So I've replied to this thread a while back and then kind of kept an eye on it over time; fascinating debates on partial pressures and how various methods will affect gases in our finished beer. I've been educated on some myths I believed to be true! That said, I've kept this question rolling around in the dark recesses of my wooden dome the last month, and this is what I come up with. If your brew is big (Belgian, stout, porter, sours, etc.) most home brewers are probably moving the beer to a bright tank for considerable aging (that's what I do), so cold crashing isn't really necessary because the aging time clears the beer. If you are brewing a "lighter" beer (IPA, bitter, pilsner, pale ale, etc.), you usually need to crash the beer to clear it, but you really wouldn't want to keep it around long enough to the point that off flavors will develop anyway. I (and all the people that come over and help me) usually push through an IPA, bitter, or pale ale within 3 weeks to a month. My point is: If the exclusion of O2 is an exercise in methodical perfection, to each his own, brew on! But in reality, I really don't think the O2 volume introduced at crashing a 5 gallon fermentor would introduce enough O2 to stale your beer in the period of time most homebrewers blow through a 5 gallon batch. That's been my experience, but I don't bottle, so maybe you have something there.
Cold crashing suck back will definitely provide enough O2 to cause perceptible oxidation. Let's say your fermentation finished at 68˚F (20˚C, 293.15˚K.) You then cold crash to 32˚F (0˚C, 273.15˚K.) When fermentation completed, your fermenter headspace contained 100% CO2 at 14.695 psia (1 atmosphere). Using the ideal gas law (PV = nRT or P/T = nR/V), nR/V is a constant for the headspace, so P1/T1 = P2/T2, and P2 = P1*T2/T1. Thus:
CO2 partial pressure after cold crash = 14.695 psia * 273.15˚K / 293.15˚K = 13.692 psia, and
O2 partial pressure = (14.695 psia - 13.692 psia) * 0.21 = 0.21 psia
After cold crashing, O2 will make up:
0.21 psia / 14.695 psia = 0.0143 => 1.43% or 14,300 ppm
of the headspace. Assuming the headspace volume is 1 gal and the fermenter volume is 6.5 gal, the O2 concentration averaged over the fermenter volume (equivalent to TPO) is:
14,300 ppm * 1 gal / 6.5 gal = 2200 ppm
Recall that from
http://www.craftbrewersconference.com/wp-content/uploads/2015_presentations/F1540_Darron_Welch.pdf (page 21), 0.15 ppm TPO is enough to cause noticeable oxidation in an IPA after 3 weeks of 68˚F storage. The cold crashed TPO is way over the limit.
So, what's preventing a cardboard nightmare? The answer is time. Even though the O2 concentration is enough to cause massive oxidation, it takes time for the O2 to diffuse into the beer, and after that it takes time for the beer to oxidize. (If you shook up the fermenter after cold crashing, the O2 would get into the beer much faster, but then only a fool would do that.) Since cold crash periods are usually short, and you get the beer into the keg in a few days, damage is limited.
If after kegging, your TPO is over the 0.15 ppm limit, time is still on your side, if you keep the beer cold. For most organic chemistry reactions, the rate of reaction is reduced about 2X for every 10˚C (18˚F) drop in temperature. If you store your keg at 40˚F instead of 68˚F, then the time to oxidize noticeably would be:
3 weeks * 2 ^ ((68˚F - 40˚F)/18˚F) = 8.8 weeks
So, the faster you drink and the colder you store, the less you need to worry about it. YMMV
Brew on
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