CO2 — even when purchased at food grade — is never 100% pure. There are many reasons for this and the solution is to buy a purifier or to do some elaborate lab tests, or both. Either way, it’s costly
Ironically, CO2 is produced plentiful during fermentation and it’s clean and natural. Wisely, the old german beer laws prohibited the addition of CO2 not produced by the beer.
What did the big boys do to filter out all taste from their product for the sake of faster turn around and profits? – They collected the CO2 produced during fermentation and re-injected it into the beer after filtering. This method is out of reach for many and I would not recommend it. Keep the taste in the beer – don’t filter. Keep it honest and real!
The classic method for natural carbonation (and good “Lager” procedure) is to transfer the beer to the conditioning tank whilst there is still a residue of fermentable sugars left (1-2%). This allows fermentation to proceed slowly, whilst producing CO2 in the tank. Generally the transfer happens quite cold, around 3-5C (which incidentally is more or less my final temperature at the end of primary fermentation for a cold lager schedule – more of that later). Having the target vessel at the same temperature as the beer will reduce foaming and preserve as much of the already existing CO2 as possible.
A manometer with a bleeder valve (“Spundapparat“) is connected to the vessel to discharge CO2 produced in excess of the desired pressure, then the temperature is gradually decreased towards -1C over the next 4 weeks. The way the Schneider brewery bottle-conditions their delicious product is not too disimilar…
Indeed, this method is pretty much the same as bottle conditioning — with the nice benefit that with a technique like “Umdrücken” no sediment is left when the beer is served or bottled (under counter-pressure) and the beer never is in contact with O2 again (which can damage it). Effectively, any excess O2 is happily consumed by the still active yeast. Nifty!
I prefer this method to force-carbonation and it is one area where I feel that the Reinheitsgebot got it right.