Cheers to Duke Wilhelm IV

Photo by Jeff Quinn for All About Beer.

Today's the big day. On April 23, 1516, the Duke of Bavaria, Wilhelm IV, issued what would only much later come to be called the Reinheitsgebot, or the “purity law.” I'm going to link again to an article I wrote for All About Beer, which may be the best article I've written (and it's about as good an article as I can write). It's a deep dive into the law and reveals a lot about it that isn't commonly discussed. It starts this way:
Originally, though, that element was downplayed—it’s just the third stipulation: “in all our towns, marketplaces and the whole of the countryside, that beer shall have no other ingredients than barley, hops, and water be used and employed.” When you read the full document, running about 320 words in translation, these 27 don’t appear to be the main point. The first two stipulations regulated prices, capping what a publican could charge, particularly during different seasons. There was also a fourth provision, again about pricing, and a comment at the end where Duke Wilhelm reserved the right to change anything in the law during grain shortages. 
In other words, the original decree had a lot more to do about money than consumer health. Indeed, even the restriction on ingredients was only partly driven by the wish to banish the use of unhealthy additives from beer (though the practice was common at the time). Wilhelm was also concerned about protecting the food supply, and limiting brewers to barley freed up the wheat crop for bakers—which also helped keep the price of bread more stable.

Even the part we think we understand is generally misinterpreted. In detailing the allowable ingredients, Wilhelm does not specify yeast, which has caused modern writers to assume he didn’t know about it. Not only is that wrong, but it leads us to miss an important element of the Reinheitsgebot’s logic. Matthias Trum, the sixth-generation family owner of Bamberg’s famous rauchbier brewery Schlenkerla, explains how we should actually understand that famous omission.

“The yeast is in fact not mentioned; that is correct.” Trum, who studied history while earning his brewing degree at Weihenstephan, points out that brewers of the day were well aware of yeast’s existence. “In the Middle Ages, they had a profession called the ‘hefener,’ so they knew exactly. The purity law lists ingredients, right? Yeast I put in there and I get more out of it. I harvest the yeast at the end and I put it into the next batch. And that was actually the job of the hefener.” It’s actually hard to imagine how they couldn’t have known about it. Why? Because after you brew, you end up with a fluffy layer of stuff at the bottom of the fermenter: “Zeug.  Zeug was the German word, which is ‘stuff.’ The hefener’s job was to harvest the yeast from the batches, to press out as much remaining beer as possible, which was sold at a low price to the poor, and then the yeast was added to the next batch. You started with a smaller amount of yeast and then you ended with a bigger amount of yeast.” An ingredient, Wilhelm’s logic went, was something that stayed in the beer.
There's a lot more at the link, so click through and read the whole thing. And of course, raise a pint to this amazing, ancient human tradition. (It predated the start of the Protestant Reformation by a year.*) Just its continued existence is staggering.

*Do not talk to me about Jan Hus, you damn pedants.