The fast-food giant has removed artificial preservatives from its American cheese, buns and Big Mac special sauce, a move that affects almost two-thirds of its burger lineup in the U.S. The pickle slices still have a synthetic preservative, and the ketchup contains high-fructose corn syrup.

“Customers are becoming more demanding around really knowing what’s in their food,” Chris Kempczinski, president of McDonald’s U.S. business, said during a conference call Wednesday.

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The Golden Arches has been on a mission to clean up its menu since Steve Easterbrook took the helm in 2015. It switched to fresh, instead of frozen, beef for its Quarter Pounders this year, and got rid of artificial preservatives from its Chicken McNuggets in 2016. It also nixed margarine from its Egg McMuffins, which sent suppliers into overdrive to make and ship millions of pounds of butter across the country.

Scrubbing ingredients isn’t easy for the world’s biggest restaurant chain with 37,000 global locations, including 14,000 in the U.S. Even a seemingly small change involves years of work within McDonald’s complex supply chain. The Chicago-based company is following moves by chains such as Chipotle Mexican Grill Inc., which shifted to a non-GMO menu, and Panera Bread, which even has beverages that are free of fake flavors, colors, sweeteners and preservatives.

McDonald’s most-recent cleanup took more than a year, and more changes are likely based on where suppliers are able to do more, Kempczinski said. He said the costs to the chain are “minimal” and that menu prices won’t be raised.

“Some of the products have proven to be a little more challenging than others,” he said.

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