Practical difficulties

According to a regulation covering China's meat reserve issued in 2007, the country stores frozen pork three times a year and keeps it for four months.

It stores frozen beef and mutton for eight months

"In China, the expiration date for frozen beef or mutton is usually between eight months and a year, compared with an average of two years in other countries," Gao said. "Meat that exceeds the time limit cannot be used."

Yang Bo, deputy director of the Changsha anti-smuggling bureau, said: "We face practical difficulties in testing the actual age of this rotten meat and cannot tell how long it has been stored."

The smuggled frozen meat usually turns up in small cities in provinces such as Yunnan, Guizhou, Sichuan and Hunan, where inspections and supervision are easier to avoid, Yang said.

Li Jun, a beef trader in Beijing, said that the meat could not be sold directly to customers.

"After being frozen for many years, the meat starts to oxidize and turn black.

"Marinating or frying it covers up the rotten taste, and restaurant customers cannot tell bad meat from good meat," Li said.

Buyers choose the meat because it is much cheaper than supplies from legitimate sources. An officer at the Beijing Entry-Exit Inspection and Quarantine Bureau said that, compared with storing frozen meat in a way that meets regulations, the cost of keeping smuggled frozen meat is very low, only 17 yuan per ton each year.

This is because the smuggled meat tends to be kept at higher temperatures than legitimate supplies, so the amount of electricity needed to keep it refrigerated is much less.

The low cost of storage has resulted in rampant smuggling of frozen meat into China, the officer said, adding that if the price of beef is below 25 yuan per kilogram, it is likely to have been smuggled from abroad.

The general administration has been cracking down on frozen meat products since the beginning of the year.

Between January and June, customs departments investigated more than 140 cases involving the smuggling of frozen meat and confiscated 420,000 tons of products, 1.3 times and 2.7 times the corresponding figures for 2014.

During the six-month operation, the customs administrations in Tianjin, Dalian and Shenyang in Liaoning province, Guangzhou in Guangdong province and Hunan province uncovered a number of major cases and detained a large number of suspects.

Customs authorities plan to step up efforts to combat the smuggling. Officers will inspect more mail parcels and intensify the examination of luggage at airports and train stations.

"We will concentrate on identifying criminal gangs and their leaders while enhancing judicial cooperation with other countries to share intelligence," said the official from the general administration's anti-smuggling department.