INGOLSTADT, Germany >> A pair of Bavarian dukes came to this pretty town on the Danube River 500 years ago and laid down what Germans claim as their source of beer-brewing prowess: the purity law.

Only hops, water and barley should go into beer, decreed the dukes. Wheat, above all, should be spared for real bread for the hungry people.

Half a millennium later, of course, there have been adjustments — yeast, for instance, has been permitted since the 17th century. But Germans, nothing if not careful conservers, still revere their purity law, or Reinheitsgebot, and insist it is the way to make beer, even though it is flexible when Germans want it to be.

“It is the oldest law still in force anywhere in the world regulating a consumed product,” said Gerd Treffer, a local tourism chief and historian. “And of course a great marketing argument for German traditions.”

But even as Germans celebrate the anniversary of the purity law, with dozens of events planned throughout the year (among them, an art show themed around “the beer gut”), not everyone is so enthusiastic about it.

Alexander Grau, a columnist for the political and cultural monthly Cicero, argued for scrapping it. Far from protecting consumers, it is a marketing ploy, “a fetish which Germans cling to like the pope to his creed,” Grau wrote. More than that, others argue, it has stifled invention and imagination.

One man who certainly hopes that Germans are ready for a change — as real ales, craft beers and other variants have boomed elsewhere around the world — is Greg Koch, 52, who opened Stone Brewing in San Diego County, California, in 1996 and now plans to distribute his craft beers brewed in Berlin around Europe.

What does the Reinheitsgebot mean to Koch? “Apart from a long and difficult-to-spell word — and I actually do know how to spell it — it was a taxation law,” he argued. Forget all those centuries of tradition.

“I bet almost no average beer drinker would be able to tell you that it only showed up on a German beer label for the first time in the 1950s, and it was only used as a marketing term in the 20th century,” he said.

It also means that, even though he will be brewing his craft beers in Germany, he will not be calling them German beers.

Still, Koch says he has invested $25 million in the belief that Germans will appreciate a break from what he calls industrial food and drink and seek the kind of variety that has developed in California. Any day now, his brewery, beer garden and slow food restaurant will open in full in a renovated 1901 red-brick building at an old gasworks in southern Berlin.

“There is something beyond the so-called corporate TV beers,” he said before showing off his location recently. He wants Berliners to see that “cheap is not perhaps an actual ideal,” and have them “embrace the idea of beer as artistry, an artisan product, not an industrial product.”

Disregarding advice that his location is too far from other parts of Berlin to become popular, Koch has espoused experimentation over German caution. “If you build it, they will come,” he said.

But when it comes to beer in Germany, others have already built — that is, a formidable reputation, which they proudly defend.

“It is a valuable brand for Germany, which we must keep,” said Josef Pfaller, 50, the production chief at Herrnbrau, part of which dates back to 1471 and is one of 30 breweries in this town of 140,000. “There are few things that enjoy more consumer confidence than beer.”

Indeed, the 81 million Germans knock back more of the brew per person than people in any other nation except the neighboring Czechs.

Germany has about 1,250 breweries — about half of them in Bavaria, where President Barack Obama swilled a morning glass (nonalcoholic, we were told) with Chancellor Angela Merkel at the G-7 summit meeting last summer and has lamented not (yet) visiting Oktoberfest.

But, even in Bavaria, as Pfaller said during a three-hour tour of his brewery (and just a few sips of his fruity jubilee beer), “the world does not stand still.”

“Of course things have changed since the 16th century,” said Pfaller, who was born about 30 miles from here, learned brewing on the job for three years and then got a university degree in, essentially, making, managing and marketing beer.

He now helps oversee a staff of about 80 and machines both ancient and modern — the old brewing vats that came to this 1974 plant when Herrnbrau moved from the center of the old town, state-of-the-art American equipment, and noisy German conveyors that marshal, clean and fill 27,000 bottles each eight-hour working day.

This mix of tradition and modernity is typical of Germany’s Mittelstand, the medium-size companies that are the backbone of the country’s economic success.

Just as German engineers strive for the better widget, blending their craftsmanship with 21st century technology, so Pfaller’s business is always monitoring shifting tastes and demands.

When Herrnbrau noticed, for instance, that younger beer drinkers (around 18) on its tours preferred milder beers than drinkers of 40, they came up with a new brew, Panther Weissbier, using the formerly banned wheat.

“We adjust within the purity law,” Pfaller said with a smile. “Many details can make the difference to a beer” — especially the water, which in his case comes from a spring 250 meters (about 820 feet) below ground and is, he said, believed to be 8,000 years old.

Even with the purity law, purity is a relative thing. A research group, the Environmental Institute of Munich, caused a stir in February when it tested 14 of Germany’s best-selling beers and found traces of glysophat, a pesticide suspected of causing cancer, in all of them.

So much for consumer protection, concluded Sophia Guttenberger, the biologist who oversaw the testing, which she said had been timed more for a coming European decision on whether to ban glysophat than the 500th anniversary of the purity law.

But it is just that kind of concern that spawned the law centuries ago.

Treffer, the tourism chief, proudly lauds a historical exhibit that shows the fortunes and population of Ingolstadt, rising and falling with the vicissitudes of history.

Ingolstadt — better known today for Audi cars — was for centuries home to a university opened in 1472, and the oldest seminary outside Rome, he noted.

Back then, both students and peasants needed protection from the substances then being thrown into beer. So, in 1516, the dukes also acted to keep highly intoxicating substances from the people, Treffer said.

Oh, and just maybe, to preserve power — for “those who drank too much could not work,” Treffer said, “and those who did not work could not be controlled.”