China stopped importing pork from six processing and six cold storage facilities in the United States on Wednesday to enforce its ban on the use of a feed additive that promotes lean muscle growth, the US Department of Agriculture said on Tuesday.

To ensure food safety, China had in March said that third-party verification was necessary to ensure that US pork shipped to the country is free of the additive ractopamine, which is sold for hog farm use as feed additive.

Ding Lixin, a researcher at the Chinese Academy of Agricultural Sciences in Beijing, said such quality control measures are commonplace in the domestic market, especially the checks to trace the presence of ractopamine in pork. The new move indicates that the government is implementing quality checks on imported pork products also.

"China is already the world's biggest pork producer, and the stringent quality control norms will encourage domestic pig farmers to increase their livestock. Pork prices have remained at relatively low levels since June, largely due to the cheap hog prices, rising pork output and shifting diet preferences for mutton and beef," Ding said.

Since most of the imported US pork products are frozen, only a few can be sold in Chinese supermarkets after they thaw. As a result, a majority of the products are processed into sausage, bacon and ham in meat plants, as well as used as food in restaurants.

China imported 584,000 metric tons of pork from overseas markets in 2013, up 11.7 percent from a year earlier. It purchased 312,138 metric tons of pork from the US. The other pork products were imported from Denmark, Belgium, Germany, the Netherlands and the United Kingdom.

Pork packing plants now ineligible to export to China include Tyson Foods plants in Perry and Storm Lake, Iowa, along with the company's facility in Logansport, Indiana, the USDA said.

Other processors listed included a Hormel Foods Corp plant in Fremont, Nebraska, Triumph Foods in St Joseph, Missouri, and Quality Pork Processors Inc in Austin, Minnesota.

Ding said the curbs will not affect US agricultural products exports to China.

zhongnan@chinadaily.com.cn