Today, we’ll take a quick look at the source of grassy flavors in beer. This off-flavor is caused by the “leaf alcohol” known as cis-3-hexenol. This compound arises in various vegetative systems (flowers, leaves, stems, etc) when unsaturated fatty acids such as linolenic acid are degraded. As you can see by the picture in that last link, linolenic acid is a fatty acid with a long 18-carbon chain (tail) with a few points of unsaturation (meaning double-bonds along the chain). These double bonds are highly reactive and the fatty acid chain can be broken here. When this happens, cis-3-hexenol can be formed as the tail-end piece and grassy flavors will result. In beer, this happens most frequently when old hops are used particularly if they haven’t been dried thoroughly or stored properly. So, if you’re growing hops at home and intend to use them in some homebrew, take note: the picked hops need to be dried down to about 30% of their original weight; roughly 8-10% moisture. So with improper hop production and storage influencing grassy flavor production, it stands to reason then, that these conditions could also lead to isovaleric acid production. All things being equal, however, the cheesiness of isovaleric acid will probably be noticed before the grassy flavors, since not only does isovaleric acid have a lower threshold than c-3-h (1ppm vs. 15ppm) but the source material for isovaleric acid (humulone; one of 3 alpha acids) is likely at a higher initial concentration than the various poly-unsaturated fatty acids (total fat content averages around 3% of the weight of hops).
There aren’t too many beers that I can think of that are heavy in grassy flavors, but I would hazard a guess that you are more likely to find them in European pilsners and lagers as they tend to be lighter in flavor (meaning they can’t hide defects as well) and they tend to use the traditional nobel hops which are used as aroma hops rather than bittering hops. This means, among other things, less source material for isovaleric acid production, as well as poorer storability and higher tendency to oxidize.
If anybody knows of any commercial beers which seem to be grassy in character, I’d love to hear from you. Shoot me an email through the “Contact” link above and I’ll see if I can track any down. For now, all my grassy beer is made by me, my stock solution of cis-3-hexenol, and my pipette.
[edit: 3/3/11, 12:53 EST, added language about hop drying.]