DMS in pale lager and how boil vigour keeps it down

DMS starts as a precursor in the malt

Pale malt carries a compound called S-methylmethionine, or SMM. When wort is heated, SMM breaks down into DMS. Pilsner and other very lightly kilned malts carry the most SMM, because heavier kilning breaks much of it down at the maltings.

So the problem is mostly in pale beers. A dark ale built on kilned malt has little precursor left. A Pilsner or Indian lager made on pale malt has plenty.

Boiling makes DMS and removes it

During the boil two things happen at once. SMM converts to DMS. DMS, which is very volatile, escapes with the steam. A commonly quoted figure puts the half-life of SMM at boiling at around 40 minutes, so a 60 to 90 minute boil converts most of it and drives off most of the DMS.

That is why Pilsner recipes traditionally call for a long boil. It is also why boil vigour matters. A rolling boil with a clear evaporation rate drives DMS out. A gentle simmer keeps it in.

Boil-off rate is your best check

Measure your evaporation rate as a percentage of the starting volume per hour. Older guidance often suggests 8 to 10% per hour. Modern kettles with efficient designs often run lower, around 4 to 6%. They still keep DMS down because the boil is turbulent and the steam vents freely. Our guide to boil evaporation rate covers how to measure it.

A few practical points:

• Keep the lid or manway open, or make sure the vent is clear. Condensate running back into the kettle carries DMS with it.
• Do not let the boil fall to a simmer to stop a boilover. Use antifoam or reduce the fill.
• Check that heating elements or steam jackets are clean. Scale cuts heat transfer and weakens the boil.

The whirlpool is where DMS creeps back

After the boil, wort sits hot in the whirlpool for 20 to 30 minutes or more. SMM that is left keeps converting to DMS, but there is no vigorous boil to drive it off. A long hot rest can push DMS back up toward or above the flavour threshold, which is commonly put at around 30 to 50 micrograms per litre.

Keep the whirlpool rest no longer than it needs to be. Start cooling promptly. A hop stand below 80 °C slows the conversion if you need a long contact time for hops.

Other sources of DMS

Not all DMS comes from the boil. Yeast can produce some from a related compound, dimethyl sulphoxide, during fermentation. Wort spoilage bacteria can produce DMS too, usually alongside vegetal or parsnip-like notes. If DMS appears suddenly in a beer that has always been clean, check hygiene and the lag phase as well as the boil.

A vigorous fermentation scrubs some volatile sulphur out with the carbon dioxide. Lager fermentations at cooler temperatures scrub less. Our guide to sulphur in lager fermentation covers the related problem of hydrogen sulphide.

Adjuncts and DMS in Indian lager

Rice and maize adjuncts dilute the SMM from the malt, since they bring extract without the precursor. That can make a high adjunct lager less prone to DMS than an all-malt Pilsner, all else equal. Corn based adjuncts can bring their own corn-like notes, which some drinkers confuse with DMS. A trained panel can tell the two apart.

Heat works against you in another way. Malt stored warm and humid for months is not a DMS cause in itself, but stale or slack malt can change how a brew behaves and confuse diagnosis. Keep malt dry, rotate stock and note the lot number on every brew sheet so a sudden change can be traced to a delivery rather than guessed at.

Testing for DMS

The lab method is gas chromatography, which many small breweries send out. On the floor, sensory checks work well. Warm a sample of fresh wort or beer gently and smell it. Sweetcorn, tomato juice or cooked cabbage are the usual descriptors. Train two or three people with a spiked reference sample so they recognise it reliably.

Sensory kits with food grade DMS standards are sold for exactly this. Taste the reference at a known level beside your beer. Note the sample point too: DMS found in wort but not in beer tells a different story from DMS that appears only after fermentation.

Your first step this week

Measure your boil-off rate on your next pale lager: volume at the start of the boil, volume at the end and the boil time. Note the whirlpool rest length too. If your boil-off is low and your rest is long, start by shortening the rest. It costs nothing and often makes the clearest difference.

Common questions

What causes DMS in beer?

Mostly the breakdown of S-methylmethionine from pale malt during heating. A weak boil or long hot whirlpool rest leaves DMS in the wort.

What does DMS taste like?

Cooked sweetcorn is the classic descriptor. Tomato juice and cooked cabbage are also used.

How long should I boil a Pilsner?

Many brewers use 60 to 90 minutes with a vigorous rolling boil and a clear vent, which converts most SMM and drives off the DMS.