The market watchdog has sealed nearly 150 kilograms of oranges from a shop in the city's northeast Yangpu District following claims that the seller was adding illegal coloring agents to make the fruits look fresher and more attractive.

Quality tests are under way to find what kind of chemicals were used, Yangpu officials said.

The scandal came to light after a resident, Zhao Zongming, complained that he had bought three oranges from the shop, only to find the orange color fading after the fruits were washed.

"The dark-color water even made my fingers turn red," Zhao said.

The owner of the fruit shop on the intersection of Siping Road and Guoshun Road proclaimed innocence, saying the fruits may have already been tainted before arriving at his shop.

The oranges were from a fruit wholesale market in Beicai, Pudong New Area, he revealed.

Several calls to the market yesterday yielded no information except that the bosses were away.

Although covering fruits with edible wax is permitted, fruit coloring is strictly banned according to food safety regulations.

Yuan Yaxiang, head of the Shanghai Fruit Association, told Shanghai Daily that fruits such as apples and oranges are often polished with wax before being sold in markets and shops since it enhances the fruits' appearance.