Dealing with DMS

Key points:

  • your malt might not be as well-modified as you think it is
  • even well-modified malt can give you DMS problems
  • yeast health, sanitation and even hops can be sources of DMS

The other day I was commenting on a stout. The stout had a DMS issue - not severe, but an issue nonetheless. After commenting, I was confronted by the brewer who insisted that it was "virtually impossible" to have DMS in a stout because of modern malts. 

That's just plain wrong. 

It frustrates me when I hear professional brewers repeat home-brew axioms because not only should they know better, it perpetuates the idea that an adequate or even excellent home-brewer can make the jump to professional brewing with no actual training, or at the very least, some serious research into a wide variety of topics. Now depending on the size of your system and the method of packaging, an excellent home-brewer can most certainly make some excellent batches. Larger systems and packaging tend to throw wrenches in the idea really quickly and that's where things get ugly. The era of "It doesn't have to be good, it just has to be local" will inevitably crash and burn given enough time.

But that aside, I'm here to raise the level of knowledge in the home brewing world. Professional brewers (who care to learn) already have a few college/trade school options available to them (though I was surprised to learn just how few actual master brewers truly exist on the continent for those looking to further the craft to apprentice under.)

Lager vs Ale Malt

Lager malt (aka "Pilsner" malt) is usually a different variety in Europe, but tends to be 2-row (usually Copeland, though that's changing) in North America. It's also not as well-modified as some brewers believe (6-row is also used, primarily for Adjunct Lagers.) The difference is in the modification. Greatly simplified, the maltster allows the embryo to grow into rootlets and the acrospire which coincides with the breakdown of the endosperm. 

Think of a barley kernel like a reservoir of 2 substances that brewers want - starches and proteins. Proteins provide enzymes which break down starches into sugars. During germination, the embryo develops into rootlets and an acrospire (or shoot), proteins are broken down into enzymes which in turn, break down sugars into starches that are consumed to further the initial growth of the plant.

When it comes to malting, if you allow the embryo to develop too much, there won't be enough sugars left and extract suffers. If you stop the embryo from growing too soon, it takes a really long time in the mash tun for a protein rest to turn those proteins into enzymes. Basically, lager malt errs on the side of too soon. Ale malt errs on the side of too late. 

The thing is, the embryo is also the source of a chemical called SMM, or S-Methyl Methionine. It's actually a vitamin and found in pretty much all plants (hence the "vegetal" aroma). As the plant grows, the embryo diminishes and when kilning happens, the rootlets and acrospire, burn off. When heated, SMM converts to DMS, or Dimethylsulfide which is highly volatile and blows away, assuming the maltster has appropriate circulation.

If you've put the dots together so far, lager malt does not allow the embryo to develop as much as ale malt, so there will be more SMM in lager malt. 

But (and this is key) we don't end there: in the presence of oxygen, DMS converts to DMSO. During the growth phase, oxygen is pumped into the steeping water so the barley doesn't suffocate and stop germinating. Longer growing time means more uptake of oxygen which means more DMSO. Ale malt grows longer, so it has more DMSO. 

DMSO is a different molecule than DMS and is not as volatile. It will however, be converted by yeast into DMS. That's a problem if you assume that ale malts can't have a DMS issue. It's also a problem because the DMS will be produced after the boil and you then have to rely on a vigorous fermentation to blow it away. If your yeast does not have a sufficient pitch rate, it's unhealthy or it's been reused too many times. Lower initial starting temperatures will also affect the vigor of the initial fermentation and although multiple studies have been done linking initial pitch temperature to DMS levels, they tends to be largely ignored in homebrew circles because they're told DMS issues come only from an insufficient boil. 

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DMS will also form from lysating yeast cells, though other problems tend to occur along with this and most home brewers already know not to let their beer sit on the trub too long. 

The last place you're going to find issues with DMS is in hops. Hops that have been sprayed with a fungucide - usually to prevent crop damage from mildew, can have detectable levels of DMS. Late boil and whirlpool additions will be susceptible to giving off levels of DMS, but a vigorous fermentation should solve that problem. The real issue will be late dry hopping where there is no or very little fermentation to drive off the DMS. If your hops smell of onions or garlic, they're susceptible to leeching DMS into your finished beer. 

If you've read this far, you're possibly overwhelmed, so here's the basics:

  • If using lager malt, a vigorous, 90 minute boil is your best option. 60, if you're using an ale malt. (Altitude is going to affect this, see here
  • A healthy, sufficient pitch rate is necessary to control DMS in ales. Pitch at 18-19C
  • Check your hops for onion or garlic odour. 
  • Make sure you don't leave your beer on the trub for too long.