In November 2012, I brewed a delicious Russian Imperial Stout with my brother, only to have it turn awfully rancid in the bottle (note: I bottle from the keg) about nine months later. I rarely bottle, so I was surprised to taste the exact same off flavor in an British Mild I brewed in July 2013 that had tasted delicious in the keg; the British Mild had been bottled in late September, and in just a month, the off flavor had developed noticeably.
Adding to the confusion, I bottled a Cream Ale on the same day I bottled the Mild. When I tasted it a few days ago, it was as if I’d added over-cooked, creamed corn to it; it was off, but not rancid like the Mild. It had also tasted excellent (for validation, judges agreed, with a score of 39.5), but had obviously started to turn, if a bit slower than the Mild.
John Palmer suggests that bacterial infection may be the cause: “When caused by bacterial infection, DMS has a more rancid character, more liked cooked cabbage than corn. It is usually the result of poor sanitation.” Based on my palate, I would definitely characterize the off flavors from the RIS & Mild as rancid vegetables, perhaps rancid grassy.
I’ve identified several possible suspects, and I’m addressing all of them. The most likely candidate so far (at least, most likely based on visual observation) is the CO2 gas side of my keg system. I’m not sure I would have guessed this could happen–and so unlucky that I’ve never noticed before now–but the inside of the gas disconnects can & do get exposed to beer during normal operation. This means, they’re going to need cleaning; perhaps, if given a little attention after each keg, it will be a simple process. As for me, I don’t recall that I’ve ever cleaned these gas disconnects in the two or three years I’ve had them.