

This is more on the approximation side of things, but the temperature and handling of a beer effect the amount of standing CO2 in a volume of beer. A beer at 60F will have about 1.1 volume of CO2 dissolved in it. A beer at 35F can have 1.7 volume of CO2.

In a nutshell, the transfer of beer from a fermenter to a keg is smoother than into a series of bottles. Less surface area to volume ratio, and a longer amount of time above the churn point allows the keg product to have as much as 1 full volume more than its bottled counterpoint after transfer.

So, if you're starting with a greater amount of preserved CO2 and don't have as much headspace volume to fill to gain equilibrium, much less sugar is needed.



...Oh, so I'm pretty sure you want a formula or something as a souvenir.

So, here's the formula for dextrose. In fermentation, half the fermentable weight is converted into CO2, the other half into ethanol.

Dextrose as we get it for brewing has a water molecule in it attached to each molecule of dextrose, making it 91% fermentable. I'd say it's completely fermentable, but 91% sounds more scientific-ish for this discussion.

As a bonus, this formula is metric in grams/liter (1 g/l = 0.5 vol)

C = F+ 0.5 * 0.91 * M / V

C: Desired Carbonation level (g/l)
F: Flat Beer Carbonation level (g/l)
M: Weight of Corn Sugar in grams
V: Volume of beer in liters.

 

sengsational

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Thanks for the entertaining response! 
brewfun said:
Thus, the headspace (in a keg) represents 1.57% of the total volume.
...
A standard American beer bottle designed to hold 12 ounces of beer is actually a 13 ounce container. A 5 gallon batch makes 53 bottles, ideally. That's 53 ounces of accumulated headspace.
So presuming the brewer racks exactly five gallons, we have 1.5% headspace in a keg.  But honestly, who always gets 5 gallons?  The example I put on homebrew stackexchange (http://homebrew.stackexchange.com/questions/12899/why-is-less-bottling-sugar-required-when-naturally-carbonating-in-a-keg-versus-b) was 4.8 gallons (5.5% headspace), and the calculator still calls for twice the sugar.

You get 1/13 headspace in bottles (7.7%) and I got 6% by the normal fill vs full to the brim bottle, so nothing too much to talk about there except I think 13 is slightly generous.

So the bottom line for me on the headspace theory...I'm still not convinced.  The primary reason is that if I reduce the volume of the beer in the calculator, the kegging sugar goes down proportionally.  If headspace was such a big deal, then it would need to ask you if you were using a pinlock (5.0 gallons) or a ball lock (5.25) or somehow get at how much headspace was in your keg after you racked.  Because in reality, you might easily exceed 10% headspace (you manage to rack off only 4.5 gallons).  So if headspace is the issue, then your naturally carbonated beer would be flatter than the calculator says.

brewfun said:
But, WAIT!
...There's more!
...
the transfer of beer from a fermenter to a keg is smoother than into a series of bottles. Less surface area to volume ratio, and a longer amount of time above the churn point allows the keg product to have as much as 1 full volume more than its bottled counterpoint after transfer.
This is an aspect of the problem that I had not considered earlier, and is to me, by far, the most interesting part of your response.

But, alas, the calculator won't cooperate with that theory; no matter what temperature I put in the calculator, the sugar for kegging is always half of that for bottling.

You've got to figure that the churning while bottling takes a fraction of the CO2 out of solution.  So if you have a really cold fermentation, then lots of CO2 will be lost to churn.  Whereas if you have an exceptionally warm fermentation, much less CO2 will be lost to churn.  So I conclude that the cold ferment would require proportionally more sugar than kegging and the warm ferment, less.  But that's not the case...it's always half, no matter the temperature.

What I've concluded is that although there are some legit points for why a little more sugar might be required for bottles, I'm still not seeing double.  I think the calculator is based on something other than pure science (i.e. rules of thumb).
 

madteacher

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I know I'm late to the game on my input here, but science and engineering are two different things. For instance, the science says that gravity is best explained by understanding Einstein's theories of relativity, but the engineers will tell you that Newton's equations are much simpler and will work in 99.9999999% of the times we will need to use them. Same here. Beersmith could possibly come up with equations that include surface area, head space, etc., but does it matter? If they overestimate what you need, your beer is overcarbed, and you will have to do a series of purges and waits (a PITA). If they underestimate, then so what, let it force carb a bit until its just right.

Just my 2 cents
 

BOB357

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Try it both ways and you'll find the definitive answer. It's done that way because, "it works".
 

dtapke

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Lol, that was a fun trip down "useless posts" lane...

why would you even care about "naturally" carbonating in a keg? gonna push it with co2 anyways, and by force carbing with co2 you won't end up with a bunch of yeast sediment in your dip tube.

 

BOB357

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It appears that some people will pose a question so they can show their superior intellect (or lack thereof) by debunking the answers they get. Most are not successful and just seem to fade into the sunset. Just saying :)
 
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