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I've had a few experiences that lead me to suspect this. A few times I've stored a small portion of wort for priming with gyle, and each time I re-boiled the wort for a short period, 10 minutes or so, which I perceived as producing a tremendous amount of DMS that wasn't in the fermenting wort. Also, a couple of times, I've had to re-start a boil, due to equipment issues or running out of propane. Each time the finished beer had a lot of DMS, even when the total boil time was over 2 hours. My suspicion from purely empirical evidence is that each time wort is heated after being cooled, it needs a full hour (or 90 minutes for pilsner) of rolling boil to drive off DMS, even if it had been previously boiled. Is this how DMS works? I had always assumed before, once you had boiled it off, it was gone, but that doesn't seem to be the case from experience.

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Yes, DMS is formed every moment the wort is hot.

DMS is formed from heating of S-Methyl Methionine (SMM) that is found in all malts in different amounts.

Your boil burns off most of the DMS that was created during mash and preboil heating.

During boil DMS production is trumped by its evaporation, and it's 35-40 minute halflife depending on pH (why we have 90 min open boils, if it's allowed to condense and fall back in, you don't get rid of it.)

At flame out your wort will continue making DMS and DMSO until chilled. DMSO can't be boiled off as it's boil temp is much higher than water.

So.. If you have a stalled boil DMSO may be your problem.

  • Thanks, this makes sense. I was beginning to think I was crazy :) – Wyrmwood Dec 29 '15 at 16:16
  • Do you have a source you can cite for this? My experience backs this up, however, you will find many people saying that once DMS is boiled off, it is gone (like in DME, for example). – Wyrmwood Aug 29 '17 at 21:48
  • @Wyrmwood howtobrew.com/book/section-4/is-my-beer-ruined/… , this is a nice summary, but there are a lot of scientific articles too with a quick Google of the full compound names for SMM, DMS, DMSO. – Evil Zymurgist Aug 30 '17 at 13:12
  • Yeah, lots to read, but have not found even a single source that references reheating, or heating multiple times. There are ubiquitous references to "there's no DMS in DME", essentially arguing that all SMM has been converted and all DMS boiled off. I am doubting that, but having a hard time finding anything to back it up, other than personal experience. – Wyrmwood Aug 30 '17 at 15:24
  • Even found letter from guy at Briess indicating this. "Hi Jon, Thanks for getting in touch. In general, our extracts are boiled gently and then evaporated. Both of these processes will drive off DMS. We essentially use a large brewing system, and of course the malt and process will be a bit different each time. If you boil the DME at all, you should be left with no excess DMS, as this is the second time this pilsen malt has been boiled. So no, 90 minutes isn't necessary. Regards, Aaron Hyde Homebrew Products Manager Briess Malt & Ingredients Co." – Wyrmwood Aug 30 '17 at 17:30
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Just read this and got to thinking. Would the DMS be the off flavor that makes me think all of my lighter beers tasted similar and not quite right? I boil with the lid on and now reading this I may have figured out why the off flavor.

  • Yes that's what is ment by an open boil, lid off. If you've been doing extract that may be why it's subtle. DME and LME have little DMS precursers since they have been boiled off in manufacturing. Do a long open boil, I think you will be pleased with results of the beer. – Evil Zymurgist Jan 14 '16 at 0:29

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