BEIJING (AP) -- China has warned dairy producers that inspectors are on alert for fresh milk tainted with the industrial chemical melamine and another toxic substance extracted from leather scraps.
Both additives -- melamine and hydrolyzed leather protein -- would make dairy products made with watered-down milk appear to have normal amounts of protein. Infant formula tainted with melamine killed six children in China in 2008 and sickened more than 300,000.
The Ministry of Agriculture said in a undated notice posted to the website of the State Council, China's Cabinet, that authorities will carry out 6,450 random checks on fresh milk this year -- underscoring official concerns that dairy producers may still be trying to use illegal and dangerous methods to boost the protein content of their milk.
All the tests will check for melamine and 30 percent will look for hydrolyzed leather protein.
In March 2009, the Chenyuan Dairy Company in central China's Zhejiang province was shut down by authorities after leather protein was found in its products, the official China Daily newspaper reported Friday. It didn't say if anyone was sickened in that case.
Peter Leedham, a China-based food testing executive, said leather protein emerged as a dairy additive in the wake of the deadly melamine scandal.
"When the melamine issue broke and everybody started being able to detect melamine in milk, unscrupulous producers tried to find an alternative way, something that supplemented the protein in milk, so what they used, very cleverly, was the hydrolyzate of bovine leather," Leedham said. "Because it's actually protein and derived from a cow, it's almost impossible to detect as an additive."
Leedham, the managing director of Eurofins Technology Service in Suzhou, a city close to Shanghai, said it's not clear how widespread the problem is.
To find out if the substance has been added to dairy, authorities look for telltale leather-curing residues. The protein extracted from cow leather is not known to be dangerous to human health, but the curing chemicals are, Leedham said.
The China Daily newspaper said the chemicals could be fatal for children in high does and put adults at risk for osteoporosis.
The Ministry of Agriculture said in a statement posted to its website on Thursday that spot checks on 7,406 batches of fresh milk last year were all free of melamine and signs of leather protein.
But powdered, rather than fresh milk, has been the main source of melamine contamination. Last year China seized 2,132 tons of melamine-tainted milk powder that was manufactured in 2008 or earlier.
China ordered all contaminated dairy, including infant formula, yogurt and other products, burned or buried, but the government did not carry out the destruction itself. Some people apparently stockpiled the tainted products.
Leedham said leather protein was also more likely to show up in milk powder and other processed dairy than in raw milk.
"I would have thought that the adulteration, if it happens, would happen in the processing later on," he said. "If it's going to be used as a fraudulent way of increasing protein levels, then the big benefits would be in the food processing downstream and it would be much easier to add hydrolyzed protein to a powder mix than to liquid milk."
He also said 6,450 tests in one year didn't seem sufficient for a country China's size.
Associated Press researcher Zhao Liang contributed to this report.