I suspect oxygen is the culprit. I had been dealing with this issue as well in all of my dry hopped beers. Beers would be fantastic at first, but anywhere from a few days up to a couple of weeks after kegging, it would turn into what I can best describe as a caramel bomb that tasted very much like an old or oxidized beer. It would only show in dry hopped beers, all non dry hopped beers would remain nice, bright an clean. Here is what I did to eliminate oxygen exposure.
- Began adding first dry hop addition 3/4 way through fermentation
- No longer rack to secondary. Instead all dry hop additions are done in the primary.
- Flush carboy with C02 before and after each dry hop addition
- No longer cold crash in the carboy. If you listen to the most recent Session, Jamil states that cold crashing can introduce as much as 4% 02 into the beer.
In addition, I'm now going to experiment with a closed transfer from carboy to a purged keg, where I will do my dry hopping and, ultimately, cold crashing. From there, I will then transfer to a second keg for serving.
There was also a Sunday Session a few years back where Nate Smith and Tasty talked about Double IPAs. They stressed how absolutely important it is to reduce all oxygen exposure during the process as the addition of dry hops can add a considerable amount of O2 to the beer.
Regardless of how careful you are minimizing O2 exposure during your process, think about how much you are potentially introducing from the following:
- Transferring to secondary
- Multiple dry hop additions
- Cold crashing (O2 is drawn into your beer during this process)
- Transferring to keg
The main difference between a pro brewer's process and a home brewer's process is their ability to transfer everything in a closed system and reducing the amount of Oxygen. By really focusing on this aspect, I think you'll begin to eliminate this off-flavor completely.
Additionally, I would also stress to make sure all of your other processes are solid as well. Pitch a nice healthy pitch of yeast, properly oxygenate your wort prior to pitching, pay attention to your mash and kettle pH, be sure you're leaving your beer on the yeast long enough to clean itself up, and ensure you are making proper water adjustments to suit the style you are brewing.