The Other Dreaded D: DMS (Dimethyl Sulfide)

Last week we tapped our second light-colored lager, a German Pils - made with 100% pilsener malt. Our fist light-colored lager, an American Premium, suffered from diacetyl to the max (not enough yeast, no diacetyl rest). This time around I managed to get rid of the butter, but handily discovered another flavor: DMS.

At first I wasn't quite sure what I was tasting. The beer is super clear and beautiful, but yet - what is that? Unlike our previous lager, this wasn't terrible. There was just too much of something unbalancing the beer. After having a couple beer-judge friends taste it, the culprit was identified - Dimethyl Sulfide.

A small level of DMS in lagers is acceptable. In fact, now that I know what it tastes like (think corn when drinking a Schlitz), I can pick it out in other beers. Ours just has a too much of it - thankfully there's enough hop bitterness in there to cover it up some, especially when it's nice and cold. But we pitched plenty of yeast, and cooled fast by pumping ice water through our immersion chiller, so....

What Causes DMS?
The first thing each judge asked us is if we boiled with the lid on. Nope. But, I don't think I boiled hard enough to drive it all off. In an effort to save gas and avoid kettle caramelization, I've been experimenting with a less-than-rolling boil - just getting it going enough to be boiling. Well, no more wimpy boils here, now that I know that a boil isn't just to sanitize and add flavor/hops to the wort, it's also supposed to drive off stuff you don't want in there. So it's time to crank it up a bit (but not so much that hot wort is leaping out of the kettle!). In general, a 10-15% evaporation rate per hour seems to be the goal. So if you start with 7.5 gallons, you want to boil off around 1 gallon or a little more in an hour, or around 1.5 gallons for a 90 minute boil. In the meantime, I'm just going to bring the iced keg to a 4th of July party, where I'm sure it'll get sucked down pretty quick on this hot Chicago day (update: it did). Cheers!