Don’t think fast food companies are taking consumers’ preference for “clean” food seriously? Just look at the war of words that has erupted this week between Chipotle and Panera Bread.

While discussing Chipotle’s recent announcement that its entire menu now is now preservative-free, CEO Steve Ells told Business Insider that chains like McDonald’s and Panera are misleading customers with that “clean” buzzword, which the industry uses to indicate that a food is free of preservatives or artificial ingredients.

Ells takes issue with competitors using the word when they still include “natural flavors” in their food, pointing out that you can’t buy those “natural flavors” at the farmer’s market.

Chipotle’s chief marketing officer Mark Crumpacker weighed in as well, saying that the biggest difference between Chipotle’s and Panera’s claims of a clean menu was that Chipotle didn’t “have any of these industrial additives of any kind.”

He went on to say that other companies have removed artificial flavors or colors, but that their menus are still “littered with colors, flavors, preservatives, dough conditioners, gums, emulsifiers, [and] humectants.”

At first, Panera responded by telling BI that it was working toward using only ingredients someone would find in a home pantry. Today, the company’s CEO joined the fray, calling Ells’ remarks “personally offensive.”

“I think that Steve Ells and the folks at Chipotle have had more than their fair share of problems,” Panera CEO Ron Shaich told Business Insider. “With the type of problems they have had, maybe they shouldn’t be throwing bombs.”

And by “problems,” he likely means Chipotle’s highly-publicized food borne illness nightmare.

Shaich says Panera has been working toward a “clean” menu for decades, and has always been straightforward with customers about what that word means.

“For us, this hasn’t been something that we woke up yesterday to try to do,” he told BI. “It’s something we have been trying to do for 20 years.”

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