However, the sauce that coats roast duck in Nanjing has a color and texture similar to soy sauce. Every restaurant and shop selling ducks in the city claim to have their own special recipe to make the sauce.

Not only does the sauce accompanying the duck have a secret recipe, so does the city's signature salted duck.

Xu Jing, who runs a duck shop in Nanjing's Pukou district, said, "The recipe is the secret that makes the ducks delicious and helps us survive in the city's competitive duck market.

"The ducks are soaked in a specially-made braising liquid for four hours after being covered with salt for three hours," she said. "The braising liquid at my shop has been used for nearly 20 years. The older it is, the better the aroma."

A range of spices, along with water, are added to the pot from time to time to ensure it remains full. New supplies of spring onion and ginger are bought every day.