Considering the biology has been independently validated by different sources, I'm disinclined to accept this as simple brewing lore.
Yes, and all scientists agree that global warming is caused by man.
My own experience is the final nail in the coffin, for me anyway. Of course 50 ppm isn't a "switch", but it's a solid rule of thumb for obtaining relatively clear beer in a sane amount of time, especially with lager yeasts that tend to be quite poor flocculators.
And it deserves the respect, or lack thereof, that all rules of thumb deserve. Rules of thumb are great for beginners and will get them a decent or perhaps even good beer. I offer up rules of thumb all the time but always with caveats. Great beers are made by questioning the rules of thumb and violating them with caution. The two biggest improvements in my brewing in recent years came from violating this particular rule of thumb for not only Boh Pils (which violates it anyway if you are trying to be authentic) but all my lagers and strict control of mash pH. When people question my use of low calcium levels and admonish me that the beer won't clear I give the same response as when they carry on about not acidifying sparge water: It's lager beer. You are supposed to lager it. Yeast and polyphenol/protein complexes drop out during lagering. That's what it is for. Another thing to keep in mind is that one of the most important things that lagering does is clean up diacetyl. Yeast in suspension clean up diacetyl better than yeast which have flocculated and this may be a good reason for keeping the calcium low i.e. to keep the yeast in suspension a bit longer. Never thought of this aspect of it before but I do not need diacetyl rests.
It's also worth mentioning that Pilsner Urquell - arguably the gold standard of pilsners - is not clear. What you see on the shelves is a filtered version. The "original" version, Pilsner Urquell Kvasnicový is only served locally and looks like a wheat beer.
This is not the original. The original had, according to a newspaper report written at the time of the broaching of the first barrels "..magnificent golden hue , with the snow-white foam floating atop....' [PU Brewery literature]. Remember that PU was made the way it was because the nascent Czech glass industry made it possible to serve in a container which showed off its crystal clarity. Once Czech glass was widely available, turbid beers were no longer acceptable to the consumer and this was a major factor in the explosive increase in the popularity of lager beers. I'm sure you have seen the well known poster of the two lovely ladies serving up mugs of PU from a wooden barrel. That beer is clear. The artist took the trouble to make sure you can see through the mugs. So no question. The Ur Quell (original source) beer was clear.
The beer served at the brewery is crystal clear. That does not mean it isn't filtered (or more probably, centrifuged). Many things have changed at PU since I was there last. They don't ferment in open wooden vessels, they don't lager in huge wooden casks and they probably don't lager for three months anymore. They may not even do triple decoctions. They do pasteurize and they probably centrifuge - all to get the beer out faster. The beer has suffered but the consumer is generally not sophisticated enough or old enough to know the difference.
Turning now to the Kvasnicovy, that term (Kvas means yeast in Slavonic languages) describes beer to which kreusen beer has been added just before packaging. If you research it a bit on the web you'll find at least one place where it is described as PU with wort and yeast added. In fact I have no idea as to whether that description is valid. It was not available the last time I was in Pilsen (long before the days of SAB). From the photos it looks like zwickle beer to me and the descriptions of it I see from people who have tasted it (I have not) indicate that that is probably the case i.e. it's simply beer that has not been fully lagered. Or it could be that the yeast are added back in. In other words, it is indeed like wheat beer - the yeast have been forced back into suspension.
If you want to make a really good pils, use soft water (violate the 50 ppm rule), set mash pH to 5.4 with acidulated malt or a sour mash, do a triple decoction mash, pitch as cold as you can, ferment at 48 and lager, making sure to bring lots of yeast over into the lagering vessel, for 3 months as close to freezing as you can get. You won't need to filter. You won't need a diacetyl rest. You'll get a beer as good as what PU used to be because that's how they used to make it. Try doing it this way. You may decide you like it. Or you may decide it's too much work and takes too long.
All in all I suppose it comes down to what a "sane" amount of time is. Back in 1853 three months was certainly considered sane. Herr Groll, without knowing about the 50 ppm rule of thumb and, therefore, inadvertently grossly violating it, managed to revolutionize the brewing world. But it took three months lagering. A modern brewery's accountant would not consider 3 months sane - not if you can get something out the door in weeks and you can. It just isn't quite as good.
