Vegetal flavors in beer often manifest themselves as cooked cabbage, broccoli, or corn.
These vegetal flavors in beer are caused by sulfur-compounds which will produce a low level cooked cabbage aromas and to a lesser extent, flavors. Do not confuse these flavors with the fresh or raw vegetable or green grass aromas and flavors you find in some beers.
The primary source of these compounds are the malt or malt extract. It can sometimes be a combination of sulfur-like compounds and DMS which combine and produce a cooked vegetable flavor and aroma in your beer. The cure for these off flavors is the same as those for DMS. To learn how to prevent DMS, click here.
Look at the quality of your malts if these flavors or aromas are found. If the flavor persists, boil your wort longer to drive off the volatile sulfur-based compounds and DMS. Bacteria may also cause these flavors in your beer. Be especially careful when re-using your yeast as this is a primary source of bacterial contamination.
I have a feeling many of these vegetal flavors in beer and aromas as well, are yeast derived and are a normal part of the flavor profile of all beers. The flavors and aromas are just below the threshold where they become noticeable. Try a different yeast and/or a different fermentation temperature if nothing else works and see if this doesn't correct the problem.
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