In this Oct. 16, 2008 file photo, workers check for industrial chemical melamine in milk products in a lab of Yili Industrial Group Co., one of China's largest dairy producers, in Hohhot, north China's Inner Mongolia region. Three dairy plant managers and one milk powder dealer in central China have been arrested for allegedly selling milk products tainted with the industrial chemical melamine, shortly after the government launched a 10-day crackdown, state media reported Wednesday, Feb. 3, 2010. (AP Photo/Alexander F. Yuan, File)

(AP) -- The resurfacing of tainted milk products in China highlights the challenges of policing the food supply in a country where close ties between local authorities and companies hamper regulation while producers are undertrained, experts said Thursday.

The problems have dealt another blow to China's efforts to restore confidence in its dairy supply after a massive contaminated milk scandal in 2008 left at least six babies dead and sickened 300,000 other children. At the time, China promised sweeping changes and punished dozens of officials, dairy executives and farmers. In November, it executed a dairy farmer and a milk salesman.

But the penalties failed to deter others, and local governments with close ties to dairy companies often shield them from being punished, leading to the new misdemeanors, said a food safety expert at Renmin University in Beijing.

"When companies violate the law, the government raises its stick high, but lets it fall down softly," said Zheng Fengtian, an agricultural economics and rural development professor. "The government coddles those companies too much and considers more the economic and employment impact that would occur if such companies suffer."

The 2008 scandal exposed the widespread practice of adding melamine, a chemical normally used in making plastics and fertilizer, to watered-down milk to increase profits and fool inspectors testing for protein. When ingested in large amounts, melamine can cause and .

At least five companies are believed to have resold milk products tainted with melamine that were supposed to have been destroyed in the earlier sweep, the Health Ministry said this week as it launched a new 10-day crackdown on the dairy industry. The ministry has not said if anyone was sickened by the latest contamination.

"Some companies and individuals are still ignoring the safety and health of the mass of the population. Their hearts are full of greed, and they committed crimes," the ministry said in a statement.

Three dairy plant managers and one milk powder dealer in central China suspected of selling melamine-tainted milk products were the first known arrests, announced Wednesday, in the crackdown after contaminated products were recently found in several provinces.

The scandal, China's worst food safety crisis in years, prompted the government to tighten regulations and vow to step up checks. But enforcement is weakened when local governments place the interests of their local dairies above regulation, allowing milk producers to be more daring, another food expert said.

"Recently there are a lot of melamine problems happening because people thought the crackdown on melamine is over and the milk powder produced two years ago will soon be expired and there are people who want to take the risk" of selling it, Chen Yu, a professor at the Beijing Agro-Business Management University, told the Southern Weekend newspaper.

The China Dairy Industry Association's chairman could not be reached for comment Thursday.

The recent spate of tainted milk reports also underscores China's struggle to effectively regulate a massive food industry full of small, scattered operations.

China adopted a food safety law last summer that places more responsibility on food producers to ensure their products are safe, but it will take more time for the law to be fully implemented, said Dr. Peter Ben Embarek, a WHO senior scientist on food safety based in Beijing.

"You have millions of food producers who all also need to be trained and educated, in some cases in some very basic food hygiene and food safety principles," Ben Embarek said.

"What is a little bit discouraging is to see that there are still producers out there who have not understood the seriousness of tampering with food safety and are continuing to put profits before safety in their products," he said.

Concerns about tainted peaked again early this year after authorities in Shanghai said they secretly investigated a dairy for nearly a year before announcing it had been producing tainted products.

The case was especially troubling because Shanghai Panda Dairy Co. was one of the 22 dairies named by China's product safety authority in the 2008 scandal, with its products having among the highest levels of melamine.

In other recent cases, officials in late January said tainted dairy products from three companies were pulled from more than a dozen convenience stores around the southern province of Guizhou. Officials said products recalled during the previous scandal somehow made it back to the market.