Dimethyl Sulfide (DMS) is a sulfur compound produced during fermentation of beer that has the aroma of cooked or creamed corn. As part of my ongoing series on flavors in beer (including the earlier articles Esters in Home Brewed Beer and Judging Beer), we’re going to go into detail this week on DMS.
DMS (Dimethyl Sulfide) is a byproduct of mashing and fermentation, so it is present to some degree in all beers. It has the aroma of cooked or creamed corn. Because people can perceive DMS even at very low flavor thresholds (of 10-150 parts per billion) it can have a significant impact on the flavor of finished beer.
DMS is primarily found in lagers for a variety of reasons we will discuss shortly. DMS is actually desirable at low levels in many lagers, but excessive levels of DMS will create a strong cooked corn aroma and flavor. German lagers contain the highest DMS concentrations (50-175 parts per billion). American lagers generally contain less than 100 ppb, and British ales contain the lowest concentrations at 10-20 ppb. (Ref: Wikipedia).
Where DMS comes from:
All malt has in it a chemical called S-Methyl Methionine (SMM) which is responsible for DMS. SMM is an amino acid formed during germination and kilning of barley as part of the malting process. The maltster can reduce SMM by slightly under-modifying the malt, and adjusting the kilning temperatures, but this is largely outside the control of homebrewers. However, two row pale malts have significantly lower SMM levels than six row pale malts, so you can reduce your DMS levels by choosing a two row pale malt base. Also very pale lager malts (such as pilsner malt) tend to have slightly higher SMM levels due to the very low temperatures used in kilning.
Heating the SMM present in pale malt will always produce some DMS. During the mashing process (and even the boil), some SMM is broken down into both DMS and a variant of DMS called Dimethyl Sulfoxide (DMSO) which is basically DMS with an oxygen atom attached. So after the mashing process we have wort that contains both DMS and DMSO (as well as some residual SMM).
The good news is that DMS itself is very volatile and a lot of it will boil off rapidly when we boil or wort during the brewing process. However DMSO is more stable, and some of it can be converted to DMS during fermenation. Vigorous ale fermentations generally produce less DMS.
Finally, infection can produce a DMS like flavor and aroma, though it will generally be an aroma closer to cabbage than corn. If you have a strong cabbage aroma or flavor you may have an infection in your finished beer.
DMS in the Beer Brewing Process
DMS is created whenever wort is heated, so some DMS is present in any beer. DMS is created in the mash, however most DMS is evaporated during the boil, so the boil is the primary place to focus if you have a DMS problem.
The half-life for DMS is 40 minutes, so half of the DMS will be boiled off in a 40 minute vigorous boil. So if we do the math, a 60 minute boil gets rid of 64.7% of the DMS and a 90 minute boil rids us of 79% of the DMS. That is why most experienced brewers recommend a 90 minute or longer vigorous boil.
Since DMS needs to evaporate off during the boil, it is important not to cover your pot. Covering a brew kettle during the boil will prevent the DMS from evaporating and create a beer with much higher levels of DMS.
Rapidly cooling your wort after boiling is also important. The SMM to DMS conversion continues at temperatures well below boiling, so DMS is produced even while the wort is cooling after the boil. However, unlike the mash, DMS produced while cooling cannot be boiled off. This conversion continues even if the hot wort is vented. For every hour you have hot wort sitting around, you will produce approximately a 30% increase in DMS.
During fermentation, CO2 bubbles actually help remove from DMS from the beer. More vigorous ale yeasts tend to produce lower DMS levels. Also different strains of yeast do tend to produce DMS during fermentation primarily by converting DMSO (which does not boil off) to DMS. Lager yeasts and yeasts fermented at lower temperatures tend to have higher DMS production. Certain wild yeasts and bacteria can create high levels of DMS as well.
DMS aromas (including some sulfur or rotten-egg aroma) during fermentation (particularly lagers) are not unusual, so you don’t need to toss your beer out just because you have a DMS aroma during fermentation. Some of this will fade naturally during the lagering process.
Finally, beers with robust flavor profiles (dark beers, strong ales, etc) tend to mask the DMS cooked corn flavor with other flavors such as roast, chocolate or caramel malts. Because of this, high DMS levels are most perceivable in lightly flavored beers such as low-adjunct pilsners, many German lagers, continental lagers and extremely light ales. DMS is rarely a problem in beers that are Amber colored or darker, and also rarely an issue with most robust beer styles.
Controlling DMS – Summary
- High DMS levels are most often perceived as a problem in light lagers such as Pilsner and many German lagers
- Choose a 2 row pale malt (over 6 row) as a base malt to minimize SMM (a DMS precursor)
- Very pale base malts (such as pilsner malt) tend to have higher SMM levels which drives higher DMS production
- Avoid using corn as an adjunct with these beers, as it can enhance the creamed corn perception
- Boil your wort for 90 minutes or longer with a vigorous rolling boil
- Don’t cover your boil pot – leave it open so the DMS can evaporate during the boil
- Minimize hot wort standing time by rapidly cooling your wort after the boil
- Select a yeast and correct fermentation temperature to minimize DMS production
- For many lagers, DMS aroma is normal during fermentation, but it should fade with time as you lager and age the beer
If you brew a beer with high DMS levels (creamed corn aroma) take a close look at the suggestions above, and focus on your boil, as that is where DMS can be most easily controlled. Thanks for joining me on the BeerSmith blog. Check out my podcast or email list for more great brewing material!
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