China's food safety watchdog announced on Tuesday that it had discovered seven companies producing salted red-yolk eggs contaminated with the dangerous red Sudan dyes.

The companies were in Beijing and the provinces of Hebei, Anhui, Henan, Zhejiang and Hubei, said the State Administration of Quality Supervision, Inspection and Quarantine in a statement.

The agency had required the companies to stop production and sales, recall all the problematic egg products and destroy them.

It asked local departments to further strengthen checks on egg products and take substantive measures to properly supervise the egg processing industry.

Sudan dyes are used legally in the leather and fabric industries, but are banned for food use. Last year, the Ministry of Agriculture reiterated the ban after the dyes were found in some brands of pepper sauce, chili oil and KFC's New Orleans roast chicken wings.

Wild ducks that eat small fish and shrimps usually produce red-yolk eggs, which are believed to be more nutritious than yellow-yolk eggs.

The contamination of duck eggs was first found in Hebei, where farmers fed ducks with the dyes to turn the yolks red.

Source: Xinhua