Do not mess with Germans and their beer.

Brewing beer is a sacred thing here. And there are rules one must follow. Serious rules.

One hugely popular tradition is the Reinheitsgebot, the “beer purity law” that says which ingredients are allowed. This past weekend it turned a sprightly 500 years old — the world’s longest-ruling food law, the German Brewers’ Federation says.

German Chancellor Angela Merkel attends the 500th anniversary ceremony of the German Beer Purity Law in Ingolstadt on April 22. Credit: Michaela Rehle/REUTERS

But while Merkel and beer lovers worldwide may cherish German purity, the old rules are becoming quite a buzzkill for upstart brewers trying to inject a dose of creative flavor. Some craft beer makers, in effect, are losing the right to call their brews beer at all.

It started with a 1516 decree by Bavarian Duke Wilhelm IV. The Reinheitsgebot mandated that beer could only contain water, barley and hops. A later tweak to the law added yeast to the list.

Hops, barley and water make the magic happen. Credit: Michaela Rehle/Reuters

By keeping impurities and additives out, brewers here credit the Reinheitsgebot with the excellent reputation German beer enjoys at home and abroad. Even non-German brewers find much to admire in this medieval statute.

"There's been a worldwide trend with foreign breweries asking my department more and more what needs to be done to brew beer according to the German Reinheitsgebot," says Frank-Jürgen Methner, a professor of brewing science at the Technical Institute of Berlin.

Consumers' growing interest in naturally sourced products has also boosted the law's stature. Beer drinkers appreciate knowing exactly what is in their brew, Methner says.

But others are less convinced. Some say the restrictive purity law is holding back innovation in Germany's craft beer scene and propping up big business.

Camba Bavaria, based in Chiemgau near the Austrian border, is one victim of strict purity law enforcement. Often cited as one of the most creative microbreweries in Germany, Camba has repeatedly come under fire with authorities for flouting the Reinheitsgebot with unorthodox ingredients like orange zest and — God help 'em — coriander.

In March, Camba even pulled from the market its coffee porter, which contains coffee beans, after Bavarian food safety authorities classified the beer as "consumer deception."

There are some workarounds, however.

"Small brewers can get away with quite a lot," says Rory Lawton, a beer consultant based in Berlin and founder of Berlin Craft Beer.

Microbreweries like Berlin's Heidenpeters have skirted these regulations by simply christening their fruit- and spice-bearing beverages as "Malzgetränke" — meaning “malty drink” — instead of beer, Lawton says.

And many flagship craft beer styles, like hoppy India pale ales, can still be brewed according to the law, he added.

Germany's notorious bureaucracy does grant room for exception. "Brewers can apply for special permission from local governments that allow them to brew Belgian-style beers with sugar," Methner says.

Still, some experts say the law is a tool that serves larger industrial breweries that aren't interested in experimenting with new flavors and tastes.

"The Reinheitsgebot is a romantic idea, but it's only there to protect larger breweries," Lawton says.

Craft beers that are handmade one batch at a time are more expensive to produce, even those like the IPAs and pale ales that fall within the confines of the Reinheitsgebot. That puts craft brewers at a disadvantage compared to Germany's industrial brewers, he says.

Lawton also thought the industry needs the myth surrounding the law to mask their sector’s unsavory truth: Germans drink less beer than they used to.

Beer sales here have been declining throughout the 21st century, from 10.7 billion liters sold in 2001 to 9.57 billion in 2015, according to Germany's Federal Statistics Office.

This trend is likely to continue. Euromonitor International, a market research firm based in London, predicted in its most recent annual report on the German beer industry that "beer consumption is expected to decline steadily" over the next five years.

To make up for this domestic decline in sales, Germany's large brewers have focused more effort on wooing consumers in developing markets such as China and India, where demand for German beers is growing, Lawton says. There, the Reinheitsgebot serves as a powerful marketing tool, with its strong romantic links to Germany's past.

"There's a lot of money there for a 'pure' product," Lawton says. "If Germany can claim that it has this long reputation for brewing the best beer in the world, then that's where the market is."

Supporters see no harm in using the Reinheitsgebot as the drinkable equivalent of "Made in Germany" to promote the country's image and history abroad.

"The Reinheitsgebot is very closely connected to Germany. It’s a great marketing tool. Why not?" Methner says. "I hope the law will remain on the books for another 500 years."