I'm at a little bit of a loss, looking for some insight. I brewed this in the spring and had to dump the batch. Rebrewed it late summer, this time without using fermcap in the boil (I was concerned maybe it prevented the DMS precursors from boiling off). Ended up with the same result, a totally vegetal and undrinkable beer.
Pretty basic czech pilsner recipe, 10 lbs bohemian pilsner malt, 8oz saas pitched in equal amounts at 60, 30, 10, and 0 during a 90 minute boil (vigorous boil, I'm in Denver and lose ~1 gal/hr to evap). Chilled with a counterflow chiller and when down to temp pitched with a 2L starter of WLP800. O.G. 1.050. Fermented at 52 for 2 weeks to hit 1.008. Raised to 68, held for 3 days (SS brew bucket with a glycol chiller for temp control). Tested for diacetyl and it was fine. No DMS present at all. Dropped 1-2 degrees per day to reach 38 degrees and held there for 2 weeks. Tasted at kegging and it was great.
Both batches tasted fine at kegging but by the time they were forced carbonated they were undrinkable, with the hops completely overwhelmed by a vegetal creamed corn taste. I clean the keg with oxyclean free, hit with a good hot water rinse, then fill it with sanitizer solution, let it sit for a period, and purge with CO2. Tap lines were completely disassembled and cleaned for each batch (I actually replaced the lines completely for this last batch). I've never had a problem with DMS before and I'm not even sure what to do different at this point.
I should add I'm using Denver tap water, treating it with a campden tablet, and adding lactic acid during the mash to hit a pH of 5.2 to 5.3.