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Question: My beer sometimes has a corn-like sulfury smell. What can I do about it?
Answer: A corn-like smell is usually due to a specific chemical compound called dimethylsulfide, or DMS for short. DMS forms in the malting process and in the boiling process while brewing any beer. It is the result of a thermal degradation of sulfur-containing amino acid in the grain called s-methylmethionine. This amino acid is present in all malts. Hence, all beers have some amount of DMS.
DMS is a highly volatile compound. This makes it an easily detectable compound in aroma, even in trace quantities. Fortunately, the volatility makes it fairly easy to scrub out of your beer.
The amount of DMS in your finished beer depends on a number of factors. First of all is the amount of s-methylmethionine in the grain. High protein malts such as six-row have higher amounts, hence more DMS. Some of the DMS is driven off by heat in the kilning process. Hence, the paler malts kilned at lower temperature have more. These two factors combined contribute to the fact that pale lagers (made from pale six-row malt), often have high levels of DMS. Not surprisingly, corn also contains a lot of s-methylmethionine. These characteristics are a hallmark of lager beers. A lager without DMS tastes rather bland.
The second factor that affects the amount of DMS is how much is removed from the wort during the boil. Every bubble of steam that comes out of the kettle will carry some DMS with it. Hence, longer boiling times and a more vigorous boil tend to reduce DMS levels. For best results, you want to keep the boil rolling for the whole time. Don't simmer your beer.
The third factor that affects DMS levels is the rate at which the beer is cooled. The decomposition reaction that converts s-methylmethionine to DMS is driven by heat. Once you shut off the heat and start cooling the wort, scrubbing action of the steam is halted. However, DMS continues to be formed until the wort is below about 160oF. Hence, a long steep will increase DMS.
The final factor (usually) is the amount of DMS that is scrubbed out during fermentation. Carbon dioxide bubbles will remove some of the DMS. Since more CO2 comes off at higher temperatures, more is scrubbed out of ales than lagers. Well-attenuated beers will be more scrubbed as well.
The plot to the right illustrates the potential DMS levels in your beer. Here we assume a 90 minute boil, a 60 minutes whirlpool at flame-out, followed by cooling and fermentation. The level of s-methylmethionine drops the longer the boil goes. The amount of DMS can rise potentially to the ending level of s-methylmethionine at flame-out if the wort isn't chilled. Conversely, a rapid chill would lead to almost zero DMS.
If you're not too careful about sanitation, you'll find that some bacteria also contribute to DMS levels. Coliform in particular is known to produce DMS. Generally, though, it will also produce other foul odors such as mercaptan (natural gas smell) and hydrogen sulfide (rotten egg smell), making the DMS level the least of your worries.
Generally, if you keep your boil vigorous and cool rapidly, DMS levels will be appropriate for style. However, the DMS level may still be too high for your taste. If you keg your beer, you can artificially scrub out DMS using CO2. This is best done before carbonating. If you have a lager, allow it to warm to room temperature first. Then, switch around the ball-lock fittings on your keg so they are backwards. Hook up the long dip tube (labeled "out") to the CO2 tank. Prop open the pressure relief and let the CO2 bubble through the beer. Slow down if it starts to foam out. Occasionally sniff the gas as it exits the keg valve. You should be able to smell the DMS at first, and then notice it fading with time. After the level has dropped, switch the ball locks back to their usual configuration. Then carbonate as usual.