Burger King has an unusual ingredient it believes will convince diners its Whopper is fresher than ever: mold.

The fast-food giant released an ad Wednesday showing mold consuming one of its signature burgers to prove that they’re now free from preservatives.

The sandwich looks ready to eat at the start of the 45-second spot, set to the Dinah Washington tune “What A Difference A Day Makes.” But its plump sesame bun and bright-green lettuce gradually wilt during a 34-day time lapse as the burger becomes covered in a stomach-churning bluish-gray fuzz.

The ad has a point to make: Burger King has rolled out a Whopper with no artificial preservatives, colors or flavors at more than 400 US restaurants with plans to reach the rest of its eateries throughout this year.

“At Burger King restaurants, we believe that real food tastes better,” said Fernando Machado, global chief marketing officer for Restaurant Brands International, Burger King’s parent company. “That’s why we are working hard to remove preservatives, colors and flavors from artificial sources from the food we serve in all countries around the world.”

Burger King did not specify which preservatives have been removed from the Whopper, but it said more than 90 percent of the company’s food ingredients at US restaurants don’t have artificial preservatives, colors or flavors. The Miami-based chain said it has also taken colors and flavors from artificial sources out of its “core” sandwiches and sides in most European countries and across the US.

Burger King is the latest fast-food chain to pare preservatives from its menu. McDonald’s boasted in 2018 that most of its hamburgers no longer had preservatives, including the iconic Big Mac. Taco Bell said last year that it planned to continue removing preservatives and other additives from its food after stripping artificial colors and flavors from its core menu.

The fast-casual chains Panera and Chipotle have bragged about serving “clean” food without any artificial preservatives. But health experts say some additives aren’t inherently unsafe or bad for diners.

“If what you’re having at Panera is a 1,000-calorie panini with a day’s worth of sodium, or a 460-calorie soda, food additives should be the least of your concern,” Michael Jacobson, co-founder of the Center for Science in the Public Interest, said in 2015 when the brand ditched dyes and other additives.