This question's answer led me to http://www.milkthefunk.com which is a great source of information.

Since there's a few methods for making sour beer, I should mention I've been doing what some call a kettle sour (or fast sour) where I do a mash, then a full boil, then sour the beer using l plantarum in a sealed keg purged with CO2, then do another full boil (topping up with water) then ferment as a standard ale, then keg for carbonation.

One of the issues I've had with sour beer is DMS, really with any pilsner as well. (This is why I am now doing a full boil before kettle souring.) It turns out, there are some reasons for it, as according to the article on DMS

For one, I'm boiling at 6500 ft elevation, so my boiling temp is ~199F/93C (instead of 212F/100C)

It has long been reported that the half-life of SMM doubles for every 6°C cooler, meaning that at 95°C the half-life is ~70 minutes

And two, especially important for sour beers

pH plays a role in the reduction of SMM to DMS, with a higher pH reducing the half-life of SMM

Meaning I really need to be boiling for >2 hours to be rid of DMS for an ale! For a sour, the pH may be in the 3.4 range, where SMM half-life is now 142 at 200F/94C.

Empirically, I've been having good success with a 75 minute boil for a standard ale. My ales do not have significant DMS with that boil time.

Does this mean I need a 284 minute (nearly 5 hour) boil for a sour beer at elevation to remove DMS!?!?!? (I am doubling the half-life after adjusting for both temperature (elevation) and acidity (pH).) If I do a full boil before souring, and a full boil after souring, do these add together? Or, not really, since each boil went above 80C, each boil needs to be figured independently?

Lastly, the question this has been leading up to, after the kettle sour portion is finished, can I raise pH to 5.2 using Calcium Carbonate to increase the DMS boil-off without altering the taste? Or will that kill the "sourness" by reducing the acidity?

  • 1
    What salts would you add to increase the pH and what would be your target pH for post souring boil? Commented Aug 18, 2017 at 13:02

2 Answers 2


The lactic acid in the sour wort will act as a pH buffer and will mean quite a lot of "salts" will be needed to actually raise the pH of the wort. The lactic acid may well form a salt precipitate in such conditions (eg. Calcium lactate) which has a very much less sour flavour compared to lactic acid. Some think it does not taste sour at all. In theory one could use Sulphuric or Phosphoric acid to reduce the pH at the very end of the process. Calcium Phosphate or Calcium sulphate would tend to precipitate out of solution over time (especially if crash cooled for a few days). That would leave much of the original lactic acid back in solution and the brew would be sour again. Probably....One might think this all more chemistry than brewing. A better answer might be to find ingredients, especially the malt, with less SMM precursor.

If one particularly wanted to boil off DMS produced then it seems one must boil for some longer time. If we take the approximate (quoted) half life of 70 mins when boiled at 94C there will be a quarter remaining after 140mins and an eight at 210mins. If an eighth of the inherent amount present is below the taste threshold then job done in 3.5 hours. Boiling it for that long or longer may well, to some degree, darken the final beer.

  • Yikes. I was hoping you'd tell me I didn't know what I was doing and this length of boil wasn't truly required...
    – Wyrmwood
    Commented Aug 18, 2017 at 16:53
  • Also, in the MTF article on DMS, 142 is the half-life for pH 3.4 @ 94C. (That's where I got the 284 minute number).
    – Wyrmwood
    Commented Aug 18, 2017 at 17:15

I think the right approach may be to boil as normal for the equipment used, then after the saccharomyces fermentation is complete, if the DMS levels are noticeable, perform a CO2 purge. According to Carl Townsend of picobrewery.com,

Generally, if you keep your boil vigorous and cool rapidly, DMS levels will be appropriate for style. However, the DMS level may still be too high for your taste. If you keg your beer, you can artificially scrub out DMS using CO2. This is best done before carbonating. If you have a lager, allow it to warm to room temperature first. Then, switch around the ball-lock fittings on your keg so they are backwards. Hook up the long dip tube (labeled "out") to the CO2 tank. Prop open the pressure relief and let the CO2 bubble through the beer. Slow down if it starts to foam out. Occasionally sniff the gas as it exits the keg valve. You should be able to smell the DMS at first, and then notice it fading with time. After the level has dropped, switch the ball locks back to their usual configuration. Then carbonate as usual.

Since the fittings are slightly different, I will try this by only switching the dip tubes, and leave everything else as normal.

This guy just flips the keg on it's end. Seems even simpler.

UPDATE: So I tried this last night and although I was unable to swap any parts around, I was able to just flip the keg over. I had to set up a couple of parallel boards, since the rim on the keg is lower than the ball lock connectors to avoid putting too much weight on the connectors, but otherwise, it worked well. I couldn't really smell the DMS, so I just kind of did it a few times, filling up a small cup with foam. In a taste test with pre-scrubbed wort, the DMS levels were significantly lower, but not absent. I may try this again tonight, then I'll dry hop it.

upside-down keg on blocks

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