What the heck is DMS?DMS is dimethyl sulfides, a compound that tastes of cooked vegetables. DMS is formed during the boil from a compound (S-methyl-methionine) that is created in the grain during the germination phase of malting grain. It is easily vaporized so during the boil you want the steam to float off into the air, not get trapped and descend back into your beer. DMS is considered an off flavor in most beers although it is usually always present in light lagers and considered part of the desired flavor, as long as it is not overwhelming. It will give the beer sort of a corn on the cob flavor. DMS can appear in your beer in other ways, such as through bacterial infection or by adding too much vegetation to your beer, such as chiles or dry hopping too long. These flavors are generally never desired.
DMS traditionally has been more of a concern for lager brewers than those of us strictly enjoying ales. Why? Lagers usually employ pilsner malt, which due to the malting process for pilsner malt tends to result in a higher amount of DMS. The usual solution is to boil pilsner malt for ninety minutes instead of the typical sixty minute boil. Recent experiments indicate modern pilsner malt is modified well enough the extended boil is unnecessary.
People have long believed DMS is formed by not cooling wort fast enough post-boil although this has mostly been rebutted by the no-chill brewers. Since DMS is formed at warm temperatures, if it forms at temperatures below boil then there's no vapor to carry it away so it all gets trapped in your beer. That's led to everybody believing you must use a chilling device and cool your beer within minutes. No-chill brewers, however, let their beer go into sanitized plastic containers right off the boil with no chilling at all and let it cool naturally. Yet in spite of their beers taking hours to cool they do not find their beers to taste like the vegetables on a cheap buffet.
For more info about DMS check out here and here.
Using the Lid During the BoilOut of fear for DMS, people often leave the lid off the boil and instruct others you should never use your lid during the boil. That is not exactly true. Using the lid does trap some of the steam but it also traps heat leaving with the steam. Keeping the lid all the way on the kettle or almost completely enclosing the kettle pre-boil is a good way to accelerate the start of the boil. Similarly, you can keep the lid partially covering the opening of the kettle to help trap some of the heat against the boil. I usually keep the lid almost all the way covering the beer as I wait for boil to begin. I leave it open just enough to see whether it is starting to foam up. Once the boil begins, I set the lid covering about half the pot.
Why do this? Well, if you are boiling on propane, it's not the cheapest method of boiling so using the lid to trap some of the heat in increases the efficiency of your set up and allows you to turn the propane a little lower. I don't know for sure how much propane I save that way but I know I use very little propane for a boil. I do the same thing on my stove top boils. I noticed when I started using the lid during the boil I could turn down the coil one or two numbers on the infinity switch. It's not a huge cost saving but over time you could save yourself a few tanks-worth of propane if you are boiling frequently.
|See the chimney on this old copper kettle?|
|More chimney on a modern system|
Don't throw the lid on completely, the steam needs to go somewhere. Putting the lid on might give you too little escape for the DMS. It's also a great way to cause boil over and nobody wants to see delicious wort spilling uselessly down the sides of their boil kettle.