Off-flavor in beer that resembles cooked vegetables is often attributed to the presence of dimethyl sulphide (DMS). This chemical compound is produced during the germination stage of the malting process, specifically when the barley is kilned. DMS is created from a precursor called S-methyl methionine (SMM), which is found naturally in barley.
During the malting process, barley is soaked in water and allowed to germinate. This germination stage activates enzymes that convert starches in the barley into fermentable sugars. As the barley germinates, SMM is also broken down by enzymes, releasing DMS as a byproduct. The DMS is then driven away during the kilning process, especially in darker malts where the kilning temperatures are higher.
However, in lighter colored beers, the kilning temperatures are lower, and some of the DMS may not be completely driven off. This can result in the presence of DMS in the finished beer, leading to an off-flavor reminiscent of cooked vegetables or creamed corn.
The perception of DMS as an off-flavor can vary depending on the individual. Some people are more sensitive to DMS and may detect it at lower levels, while others may not notice it at all. Additionally, the presence of other flavors and aromas in the beer can also influence how noticeable the DMS is.
To mitigate the presence of DMS in beer, brewers can take certain measures during the brewing process. One common method is to perform a vigorous boil during the wort production stage. Boiling the wort helps to drive off volatile compounds, including DMS. Additionally, using a well-modified malt that has undergone thorough kilning can also help reduce the presence of DMS.
It's worth noting that some beer styles, such as certain lagers, may have a slight amount of DMS as part of their flavor profile. In these cases, the DMS is intentionally present and can contribute to the overall character of the beer.
In my personal experience as a brewer, I have encountered the presence of DMS in lighter colored beers, particularly those brewed with pale malts. The cooked vegetable or creamed corn aroma can be quite noticeable, and it can be a challenge to balance it with other flavors in the beer. Through careful attention to the brewing process and ingredient selection, it is possible to minimize the presence of DMS and achieve a more desirable flavor profile in the finished beer.