Local officials have shut down all factories following the CCTV report, which said some plants use the chemical to accelerate the process.

The report, aired on Friday, reveals that some food processors are found to use the chemical, mostly used for industrial purposes as a catalyst.

The industrial substance, which contains poisonous ingredients, is banned according to China's Food Safety Law.

During the massive shutdown, 30 legitimate plants have also been affected as their licences were revoked.

"All the plants are being investigated. The eggs and the industrial copper sulphate that have been confiscated have been sent for lab tests," a county official surnamed Xiong was quoted by the Beijing Times as saying on Saturday.

Generally, preserved eggs, a pungent appetizer usually served with pickled ginger or cooked in congee, become edible after duck eggs are preserved using an alkali, salt and clay for some two months.

But with copper sulphate, the preservation stage will be halved, and therefore, brings more profit.

"Chemical ingredients sometimes can be used in food processing under special standards. But it is not allowed to add industrial additives to food," Zhang Yongjian, chief of the Research Center for Development and Regulation of Food and Drug Industry from the Chinese Academy of Social Sciences, told the Global Times.

For preserved egg producers, there is no copper sulphate specially made as food additives. "You can't find the chemical for food use," a worker with the Nanchang-based preserved egg plant, Shenzhu Tianyuan Food Company, who declined to be named, told the Global Times, adding that it is no secret that almost all preserved egg processors use industrial copper sulphate as substitute.

Liu Dong, an SFDA official, said so far, no company in the country is licensed to produce copper sulfate as a food additive.