BEIJING - Fake peanut oil that could potentially cause infertility was confiscated in South China on Tuesday, a fresh warning of food safety in the world's most populous country.

Authorities in Dongguan, Guangdong province, seized nearly 600 substandard and fake edible oil products after they checked several markets, Shao Xiongfeng, with the Dongguan administration for industry and commerce, told China Daily on Wednesday.

Selling fake oil products appeared to be rampant in some of the markets, Shao said.

"In one shop, six out of 20 brands of peanut oil products are problematic," he added.

All the confiscated products were produced by Yonglong Food Factory, and authorities conducted the checks after an undercover reporter from the Southern Metropolis Daily discovered the factory processed more than 4 tons of substandard and possibly hazardous peanut oil daily late last month.

The factory, in Zhongtang township, blended soybean oil, palm oil of unknown origins, essence and crude cottonseed oil. Essence was added to let the mixture smell like peanut oil while cottonseed oil was used to make the liquid clearer to look like peanut oil.

Doctors said crude cottonseed oil can lead to infertility.

"Unrefined cottonseed oil is toxic, and experiments have proved it poisons sperm cells and can cause male infertility," said Wang Xingguo, director of nutrition department at Dalian City Central Hospital.

The price of 5 liters of peanut oil is about 100 yuan ($15.9). Fake oil from the factory sells at 92 yuan. But the cost of soybean oil and palm oil products - even if they are qualified - is about one-third the price of genuine peanut oil.

The factory owner, surnamed Liu, displayed a business license to the undercover reporter, according to the Southern Metropolis Daily report. But China Daily did not find the factory in the search system of registered businesses in Dongguan.

The oil products, after being delicately packaged and labeled, are difficult to differentiate from genuine ones such as those under famous brands including Master Kong, a brand of Tingyi (Cayman Islands) Holding Corp.

The fake oil was sold to food processing mills and food markets in Guangdong, Hunan and Jiangxi.

This is not the first time that fake peanut oil has been found in Guangdong.

In March 2011, police in Shaoguan city demolished an illegal shop manufacturing "peanut oil" by blending soybean oil, essence and coloring. More than 700 fake products were seized.

In 2008, official checks found more than 70 percent of peanut oil product samples purchased from local markets failed to meet national standards.

An expert from the Chinese Cereals and Oils Association, who declined to be named, said there are at least 30 edible oil factories producing fake peanut oil in the province.

"And the majority of them are registered businesses," he said.