Originally Posted by TheFlatline
You sound like you're making extract beers. DMS isn't as big of an issue if you're going with extracts. It *can* make a difference if you're going all grain. Note I didn't say it *would*, I said it *can*.
It's generally just a good habit to get into. Unless you have a tempered glass lid you're at risk of a boilover without realizing it if you boil with the lid on.
If you're having trouble getting water to boil, or even wort to boil, putting the lid on to get it up to a boil isn't a bad thing. DMS forms during the boil. Even I do it with my turkey fryer just to save on fuel and cut down the immense amount of time a brew can take.
PS: Get yourself a turkey fryer. When you boil 5 gallons of water in 25 minutes you'll thank yourself.
PPS: Lid on/lid off isn't exactly "new". It's been common knowledge for years.
Just to be completely accurate, DMS doesn't form just during the boil. DMS FORMS, if there are significant DMS precursers in your wort, anytime the wort is over 140 degrees. But, during the boil is when DMS that has formed is released through the bubbles that come out of the wort while boiling.
Where you have significant precurseres is dependent mostly on what type of malt you use. The lower degree to which barley is malted, the more DMS potential of that malt. So, if you are using Pilsner Malt, then you have significant chance of creating DMS. If you are using 2-row, it is reduced and so on with other malt types.
It is very true though that if you are brewing extract brews, DMS is not as much of a concern and that is probably why Baccus is not noticing any problem with his beers even though he boils with the lid on.
For all-grain though, DMS is a very real concern and should be avoided if at all possible. DMS is not merely an academic subject in the world of brewing. Comercial brewers deal with it by venting the tops of their brewhouses and collecting condensation from the inside of the brew kettle so as to prevent DMS from falling back into the wort.
All this is simply the long way of saying what has already been said. Definitely use a lid to get your wort up to a boil if needed, but once you have reached a boil, you should remove the lid in order to facilitate removal of DMS from your wort.
Additionally, DMS is one of the reasons that it is so important that you cool the wort quickly after boiling. As long as the wort is between 212 and about 140, new DMS is being created and since you are no longer boiling it will not boil off. Therefore, the faster you get that temperature down, the less DMS will be in the beer.