These are the points I considered when I decided to use an air pump rather than O2. My belief is if I add air containing oxygen over time while the yeast is in it's respiratory phase, it will be what it needs, as it needs it.
I'm not saying adding O2 is bad, but I can imagine oxidizing some compounds in the cool wort simply by the introduction of concentrated O2. O2 is vital to us, but breathing pure O2 is often irritating to mucous membranes. My hypothesis/belief is that it can be similarly detrimental to more sensitive compounds in the beer.
Now don't get me wrong, I am NOT saying people adding O2 are wrong. I am saying that there can be too much of a good thing, and that adding air over a longer period of time has a wider margin of error than adding O2 over a very short period of time. I'm not in a rush when brewing, I have plenty of things to do while the air pump runs.
Interesting discussion!
I wonder how we could measure if adding O2 actually did negatively impact the beer, aside from subjective tests? I suppose it would first have to be one of extremes: A split batch, one half oxygenated for the standard time (30 seconds?) and one half for let's go crazy and say 10 minutes. Put them in front of a panel of judges and see what happens.
Adding air with aquarium pump is definitely Ok, as long as you accept the following:
1. You are not really pumping air/oxygen into the yeast directly, you are dissolving it into the water, for yeast to use. So at the same ppm, it doesn't matter how it got there. You analogy with directly breathing 100% oxygen vs. breathing air with 20% is wrong. If you went into the space station with 22% or whatever partial pressure of O2, you wouldn't care too much if it came from pure oxygen or from air.
2. You can never get to 9-10ppm of O2 with aquarium pump.
3. You may need to run your aquarium pump for 20-30 min, while pure oxygen takes 30-60 seconds, to get to comparable values of oxygenation. Will you be sitting there watching your pump working for 20-30min?
4. You run a much higher risk of pumping your wort full of bacteria and wild yeast if you use air pump running for 20-30min than using oxygen from the tank for say 60 seconds. Especially if you don't filter your air.
In either case you should use a filter, but still, the risk with pumping room air is always higher than with pure O2 from the can.
For $10 O2 canisters, which lasts half a dozen of batches or so, for me - it's no brainer. Use O2, just don't walk away and leave it on for too long.
To me the question of - can you "over-oxygenate a wort" is sort of like asking "can you overcook and burn a quesadilla" - of course you can - if you are negligent and leave it for way, way, too long.
So in summary: Don't be negligent, follow the instructions. Don't oxygenate your standard 5G batch for more than 60 seconds or so. 2 minutes is probably still ok (think well-browned quesadilla). But 10 min or an hour is definitely NOT ok - think "burned to a crisp" quesadilla.
