Recipe questions

Not in this context.

Wiki Page on Partial Pressures

"The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids."

 

Henry's Law: (from your link)
" the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution" (in this case...that gas is CO2)

Yes the O2 will diffuse and dissolve over time, but it is not instantaneous nor significant in the context of most standard homebrewing timeframes and procedures.

 

Nothing happens instantly. But significance is a matter of opinion. Some of us believe every post-yeast-growth exposure to O2 is worth eliminating or minimizing. That's opinion too. But I bet you won't find anyone who, for example, racks under CO2 pressure in a closed system and says their beers didn't improve after they started doing that, as compared to racking into an open keg that had just been purged with a CO2 "blanket."

 

Most people who rack to an open, purged keg are still siphoning (which is the real culprit). Put me down as one of those people who didn't see a significant difference. (I used BYO Dec. 2014 closed racking system with a bucket, figure 3, pg 93)

O2 should be minimized, but it can never be totally eliminated...no matter how hard you try...and at some point follows the law of diminishing returns.

 

Many people equate oxidation with sherry and cardboard. It takes several months for this to be noticeable. The short term affect of oxygen exposure is a diminished hop presence and beer that tastes somewhat stale and older than it is.

The reason why cold crashing is potentially detrimental is presumably you removed the lid to check gravity before you crashed. Now the headspace is mostly air. The cold temperatures pull gas into the beer more efficiently than warm temperatures.

Now, it's certainly possible to cold crash and the end result is a beer that tastes good. However, the same beer would be in better condition without a cold crash. Some styles tolerate oxygen pickup a lot better than others. A big stout will be more oxygen tolerant in terms of subjective results than an IPA.