From what I've read and heard from Pope Jamil Z the undesirable production of DMS is the effect of the wort staying hot for to long.
Description: Tastes and smells of cooked vegetables/corn/cabbage or shellfish/seafood.
Cause: Bacterial infection can cause DMS, as well as covering up your brewpot during the boil. DMS is removed from the wort during the boil, so covering it up is obviously going to cause the DMS-laced condensation to fall back into your brewpot.
Remedy: Keep good sanitation procedures, and make sure to keep a rolling, uncovered boil when brewing. Do not cover your brewpot during the boil!
The lighter the beer the more noticable DMS will be although a little is sometimes considered desirable in some light lagers.
DMS is produced anytime the wort is above 140F. The idea is to get the wort below 140 ASAP. During the active boil it's being driven off, but as soon as the boil stops it forms and stays in the wort. If you can't get the wort below 140F in 20 mins, you can also boil for 90 mins to drive off all the SMS which is the raw material for DMS.
I started with a counterflow chiller in my 5gal days and it was perfectly adequate for that size batch. With 10 g batches it took too long so I switched to an immersion chiller and now my wort is to 68F in 20 min.
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