I ferment in a keg, mainly as I want to minimise exposure to oxygen following fermentation and during packaging. I have two methods I currently use. One is more faff but gives more control of the dry-hopping process, but the jury’s still out as to whether it’s worth the extra effort!
1. The easy one!
Once you have pitched your yeast, make up your dry hop charge by placing a sanitised stir-bar inside a sanitised hop bag. Place this on the inside of your fermentation bucket/keg/lunchbox lid, and secure it by putting a neodymium magnet on the other side of the lid. It should stay there quite happily depending on the strength of your magnet and weight of your dry hops until you want to drop it in your beer. To do so, simply remove the magnet from the outside of the lid, and you hops - which have been happily purged of oxygen due to the CO2 off gassing created through fermentation - will fall into your beer. Simple!
2. More cleaning, but more control.
If I am not serving directly from my fermenting keg, I will add a stainless steel hop tube with my dry hop charge directly into an empty keg. Once I have pinched my yeast into the fermentation keg, I attach a tube from the gas out of my fermentation keg to the beer in of my empty keg, and a further tube from the gas out of the empty ‘dry hop’ keg into my serving keg, connecting a blow-off tube or spunding valve to the final keg. I let the CO2 from the fermenting keg purge the ‘dry hop’ keg as well as the serving keg. When I am ready to dry hop, I rack the beer from the fermenting keg into the dry hop keg. After three days or so I will then rack from the dry hop keg into the serving keg.
My thoughts on this process are… mixed. I like the sense of control it gives me, but am yet to find any perceived benefit in the finished beer, and it’s a lot more work and an extra keg to clean. I don’t notice any grassy off-flavours when serving off a keg with dry hops in, even when left for up to six weeks.
Anyway - two methods which work well for reducing oxygen exposure whilst dry hopping. Hope they help.