"I have no problem spending money on good and healthy food," he said of the Gorilla natural fast-food restaurant's lunch offering.
"I prefer good quality over quantity." Mueller, 40, smiled and pointed at his paunch: "It also helps me lose a little weight."
Despite skyrocketing world food prices, consumers such as Mueller have helped fuel a doubling of organic food consumption in Germany between 2001 and 2007, according to the Bonn-based research group ZMP, which tracks food prices and production in Germany.
Matthias Rischau opened the first Gorilla in 2006, and business in the last year has been so good that he has expanded into a mini organic-food chain, adding four more restaurants in the German capital.
At Rischau's restaurant where Mueller ate in downtown Berlin, a poster on the wall is a take on the famous illustration of the evolution of man from apes in reverse, with modern man devolving into the chain's trademark gorilla.
"I named it Gorilla because I saw this giant animal in the zoo reaching out for leaves and other greens around him, and that's all he needed to be so strong and powerful," Rischau said.
He said his business has been helped as people have watched superstars such as former Police front man Sting and actor Leonardo DiCaprio publicly adopt healthy lifestyles.
The stars "want to eat healthy food, and to support our environment", Rischau said.
Amid the global rise in food prices, Germany has seen food costs increase about 8 percent during the past year.
But that doesn't seem to have had a major impact on the organic food industry, despite the premium prices on its products.
Europe saw annual market growth in organic food of more than 15 percent in 2007 over the year before, said Helga Willer, a spokeswoman for FiBL, a Swiss-based research institute that tracks organic agriculture. Germany was the biggest consumer, accounting for about one quarter of the European market share.
Thanks to its trendy appeal and relatively cheap real estate for a major European city, Berlin has been at the heart of this, said Ulrike Netzker, spokeswoman for an organisation that promotes organic products in Berlin.
"Many families and young people live here," Netzker said. "They are more aware of healthy food."
As well as Gorilla, there is also RNBS, offering low-fat low-calorie Asian dishes with no additives, which has two locations in Berlin. The capital has many other smaller organic-food establishments, not to mention scores of organic supermarkets.
Rischau said part of his goal is to offer exclusively local foods at Gorilla - giving customers fresher products and also reducing his chain's carbon footprint by lessening the distance goods need to be trucked in.
"We don't want and need fancy fruits and veggies from elsewhere. We only take what grows around us. Seasonal and regional food, like the gorilla does," Rischau said.
Still, he acknowledges that sometimes the ingredients for his specialities - such as the mango in his popular spicy quinoa greens and mango salad - would have a hard time growing in the region around often chilly Berlin.
"We're not perfect," Rischau said. "Yet."