||12-31-2009 01:14 AM
What's the best way to purge oxygen from a keg?
There is a generally accepted belief that beers that are drunk a few weeks after kegging or bottling wonít show oxidation-related defects. I generally donít have beers hanging around for months, but now and then I do age an IPA or a holiday ale for 6 to 12 months. I havenít noticed any oxidation-related flavors, in any of my beers, so Iím content that my method of purging my corny kegs of oxygen is good enough, but I want data, not beliefs. Also, I donít have the most sensitive palate, so maybe Iím missing the flavors.
Iím not interested in resuming this debate, so letís assume for the sake of this thread that oxidation defects in home brewed beer are real, and that one source of oxygen is encountered during kegging.
So I need some help that only you brewing mavens can provide.. There is an on again-off again discussion on this and other boards about the best or right way to purge a corny keg to minimize oxygen content in the purged keg. Some folks pressurize and release the pressure several times (Method 1), some fill with CO2 from the liquid post to push the air out through the pressure release valve (Method 2), and some fill with sanitizer and push it out with CO2 (Method 3). No one seems to have any real data or can offer a diffusion equation that can be used calculate/estimate the amount of O2 that ends up in the CO2 purge gas. I want to find a diffusion rate constant for O2 diffusing into CO2, so I can calculate how much O2 is getting into my purge gas.
My belief is that Method 1, pressurize and release, is the least effective purge method, since it would take five 30 psi purges to get to 0.1 % O2, which I suspect is enough oxygen to cause trouble, even when most of the gas in the keg is pushed out by the incoming beer.
So, hereís my math to back up that statement. 30 psi (2 atmospheres) CO2 dilutes a corny full of air (one atmosphere of air) a dilution of 2:1. So starting with a keg full of air at atmospheric pressure and pressurizing and releasing with 30 psi CO2 reduces the O2 content in the keg like this, assuming good mixing in the keg: First purge cycle: 22% O2 to 7% O2, second purge gives 2.67% O2, third purge leaves 0.9 %, a fourth leaves 0.3 %, a fifth 0.1 %. If the purge gas pressure is lower than 30 psi, then even more oxygen is left in the keg after purging. Also I suspect few of us do more than 3 purge cycles, we leave even more O2 in the keg. Also this method uses a lot of CO2, roughly 88 grams, or about 3 ounces per purge, so three purges at 30 psi is a half pound of CO2.
I use Method 2, purging empty, sanitized cornies through the liquid in line with the cover in place and the pressure release valve open. I want to figure out how much oxygen is diffusing into the CO2 as I purge this way. I purge slowly enough that I think I can ignore gas mixing. It takes about ten minutes for me to get a strong CO2 odor at the pressure release valve, so I know Iím purging at about 2 liters per minute. I know that gases diffuse rapidly into other gases, but I canít find any web based reference that gives a definite rate constant, and I canít find my old physical chemistry text, so if you have any references, letís have them. Iíd like to know if my method is useless, or practical.
It seems intuitive to me that the most effective purge is Method 3, fill the keg with sanitizer, and push it out with CO2, since most of the air is already displaced with sanitizer. But, any air left in the keg will still let some O2 diffuse into the purge gas, or mix the air with the CO2 if the purge gas goes into the keg too fast. But how much?
Anyone here who can help? I believe that my method is adequate, because I donít detect oxidation defects, and no one else has mentioned them, but I still want to be able to compare these methods on a quantitative basis.
Oh, and if you have a different purge method, what is it, and why do you think it effectively gets the O2 out of the keg?