Dry hopping without oxidation

My last three IPAs smelled great on day three and like wet cardboard by day ten. The culprit was obvious in hindsight: opening the fermenter to throw in hops lets in exactly the oxygen the yeast had spent a week scrubbing out.

I now ferment as usual and transfer to a keg that has been filled with sanitiser and pushed out with CO2, so it never sees air. The hops go into a weighted stainless canister hung from the lid before the purge.

Same recipe, same yeast, same hop lot. The citrus and stone fruit now hold for close to four weeks at cellar temperature, and the colour stays pale gold instead of drifting toward amber.