Hello all.
I have heard that bottle co2 has a bit of oxygen in it. And fermentation gas does not
Does it matter? Is the amount enough such that we should choose one over the other?
Food/beverage grade CO2 is 0.01% other gasses and not all of that is oxygen. The best thing about fermentation gas is that it's free.
Food grade CO2 is at least 99.5% pure, and can contain up to 50 ppm of O2. Beverage grade CO2 is at least 99.9% pure, and can contain up to 30 ppm O2.
ref In all cases, the O2 can be significantly less than the spec limits. I believe 30 ppm O2 is about the practical limit for measuring O2 with relatively inexpensive meters, so producers don't want a lower spec, even if their product is much lower in O2 - it's just too expensive to routinely prove that it is much lower.
If you liquid flush a keg with beverage grade CO2, then the keg will contain less than 30 ppm of O2. O2 gas has a density of 1.43 g/L at STP (and will be slightly lower at room temp, so we'll just use the STP value as a worst case.) If you fill a purged keg with 19 L (about 5 gal) of beer leaving 1.5 L of headspace, the headspace will contain at most 1.43 g/L * 1.5 L * 30e-6 = 64.3 micrograms of O2. 19 L of 1.010 SG beer will weigh 19 L * 0.9982 g/L * 1.010 = 19155 g. If the entire ~65 micrograms of O2 from the headspace dissolved in the beer, the concentration in the beer would be 65E-06 g / 19155 g = 3.36 parts per
Billion. So, beverage grade, or even food grade, CO2 is more than adequate for purging kegs prior to filling with beer.
But, as noted, fermentation CO2 is free, if you have the equipment to harvest it.
Brew on
