Thanks for the update
@Proday137 , and the follow up,
@mattdee1
The hops are fresh and properly stored. I'm leaning towards O2. I'm not fermenting under pressure, so I imagine when I cold crash, O2 gets in via the vacuum. While my airlock isn't pulling liquid in, I do see it reverse bubbling during this stage. I don't have room in my chamber for another vessel during fermentation so I can't do the keg hookup. I'll have to figure out another way to keep this fully under pressure. Maybe the NorCal CO2 harvester?
You can try one of those "balloon" type rigs that capture fermentation CO2 and save it so that only CO2 is sucked back during cold crash. Never tried that myself, so don't know how well it works. I'm inclined to think that you'd need a pretty good seal on your bucket lid for it to work properly and frankly, it just sounds like a monumental PITA.
If you have no means of maintaining pressure in the fermentor, then the easiest way to avoid O2 intrusion from cold crashing is to not cold crash at all.
IMO the only "good" reason to cold crash is to get the dry-hop matter to settle so it doesn't clog up your transfer to the keg. In other words, if you have some other way of preventing the clogging, then don't cold crash because it just creates a completely avoidable vector for O2 to get in.
The first time I tried a no-cold-crash closed transfer of a dry-hopped IPA without doing anything to prevent clogging, it was a nightmare. It clogged every 15 seconds.
So I thought about it for a while and came up with the (dirt cheap) system shown below that might give you some ideas. I've used it a dozen or so times now and it works incredibly well; I am able to drain the entire fermentor full of dry-hopped IPA down to the dregs without a single visible piece of hop matter getting in the way, and all without ever opening the bucket lid or cold crashing.
I insert the sanitized hop filter into the fermentor bucket on brew day when I'm filling the fermentor, and it stays there the whole process. The loop of red tubing is just there to hold it in the correct position. For dry-hopping, the bucket lid has a port with rubber stopper that is directly over the opening in the filter so when I drop the dry hops in, they fall into the filter. Then when I'm doing a closed transfer to the purged keg, I put the bucket up onto a tilting table with the hop filter full of dry hops at the "back" (see image). The racking cane is inserted into a separate port at the "front." This configuration keeps the hop matter in the hop cylinder and only beer goes through the cane.
I actually have separate bucket lids that I use for dry hopped batches and non-dry-hopped batches. The picture shows the non-dry-hop lid, but you just need to imagine a port with stopper where the airlock is, and have the airlock off to the side.
