Guide to Off-Flavors: DMS

A problem that all brewers face is managing and eliminating off-flavors. For the first post of this series, we'll take a look at DMS.

What is it?

DMS (dimethyl sulfide) exhibits itself as a canned vegetable or creamed corn flavor in beer.  

How does it form in beer?

All malt contains a variant of the amino acid methionine called S-methyl methionine (SMM).  SMM is pulled into the wort during mashing and lautering and as the wort is heated the SMM degrades and is converted into DMS.  DMS forms at 180°F and above in the brewhouse. This is why it is important to boil vigorously in the kettle to volatilize the DMS, and also why it is important to not hold wort in the whirlpool for excessive amounts of time - the DMS formed will not be flashed off.  Any delays seen in the kettle and whirlpool process, the brew should be held in the kettle and then vigorously reboiled for 15 minutes before transferring to the whirlpool.

How do you solve the problem?

The volatility of DMS allows it to be removed from the wort and out the ventilation stack during the boil.  Even after the wort is chilled to the point where SMM is no longer being converted into DMS, the whole corny story isn’t over yet. During fermentation, the carbon dioxide that is produced by the yeast has a scrubbing effect on the DMS, carrying some of it out of the beer. This happens more efficiently at higher temperatures since the fermentations are more vigorous, and for this reason ale fermentations are better at this than lagers. This is why many lagers tend to have somewhat higher levels of DMS than ales do.

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