A reference pilsner entered the showdown, and thankfully wasn’t in a green bottle. We thought it looked great, like a bar of GOLD!! Smelling it, we caught diacytel. While that’s to style when it’s “restrained” we thought it was not restrained at all and it really put us off the beer. Matt successfully nailed it and called the beer, but the off flavors put the whole group off enough to rank it one of the lowest in the showdown.It even fared more poorly than several lightstruck examples. The indignity!
Truth is, writers of style guidelines don't know how to handle Urquell. This is the "Bohemian-style pilsener" entry from the 2016 Brewers Association style guidelines, the one used to judge beers at the GABF (bolds mine):
Very low diacetyl and DMS aromas, if perceived, are characteristic of this style and both may accent malt aroma.... Very low levels of diacetyl and DMS flavors, if perceived, are characteristic of this style.The folks judging pilsners may well have been looking at these very words while they sampled Urquell, because one thing is for sure: the diacetyl note* in that beer is not "very low." It's huge and aggressive, and because Urquell, a světlý ležák (12° beer), is so under-attenuated--it's just 4.4% ABV--that diacetyl really comes through as sticky and sweet. Budvar, by contrast, is also a 12° beer, but it's 5% ABV.
Writers of style guidelines have never known what to do with this, because breweries typically don't like their beers to taste of diacetyl. And no one would roll out a beer like Urquell now--it's just too weird and un-pilsnery. Indeed, German pilsners, which form the far more common template for international pale lagers, are specifically prohibited by style guidelines from having diacetyl. In my two trips to the Czech Republic, I managed to sample maybe two dozen pale lagers. I would not therefore forward myself as any kind of authority, but within that sample I rarely encountered diacetyl, and never anything like what you find in Urquell. The Czechs are very deferential to the first pilsner, and my guess is that they don't want to appear to be aping the original.
In conclusion, we should probably write style guidelines that say things like, "if your pilsner is awash in what seems like inappropriately slicky, buttery diacetyl, we'll consider it a fault unless the sample came from Pilsner Urquell." Splitting the baby and saying it's okay in low levels accounts for neither the way diacetyl appears in Pilsner Urquell (where it's massive) nor in most other pilsners (where it's absent). Otherwise you end up in a situation where people are eliminating the original pilsner from pilsner competitions because it's not brewed to style.
*Diacetyl is slick on the tongue, a bit full, and tastes like butter or butterscotch. It's so buttery, in fact, that it's commonly added to foods to make them taste like butter.