During the mashing process the S-Methyl Methionine (SMM) present in the grain will break down into dimethyl sulfide (DMS) and DMSO - an oxide variation of DMS.
Some grain contains more SMM than others which is primarily (as afar as I'm aware) due to the kilning temp of the grain. Lower temps means more SMM. Pilsner malt therefore contains more SMM than Munich for example.
DMS is a problem in beers as it has a flavour of cooked/creamed corn. All beers contain DMS, but some more than others and usually you want to reduce it as much as possible.
You can do this by boiling your wort for an extended period of time (60 min or more) as DMS is fairly volatile, which means it boils off at a low temp and is removed from your beer.
The half life during a rolling boil of DMS is 40 min so theoretically you reduce the DMS in your beer by half by boiling for 40 min, 75% after 80 min 87% by 120 min. Due to this evaporation of DMS, you should keep the lid off your boil kettle so it's doesn't just drip back into your beer.
The oxide variation of DMS isn't as volatile unfortunately and may be converted to DMS during fermentation. This is more common for cold fermenting lager yeasts. Ale yeasts if fermented vigorously can scrub some of the DMS out.
DMS can also be formed from SMM after the boil while the wort is still hot, so it's beneficial to cool the wort as quickly as possible to prevent this creating too much DMS.
I no-chill some of my beers (mostly lagers) and haven't had any issues with DMS, but I always do a 90min boil.
So, theoretically and generally speaking:
- Pils malt will give you more DMS
- lager yeast will convert more SMM into DMS during fermentation
- slower chill times post boil will increase DMS in your beer (like I said i haven;t had any issues with nochilling my lagers, but that may be due to some other reason, I'm not sure.)
ways to reduce DMS
- boil with the lid off your kettle
- boil for longer
- chill as fast as you can (also helps prevent infection and can help preserve hop flavour/aroma but i don't mean to discuss that as no-chill works very well for most people who do it, and can produce very hop heavy beers)
- use grain that contains less SMM - but this obviously isn't practical if you want to brew light lagers.
- using a vigorous ale yeast can reduce DMS in your beer
The reason you don't need to boil for 90 min with extract is because its likely been boiled for an extended period prior to packaging and the concentration stage (either to dry powder or thick gooey liquid malt) would have reduced most of the DMS for you.
That's a basic run down, hope it helps.
Edit: sorry i just read the last two posts, i didn't see them when i wrote my response. I should also add, that if you use 50% of your fermentables from kit/extract cans/powder then your DMS content would be dramatically reduced.
Edited by Alex.Tas, 19 January 2015 - 10:45 AM.