I've been thinking (possibly dangerous) and thought, if I have a fully carbonated 4 or 5 litre mini keg and are going to consume it in the one session, would a pump for dispensing cause any detrimental effect to the quality of the beer? Initially the head space would have co2 but air from the pump would firstly mix with it but would it immediately oxygenate the brew? How long would it take to do this. Now there must be a problem? Cause why aren't we all doing it if the contents are going to be consumed in the one session.?