When some KFC restaurants secretly switched to frying oil free of trans fats, customers didn’t notice. Attempts to rid the chain’s popular biscuits of a shortening filled with the artery-clogging substance did not go as smoothly.

KFC announced Monday that it would no longer fry chicken in artery-clogging trans fats, but unless the company finds a substitute for the shortening, the biscuits could be outlawed in New York City — just one casualty of the city’s proposal to ban trans fats.