Thirty companies producing preserved eggs have been closed by authorities in Jiangxi province after a media exposé that toxic chemicals were used to speed up production, the country's latest food-safety scandal. China National Radio reported yesterday that all eggs being processed at the plants in Nanchang county had been locked away for further testing, and that authorities were still screening small processing workshops without licences. A report by state broadcaster China Central Television on Friday showed that three plants producing preserved duck eggs, a popular delicacy also known as thousand-year eggs, were using industrial copper sulphate to halve the curing period to a month. Two of the three plants' licences were in order, raising concerns that the practice might be common among all processers in Nanchang county, which produces 300,000 tonnes of preserved eggs annually, or about 15 per cent of the country's total, according to CCTV. Industrial copper sulphate usually contains high levels of toxic heavy metals, including arsenic, lead and cadmium, so is banned for use as a food additive. Liu Dong , of the China Food and Drug Administration, said there was no copper sulphate produced as a food additive in China, according to the broadcaster. The eggs are usually preserved with baking soda, salt, and quicklime for about two months. The process turns yolks dark green and the egg white into a stiff, dark jelly. Using copper sulphate could significantly reduce the processing time while achieving the same effect. A plant boss told CCTV that virtually all factories in Nanchang used the chemical. "There won't be a problem if you don't eat too many of them," the boss said. After a string of food-safety scandals, Premier Li Keqiang vowed last month to "make the lawbreakers pay an unaffordable price for their illegal practices". But the public seemed unconvinced after this latest scandal. "Why do we only learn about these food-quality problems from media reports? What are the regulators doing?" asked an internet user in an online post.