DMS appearing after kegging

cassidymiller

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.

doug293cz

I think your altitude may be the issue. The degradation rate of SMM, the precursor to DMS, is a function of temperature, and to a smaller degree pH. Boil vigor is not a factor, since the temp for a slow boiling is the same as for a roiling boil. SMM does not "boil off" but gets converted to DMS. DMS boils at about 100°F, so dissipates rapidly once formed (as long as there is good convection in the liquid so the DMS can get to the surface.)
At 212°F, the half life of SMM in wort is about 30 - 40 minutes depending on pH. In Denver, water boils somewhere around 204°F, or about 5°C lower than at sea level. If we assume SMM degradation has a similar activation energy to most other organic reactions, then the rate doubles, or halves, with each 10°C change in temp. Since the temp effect is exponential, rather than linear, at 204°F, the rate of conversion of SMM to DMS is about 0.6 what it is at 212°F, so the half life of SMM is about 1.6 times longer.
So, to get the same effect as a 90 min boil at sea level in Denver, you need to boil for about 90 * 1.6 = 145 minutes (about 2.5 hours)!
I suspect that you have a lot of residual SMM in your finished beer. There is a mechanism for SMM to convert to DMS in finished beer (but I don't remember the details), but that is probably what is happening in your case.
Brew on

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cassidymiller

I wonder about that but then Bierstadt, Prost, and Cohesion seem to be able to brew pilsners without a problem! Maybe I'll shoot a couple of their brewers a message and get their thoughts.
How would I manage a 2.5 hour boil and keep O.G. reasonable? Add some boiling water during the boil, or just at the end to dilute it to the proper O.G.? And how to manage hop rate in all of this?
I guess what really gets me is that it's not noticeable until I keg... but maybe it's the CO2 coming out of solution and carrying the DMS with it?

doug293cz

The malt used can make a difference, due to different levels of SMM in different malts. More SMM will take longer to convert "fully" to DMS.
Lots of ways to approach this: use a less vigorous boil (remember it's the temperature that matters, not the boil vigor), start with more pre-boil volume, dilute during/after boil, etc.
Temperature also controls the rate of isomerization of alpha acids from the hops. Lower temps require longer times to get the same amount of isomerization compared to higher temps. Relative times should be similar to what is required for SMM breakdown.
It's the creation of DMS from residual SMM that is the problem, not CO2 bringing out the DMS.
Brew on

doug293cz

There is a discussion of your issue here: DMS formation post packaging - ProBrewer Discussion Board
I'm attaching a couple of papers on SMM and DMS.
Brew on

cassidymiller

Thank you everyone! I'll do some reading, plan for my next batch, and report back in a few months with results.
Cassidy