While I'm not sure if there is a market ready apparatus/procedure for this out there, I would assume that bubbling nitrogen through the bottom and allowing it to push O2 out would be a better method. I say this since N2 is lighter than air, and if you were to flush it from the top, the air trapped in the liquid might not dissipate.Further, short of a continuous stream of N2, how do you plan on stopping the reintroduction of air into your container? Would you be using a vacuuming procedure?