The latest case adds to the growing number of discoveries in recent weeks of products tainted with the toxic chemical that were repackaged for sale and put back on the market in China instead of being destroyed after the 2008 melamine scandal, whereby milk powder laced with melamine killed six children and sickened an estimated 300,000.
Melamine, used in the manufacture of plastics, was cut into milk powder to boost protein levels and increase profits of suppliers in the country.
And the 2008 scandal culminated in the bankruptcy of state-owned dairy Sanlu and the execution of two people in November last year.
Companies involved in the string of recent melamine discoveries have been based in various provinces of the country, including Shanghai, Guizhou and the northeastern province of Liaoning.
And the Chinese authorities launched an emergency crackdown eight days ago as a result, with health ministry officials saying the probe would “thoroughly check potential problems in food safety”, said one report.
The investigation is set to end on Wednesday.
In the latest case, two dairy companies in Ningxia were shut down following the seizure of 72 tonnes of the milk product, with a hunt on to trace the remainder of the powder as it was repackaged by the Ningxia Tiantian Dairy Company and sold to factories in neighbouring regions, reports the China Daily newspaper.
The report said the tainted powder should have been destroyed in 2008, and that Ningxia Tiantian Dairy got it from an unnamed company as a debt payment.
The newspaper also quoted a representative from the Ningxia Dairy Industry Association, who claims that many small dairies in China do not have the technology to test for melamine.
All this raises questions about the determination of Chinese industry to reform and the ability of authorities to control the problem.
Following the 2008 scandal, China passed food-safety laws establishing standards, better testing systems and a method to recall problem products, placing more responsibility on food producers themselves to ensure their products were safe.
Officials also issued guidelines on how to destroy the tainted products, suggesting that they be burned in large-capacity incinerators or that small amounts be buried in landfills.
But the new law, which went into effect in June, has so far failed to prevent the sale of melamine laced products.